VIN+
ADC081000/
ADC081500
VCMO SPI
RG1
RG2
RF1
RF2
LMH6555 VCM_REF
VOUT+
VOUT = 0.8 VPP
VIN-
VIN a
+
-
RS1
50:
RS2
50:
RT2
50:
RT1
50:
340 mVPP
-
+
OPT LMV321
3.3V
OPT
LMH6555
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SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
LMH6555 Low Distortion 1.2 GHz Differential Driver
Check for Samples: LMH6555
1FEATURES DESCRIPTION
The LMH6555 is an ultra high speed differential line
2• Typical Values unless Otherwise Specified. driver with 53 dB SFDR at 750 MHz. The LMH6555
•−3 dB Bandwidth (VOUT = 0.80 VPP) 1.2 GHz features a fixed gain of 13.7 dB. An input to the
• ±0.5 dB Gain Flatness (VOUT = 0.80 VPP) 330 device allows the output common mode voltage to be
MHz set independent of the input common mode voltage in
order to simplify the interface to high speed
• Slew Rate 1300 V/μsdifferential input ADCs. A unique architecture allows
• 2nd/3rd Harmonics (750 MHz) −53/−54 dBc the device to operate as a fully differential driver or as
• Fixed Gain 13.7 dB a single-ended to differential converter.
• Supply Current 120 mA The outstanding linearity and drive capability (100Ω
• Single Supply Operation 3.3V ±10% differential load) of this device are a perfect match for
driving high speed analog-to-digital converters. When
• Adjustable Common-Mode Output Voltage combined with the ADC081000/ ADC081500 (single
or dual ADC), the LMH6555 forms an excellent 8-bit
APPLICATIONS data acquisition system with analog bandwidths
• Differential ADC Driver exceeding 750 MHz.
• Texas Instruments ADC081500/ ADC081000 The LMH6555 is offered in a space saving 16-pin
– (Single or Dual) Driver WQFN package.
• Single Ended to Differential Converter
• Intermediate Frequency (IF) Amplifier
• Communication Receivers
• Oscilloscope Front End
TYPICAL APPLICATION
Figure 1. Single Ended to Differential Conversion
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2006–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
ABSOLUTE MAXIMUM RATINGS (1)(2)
ESD Tolerance (3) Human Body Model 2000V
Machine Model 200V
VS4.2V
Output Short Circuit Duration(one pin to ground) Infinite
Common Mode Input Voltage −0.4V to 3V
Maximum Junction Temperature +150°C
Storage Temperature Range −65°C to +150°C
Soldering Information Infrared or Convection (20 sec.) 235°C
Wave Soldering (10 sec.) 260°C
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For specifications, see the Electrical
Characteristics tables.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of
JEDEC)Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).
OPERATING RATINGS (1)
Temperature Range (2) −40°C to +85°C
Supply Voltage Range +3.3V ±10%
Package Thermal Resistance (θJA)(2) 16-Pin WQFN 65°C/W
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For specifications, see the Electrical
Characteristics tables.
(2) The maximum power dissipation is a function of TJ(MAX),θJA and TA. The maximum allowable power dissipation at any ambient
temperature is PD= (TJ(MAX) — TA)/ θJA. All numbers apply for package soldered directly into a 2 layer PC board with zero air flow.
Package should be soldered unto a 6.8 mm2copper area as shown in the “recommended land pattern” shown in the package drawing.
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'VOUT
'VIN
DC,
LMH6555
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3.3V ELECTRICAL CHARACTERISTICS (1)
Unless otherwise specified, all limits are specified for TA= 25°C, VCM_REF = 1.2V, both inputs tied to 0.3V through 50Ω(RS1 &
RS2) each (2), VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS AND SPECIFICATIONS
(ALPHABETICAL ORDER) for definition of terms used throughout the datasheet. Boldface limits apply at the temperature
extremes.
Symbol Parameter Conditions Min(3) Typ(4) Max(3) Units
AC/DC Performance
SSBW −3 dB Bandwidth VOUT = 0.25 VPP 1200 MHz
LSBW VOUT = 0.8 VPP 1200
Peak Peaking VOUT = 0.8 VPP 1.4 dB
GF_0.1 dB Gain Flatness ±0.1 dB 180 MHz
GF_0.5 dB ±0.5 dB 330
Ph_Delta Phase Delta Output Differential Phase Difference < ±0.8 deg
f≤1.2 GHz
Lin_Ph Linear Phase Deviation Each Output < ±30 deg
f≤2 GHz
GD Group Delay Each Output 0.75 ns
f≤2 GHz
P_1 dB 1 dB Compression 1 GHz 1 VPP
TRS/TRL Rise/ Fall Time VOUT = 0.2 VPP Each Output 320 pS
OS Overshoot VOUT = 0.2 VPP Each Output 14 %
SR Slew Rate 0.8V Step, 10% to 90%,(5) 1300 V/µs
tsSettling Time ±1% 2.2 ns
AV_DIFF Insertion Gain (|S21|) 13.2 13.7 14.0
13.1 14.1 dB
TC AV_DIFF Temperature Coefficient of −0.9 mdB/°C
Insertion Gain
ΔAV_DIFF1 Insertion Gain Variation with VCM_REF Input Varied from 0.95V to −0.04 ±0.50 dB
VCM_REF 1.45, VOUT = 0.8 VPP ±0.58
ΔAV_DIFF2 Insertion Gain Variation with VI_CM −0.3 ≤VI_CM ≤2.0V ±0.03 ±0.48 dB
±0.55
Distortion And Noise Response
HD2_L 2nd Harmonic Distortion 250 MHz (6) −60
HD2_M 500 MHz (6) −62 dBc
HD2_H 750 MHz (6) −53
HD3_L 3rd Harmonic Distortion 250 MHz (6) −67
HD3_M 500 MHz (6) −61 dBc
HD3_H 750 MHz (6) −54
OIP3 Output 3rd Order Intermodulation f = 1 GHz 27.5 dBm
Intercept POUT (Each Tone) ≤–8.5 dBm(6)(7)
OIM3 3rd Order Intermodulation Distortion f = 1 GHz −67 dBc
POUT (Each Tone) = −6 dBm(6)(7)
eno Output Referred Voltage Noise ≥1 MHz 19 nV/√Hz
(1) Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ= TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self-heating where TJ> TA.
(2) Quiescent device common mode input voltage is 0.3V.
(3) Limits are 100% production tested at 25°C. Limits over the operating temperature range are specified through correlation using
Statistical Quality Control (SQC) methods.
(4) Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
(5) Slew Rate is the average of the rising and falling edges.
(6) Distortion data taken under single ended input condition.
(7) 0 dBm = 894 mVPP across 100Ωdifferential load
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vO_CM
vOUT
f = 750 MHz,
'VO_CM
'VOUT
DC,
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3.3V ELECTRICAL CHARACTERISTICS (1) (continued)
Unless otherwise specified, all limits are specified for TA= 25°C, VCM_REF = 1.2V, both inputs tied to 0.3V through 50Ω(RS1 &
RS2) each (2), VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS AND SPECIFICATIONS
(ALPHABETICAL ORDER) for definition of terms used throughout the datasheet. Boldface limits apply at the temperature
extremes.
Symbol Parameter Conditions Min(3) Typ(4) Max(3) Units
NF Noise Figure Relative to a Differential Input 15.0 dB
≥10 MHz
Input Characteristics
RIN CM Input Resistance Each Input to Ground 45 50 55 Ω
RIN_DIFF Differential Input Resistance Differential 66 78 100 Ω
CIN Input Capacitance Each Input to GND 0.3 pF
CMRR Common Mode Rejection Ratio −0.3 ≤CMVR ≤2.0V 40 68 dB
36
Output Characteristics
VOOS Output Offset Voltage Differential Mode 15 ±50 mV
±55
TCVOOS Output Offset Voltage (8) ±100 μV/°C
Average Drift
ROOutput Resistance RT1 and RT2 43 50 53 Ω
BAL_Error_DC Output Gain Balance Error −57 −38
dB
BAL_Error_AC −48
BAL_Error_AC_ Output Phase Balance Error f = 750 MHz, ±0.6 deg
Phase VOUT+- VOUT−Phase
|ΔVO_CM/ΔVI_CM| Output Common Mode Gain DC −26 −22 dB
−21
VCM_REF Characteristics
VOS_CM Output CM Offset Voltage VOS_CM = VO_CM – VCM_REF −4 ±60 mV
±85
TC_VOS_CM CM Offset Voltage Temp −0.2 mV/°C
Coefficient
IB_CM VCM_REF Bias Current 0.95V ≤VCM_REF ≤1.45V (9) −25 ±390 μA
±415
RIN_CM VCM_REF Input Resistance 3.5 5.8 kΩ
Gain_VCM_REF VCM_REF Input Gain to Output ΔVO_CM/ΔVCM_REF 0.97 0.99 1.00 V/V
Power Supply
ISSupply Current RS1 & RS2 Open (10) 120 150 mA
156
PSRR Differential Power Supply Rejection DC, ΔVS= ±0.3V, ΔVOUT/ΔVS−27 −44 dB
Ratio −25
PSRR_CM Common Mode PSRR DC, ΔVS= ±0.3V, ΔVO_CM/ΔVS−29 −39 dB
−27
(8) Drift determined by dividing the change in parameter at temperature extremes by the total temperature change.
(9) Positive current is current flowing into the device.
(10) Total supply current is affected by the input voltages connected through RS1 and RS2. Supply current tested with input removed.
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'VO_CM
'VOUT
Balance Error. See
¨
¨
©
§
¨
¨
©
§
16 131415
4
3
2
1
VOUT-
9
10
11
12
78
6
5
GND
GND
VIN+
VIN-GND GND VOUT+
VCC
VCM_REF
VCC
VCC
GND
GND
GND
GND
+
-
RT2
RT1
RG1
RG2
RF2
RF1
LMH6555
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SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
CONNECTION DIAGRAM
Figure 2. 16-Pin WQFN
DEFINITION OF TERMS AND SPECIFICATIONS (ALPHABETICAL ORDER)
Unless otherwise specified, VCM_REF = 1.2V
1. AV_CM (dB) Change in the differential output voltage (ΔVOUT ) with respect to the change in input common
mode voltage (ΔVI_CM)
2. AV_DIFF (dB) Insertion gain from a single ended 50Ω(or 100Ωdifferential) source to the differential output
(ΔVOUT)
3. ΔAV_DIFF (dB) Variation in insertion gain (AV_DIFF)
4. BAL_ERR_DC & BAL_ERR_AC
5. CM Common Mode
6. CMRR (dB) Common Mode rejection defined as: AV_DIFF (dB) - AV_CM (dB)
7. CMVR (V) Range of input common mode voltage (VI_CM)
8. Gain_VCM_REF (V/V) Variation in output common mode voltage (ΔVO_CM) with respect to change in VCM_REF input
(ΔVCM_REF) with maximum differential output
9. PSRR (dB) Differential output change (ΔVOUT) with respect to the power supply voltage change (ΔVS) with
nominal differential output
10. PSRR_CM (dB) Output common mode voltage change (ΔVO_CM) with respect to the change in the power supply
voltage (ΔVS)
11. RIN (Ω) Single ended input impedance to ground
12. RIN_DIFF (Ω) Differential input impedance
13. RL(Ω) Differential output load
14. RO(Ω) Device output impedance equivalent to RT1 & RT2
15. RS1, RS2 (Ω) Source impedance to VIN+and VIN−respectively
16. RT1, RT2 (Ω) Output impedance looking into each output
17. VCM_REF (V) Device input pin which controls output common mode
18. ΔVCM_REF (V) Change in the VCM_REF input
19. VI_CM (V) DC average of the inputs (VIN+, VIN−) or the common mode signal at those same input pins
20. ΔVI_CM (V) Variation in input common mode voltage (VI_CM)
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'VO_CM
'VOUT
AC version of the DC balance error
¨
¨
©
§
¨
¨
©
§test
vO_CM
vOUT (dB)
'VO_CM
'VOUT (dB)
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
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21. VIN+, VIN−(V) Device input pin voltages
22. ΔVIN (V) Terminated (50Ωfor single ended and 100Ωfor differential) generator voltage
23. VO_CM (V) Output common mode voltage (DC average of VOUT+and VOUT−)
24. ΔVO_CM (V) Variation in output common mode voltage (VO_CM)
25. Balance Error. Measure of the output swing balance of VOUT+and VOUT−, as reflected on the
output common mode voltage (VO_CM), relative to the differential output swing (VOUT). Calculated
as output common mode voltage change (ΔVO_CM) divided into the output differential voltage
change (ΔVOUT which is nominally around 800 mVPP)
26.
27. VOOS (V) DC Offset Voltage. Differential output voltage measured with both inputs grounded through 50Ω
28. VOS_CM (V) Difference between the output common mode voltage (VO_CM) and the voltage on the VCM_REF
input, for the allowable VCM_REF range
29. VOUT (V) Differential Output Voltage (VOUT+- VOUT−) (Corrected for DC offset (VOOS))
30. ΔVOUT (V) Change in the differential output voltage (Corrected for DC offset (VOOS))
31. VOUT+, VOUT−(V) Device output pin voltages
32. VS(V) Supply Voltage (V+- V−)
33. ΔVS(V) Change in VCC supply voltage
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10 100 1000
-1
0
1
2
3
4
5
6
OUTPUT POWER (dBm)
FREQUENCY (MHz)
0 dBm = 894 mVPP
0 1 2 3 4 5 6 7 8 9 10
VOLTAGE (V)
TIME (ns)
OUTPUT
(100 mV/DIV)
INPUT
(50 mV/DIV)
110 100 1000
FREQUENCY (MHz)
-30
-25
-20
-15
-10
-5
0
5
10
15
PHASE (°)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
GROUP DELAY (ns)
LINEAR PHASE DEVIATION
GROUP DELAY
110 100 1000 10000
FREQUENCY (MHz)
-70
-60
-50
-40
-30
-20
-10
0
DELTA_GAIN (dB)
PHASE
GAIN
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
DELTA_PHASE (°)
10
FREQUENCY (MHz)
-20
-12
-4
4
NORMALIZED GAIN (dB)
1000
100
0
-8
-16
2
-2
-6
-10
-14
-18
GAIN
PHASE
-550
-350
-150
50
-50
-250
-450
0
-100
-200
-300
-400
-500
NORMALIZED PHASE (°)
10 100 1000
FREQUENCY (MHz)
-1.5
-1
-0.5
0
0.5
1
1.5
AV_DIFF NORMALIZED (dB)
+0.5 dB
-0.5 dB
LMH6555
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SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
TYPICAL PERFORMANCE CHARACTERISTICS
Unless otherwise specified, RS1 = RS2 = 50Ω, VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS
AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used throughout the datasheet.
Frequency Response ±0.5 dB Gain Flatness
Figure 3. Figure 4.
Bal_Error
vs.
Linear Phase Deviation & Group Delay Frequency
Figure 5. Figure 6.
−1 dB Compression
vs.
Frequency Step Response (VOUT+)
Figure 7. Figure 8.
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12.5 13 14 14.5
AV_DIFF (dB)
0
5
10
15
25
30
35
PERCENTAGE (
20
13.5
VS = 3.3V
'VIN = 160 mV
0 20 40 60 80 100 120 140 160 180 200
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
NORMALIZED AV_DIFF (dB)
|VIN (mV)|
25°C
85°C
AV_DIFF NORMALIZED
to VIN = 160 mV @ 25°C
-40°C
-25 -20 -15 -10 -5 0
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
2-TONE SPURS (dBc)
SINGLE TONE POUT (dBm)
f _CENTER = 1 GHz
SINGLE-ENDED INPUT
RL = 100: (DIFFERENTIAL)
0 dBm = 894 mVPP
-0.3 0.2 0.7 1.2 1.7 2.2
-0.02
-0.01
0
0.01
0.02
0.03
0.04
AV_DIFF NORMALIZED (dB)
VI_CM (V)
77
78
79
80
81
82
83
RIN_DIFF (:)
01 1.5 2 2.5 3 3.5
TIME (ns)
-4
-2
0
2
4
±SETTLING (%)
0.5
+1%
-1%
100 1000
FREQUENCY (MHz)
-100
-90
-80
-70
-60
-50
-40
-30
-20
HD (dBc)
HD2
HD3
RL = 100:
VOUT = 800 mVPP (DIFFERENTIAL)
SINGLE ENDED INPUT
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Unless otherwise specified, RS1 = RS2 = 50Ω, VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS
AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used throughout the datasheet.
Harmonic Distortion
vs.
Step Response Settling Time Frequency
Figure 9. Figure 10.
AV_DIFF & RIN_DIFF
vs.
3rd Order Intermodulation Distortion VI_CM
Figure 11. Figure 12.
Insertion Gain Variation
vs.
Insertion Gain Distribution Input Amplitude
Figure 13. Figure 14.
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-50 -25 0 25 50 75 100
TEMPERATURE (°C)
-6
-4
-2
0
2
4
6
'VOOS (mV)
UNIT 3
UNIT 2
UNIT 1
UNIT 2
110 100 1000
FREQUENCY (MHz)
-70
-60
-50
-40
-30
-20
-10
0
S_PARAMETER (dB)
SINGLE-ENDED INPUT
TO EACH OUTPUT
S22
S11 S12
0.1 1 10 100 1000
FREQUENCY (MHz)
20
30
40
50
60
70
80
CMRR (dB)
160
0.01 11000
FREQUENCY (MHz)
0
60
100
10
0.1
120
100
40
20
80
140
40
0
15
30
25
10
5
20
35
OUTPUT NOISE (nV/
Hz)
NF (dB)
SE = SINGLE-ENDED INPUT
DI = DIFFERENTIAL INPUT
RS = 50: (SE) or 100: (DI)
OUTPUT NOISE
NF, SE
NF, DI
-0.4 0.1 0.6 1.1 1.6 2.1
40
50
60
70
CMRR (dB)
VI_CM (mV)
25°C
-40°C
85°C
0.01 0.1 1 10 100
-60
-50
-40
-30
-20
-10
0
PSRR (dB)
FREQUENCY (MHz)
PSRR
PSRR_CM
LMH6555
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SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Unless otherwise specified, RS1 = RS2 = 50Ω, VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS
AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used throughout the datasheet.
PSRR & PSRR_CM CMRR
vs. vs.
Frequency VI_CM
Figure 15. Figure 16.
CMRR
vs.
Frequency Noise Density & Noise Figure
Figure 17. Figure 18.
S_Parameters
vs. Differential Output Offset Variation for
Frequency 3 Representative Units
Figure 19. Figure 20.
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0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
VCM_REF (V)
-20
-10
-5
0
10
15
20
'VOS_CM (mV)
-15
5
-40qC
85qC
25qC
'VOS_CM RELATIVE TO
VCM_REF = 1.2V @ 25qC
-50 -25 0 25 50 75 100
TEMPERATURE (°C)
114
116
118
120
122
124
126
SUPPLY CURRENT (mA)
VS = 3.3V
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Unless otherwise specified, RS1 = RS2 = 50Ω, VS= 3.3V, RL= 100Ωdifferential, VOUT = 0.8 VPP. See DEFINITION OF TERMS
AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used throughout the datasheet.
Common Mode Offset Voltage Variation Supply Current
vs. vs.
VCM_REF Temperature
Figure 21. Figure 22.
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VIN+
ADC081000/
ADC081500
VCMO SPI
RG1
RG2
RF1
RF2
LMH6555 VCM_REF
VOUT+
VOUT = 0.8 VPP
VIN-
VIN a
+
-
RS1
50:
RS2
50:
RT2
50:
RT1
50:
340 mVPP
-
+
OPT LMV321
3.3V
OPT
LMH6555
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SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
APPLICATION INFORMATION
See DEFINITION OF TERMS AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used.
GENERAL
The LMH6555 consists of three individual amplifiers:
1. VOUT+driver
2. VOUT−driver
3. The common mode amplifier
Being a differential amplifier, the LMH6555 will not respond to the common mode input (as long as it is within its
input common mode range) and instead the output common mode is forced by the built-in common mode
amplifier with VCM_REF as its input. As shown, in Figure 23 below, the VCMO output of most differential high speed
ADC’s is tied to the VCM_REF input of the LMH6555 for direct output common mode control. In some cases, the
output drive capability of the ADC VCMO output may need an external buffer, as shown, to increase its current
capability in order to drive the VCM_REF pin. The Electrical Characteristics Table shows the gain (Gain_VCM_REF)
and the offset (VOS_CM) from the VCM_REF to the device output common mode.
Figure 23. Single Ended to Differential Conversion
The single ended input and output impedances of the LMH6555 I/O pins are close to 50Ωas specified in
Electrical Characteristics Table (RIN and RO). With differential input drive, the differential input impedance
(RIN_DIFF) is close to 78Ω.
The device nominal input common mode voltage (VI_CM) is close to 0.3V when RS1 and RS2 of Figure 23 are
open. Thus, the input source will experience a DC current with 0V input. Because of this, the differential output
offset voltage is influenced by the matching between RS1 and RS2. So, in a single ended input condition, if the
signal source is AC coupled to one input, the undriven input needs to also be AC coupled in order to cancel the
output offset voltage (VOOS).
In applications where low output offset is required, it is possible to inject some current to the appropriate input
(VIN+or VIN−) as an effective method of trimming the output offset voltage of the LMH6555. This is explained
later in this document. The nominal value of RS1 and RS2 will also affect the insertion gain (AV_DIFF).
The LMH6555 can also be used with the input AC coupled through equal valued DC blocking capacitors (C) in
series with VIN+and VIN−. In this case, the coupling capacitors need to be large enough to not block the low
frequency content. The lower cutoff frequency will be 1/(πREQC)Hz with REQ= RS1+ RS2 + RIN_DIFF where RIN_DIFF
≈78Ω.
The single ended output impedance of the LMH6555 is 50Ω. The LMH6555 Electrical Characteristics shows the
device performance with 100Ωdifferential output load, as would be the case if a device such as the ADC081000/
ADC081500 (single/ dual ADC) were being driven.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LMH6555
V+
VIN+RG1
+-
ACM
-A
VOUT--A VOUT+
VIN-
V+
A1
RC1 VCM_REF RC2
D1 D2
Q1 Q2
Vx
RE1 RF1 RF2 RE2
RG2
Vy
A2 RT1
50:
RT2
50:
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
CIRCUIT ANALYSIS
Figure 24 shows the block diagram of the LMH6555.
RG1 = RG2 = RG= 39Ω
RE1 = RE2 = RE= 25Ω
RF1 = RF2 = RF= 430Ω
ICQ1 = ICQ2 = 12.6 mA
Figure 24. Block Diagram
The differential input stage consists of cross-coupled common base bipolar NPN stages, Q1 and Q2. These
stages give the device its differential input characteristic. The internal loop gain from Vxand Vyinternal nodes
(Q1 and Q2 emitters) to the output is large, such that these nodes act as a virtual ground. The cross-coupling will
ensure that these nodes are at the same voltage as long as the amplifier is operating within its normal range.
Output common mode voltage is enforced through the action of “ACM” which servos the output common mode to
the “VCM_REF” input voltage.
The discussion that follows, provides the formulas needed to analyze single ended and differential input
applications. For a more detailed explanation including derivations, please see Appendix at the end of
the datasheet.
12 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
VIN+
VIN-
RS1
50:
RS2
50:
VIN
0.3 VPP
RL
100:
VOUT-
VOUT+
LMH6555
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
SINGLE-ENDED INPUT
The following is the procedure for determining the device operating conditions for single ended input applications.
This example will use the schematic shown in Figure 25.
Figure 25. Single-Ended Input Drive
1. Determine the driven input’s (VIN+ or VIN−) swing knowing that each input common mode impedance to
ground (RIN) is 50Ω:
VIN+ (or VIN−) = VIN · RIN/(RIN + RS) (1)
whitespace
For Figure 25:
VIN+ = 0.3 VPP · 50/(50+50) = 0.15 VPP (2)
whitespace
2. Calculate VOUT knowing the Insertion Gain (AV_DIFF):
VOUT = (VIN/2) · AV_DIFF
AV_DIFF = 2 · RF/ (2RS+ RIN_DIFF)
where
• RF= 430Ω
• RIN_DIFF = 78Ω(3)
whitespace
For Figure 25:
RS= 50Ω → AV_DIFF = 4.83 V/V
VOUT = (0.3 VPP/2) · 4.83 V/V= 724.5 mVPP (4)
whitespace
3. Determine the peak-to-peak differential current (IIN_DIFF) through the device’s differential input impedance
(RIN_DIFF) which would result in the VOUT calculated in step 2:
IIN_DIFF = VOUT/ RF(5)
whitespace
For Figure 25:
IIN_DIFF = 724.5 mVPP/ 430Ω= 1.685 mAPP (6)
whitespace
4. Determine the swing across the input terminals (VIN_DIFF) which would give rise to the IIN_DIFF calculated in
step 3 above.
VIN_DIFF = IIN_DIFF · RIN_DIFF (7)
whitespace
For Figure 25:
VIN_DIFF = 1.685 mAPP · 78Ω= 131.4 mVPP (8)
whitespace
5. Calculate the undriven input’s swing, based on VIN_DIFF determined in step 4 and VIN+ calculated in step 1:
VIN−= VIN+ - VIN_DIFF (9)
whitespace
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LMH6555
RS1
50:
LMH6555
VOUT-
VOUT+
RL
100:
V1
V2RS2
50:
VIN-
VIN+
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
VOLTAGE (V)
TIME
VIN
VIN+
VIN-
150 mVPP @
138 mV DC
18.6 mVPP @
138 mV DC
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
For Figure 25:
VIN−= 150 mVPP - 131.4 mVPP = 18.6 mVPP (10)
whitespace
6. Determine the DC average of the two inputs (VI_CM) by using the following expression:
VI_CM = 12.6 mA · RE· RS/ (RS+ RG+ RE)
where
• RE= 25Ω
• RG= 39Ω(both internal to the LMH6555)
For Figure 25 (11)
RS= 50Ω → VI_CM = 15.75 / (RS+ 64)
VI_CM = 15.75/ (50+64) = 138.2 mV (12)
whitespace
The values determined with the procedure outlined here are shown in Figure 26.
Figure 26. Input Voltage for Single-Ended Input Drive Schematic
DIFFERENTIAL INPUT
The following is the procedure for determining the device operating conditions for differential input applications
using the Figure 27 schematic as an example.
Assuming transformer secondary, VIN, of 300 mVPP
Figure 27. Differential Input Drive
1. Calculate the swing across the input terminals (VIN_DIFF) by considering the voltage division from the
differential source (VIN) to the LMH6555 input terminals with differential input impedance RIN_DIFF:
VIN_DIFF = VIN · RIN_DIFF/ (2RS+ RIN_DIFF) (13)
whitespace
For Figure 27:
VIN_DIFF = 300 mVPP · 78 / (100 + 78) = 131.5 mVPP (14)
14 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
VOLTAGE (V)
TIME
V1
VIN+
VIN- 65.7 mVPP @
138 mV DC
V2
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
whitespace
2. Calculate each input pin swing to be ½ the swing determined in step 1:
VIN+=VIN−= VIN_DIFF/ 2 (15)
whitespace
For Figure 27
VIN+=VIN−= 131.5 mVPP/ 2 = 65.7 mVPP
whitespace
3. Determine the DC average of the two inputs (VI_CM) by using the following expression:
VI_CM = 12.6 mA · RE· RS/ (RS+ RG+ RE)
where
• RE= 25Ω
• RG= 39Ω(both internal to the LMH6555) (16)
whitespace
For Figure 27:
RS= 50Ω → VI_CM = 15.75 / (RS+ 64)
VI_CM = 15.75/ (50+64) = 138.2 mV (17)
whitespace
4. Calculate VOUT knowing the Insertion Gain (AV_DIFF):
VOUT = (VIN · / 2) · AV_DIFF
AV_DIFF = 2 · RF/ (2RS+ RIN_DIFF)
where
• RF= 430Ω
• RIN_DIFF = 78Ω(18)
whitespace
For Figure 27:
RS= 50Ω → AV_DIFF = 4.83 V/V
VOUT = (0.3 VPP/2) · 4.83 V/V= 724.5 mVPP (19)
whitespace
The values determined with the procedure outlined here are shown in Figure 28.
Figure 28. Input Voltage for Figure 27 Schematic
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LMH6555
35 40 45 50 55 60 65 70
1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
2.80
2.90
|VOUT/VIN| (V/V)
RS1 (:)
-100
-60
-20
20
60
100
VOOS (mV)
RS2 = 50:
GAIN
VOOS
(A)
SINGLE ENDED INPUT
APPLIED THROUGH RS1
35 40 45 50 55 60 65 70
1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
2.80
2.90
|VOUT/VIN| (V/V)
RS2 (:)
-100
-60
-20
20
60
100
VOOS (mV)
RS1 = 50:
SINGLE ENDED INPUT
APPLIED THROUGH RS1
GAIN
VOOS
(B)
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
SOURCE IMPEDANCE(S) AND THEIR EFFECT ON GAIN AND OFFSET
The source impedances RS1 and RS2, as shown in Figure 25 or Figure 27, affect gain and output offset. The
Electrical Characteristics and TYPICAL PERFORMANCE CHARACTERISTICS are generated with equal valued
source impedances RS1 and RS2, unless otherwise specified. Any mismatch between the values of these two
impedances would alter the gain and offset voltage.
OUTPUT OFFSET CONTROL AND ADJUSTMENT
There are applications which require that the LMH6555 differential output voltage be set by the user. An example
of such an application is a unipolar signal which is converted to a differential output by the LMH6555. In order to
utilize the full scale range of the ADC input, it is beneficial to shift the LMH6555 outputs to the limits of the ADC
analog input range under minimal signal condition. That is, one LMH6555 output is shifted close to the negative
limit of the ADC analog input and the other close to the positive limit of the ADC analog input. Then, under
maximum signal condition, with proper gain, the full scale range of the ADC input can be traversed and the ADC
input dynamic range is properly utilized. If this forced offset were not imposed, the ADC output codes would be
reduced to half of what the ADC is capable of producing, resulting in a significant reduction in ENOB. The choice
of the direction of this shift is determined by the polarity of the expected signal.
Another scenario where it may be necessary to shift the LMH6555 output offset voltage is in applications where it
is necessary to improve the specified Output Offset Voltage (differential mode), “VOOS”. Some ADC’s, including
the ADC081000/ ADC081500 (and their dual counterparts), have internal registers to correct for the driver’s
(LMH6555) VOOS. If the LMH6555 VOOS rating exceeds the maximum value allowed into this register, then
shifting the output is required for maximum ADC performance.
It is possible to affect output offset voltage by manipulating the value of one input resistance relative to the other
(e.g. RS1 relative to RS2 or vice versa). However, this will also alter the gain. Assuming that the source is applied
to the VIN+side through RS1,Figure 29(A) shows the effect of varying RS1 on the overall gain and output offset
voltage. Figure 29(B) shows the same effects but this time for when the undriven side impedance, RS2, is varied.
Figure 29. Gain & Output Offset Voltage vs. Source Impedance Shift for Single Ended Input Drive
As can be seen in Figure 29, the source impedance of the input side being driven has a bigger effect on gain
than the undriven source impedance. RS1 and RS2 affect the output offset in opposite directions. Manipulating the
value of RS2 for offset control has another advantage over doing the same to RS1 and that is the signal input
termination is not affected by it. This is especially important in applications where the signal is applied to the
LMH6555 through a transmission line which needs to be terminated in its characteristic impedance for minimum
reflection.
For reference, Figure 30 shows the effect of source impedance misbalance on overall gain and output offset
voltage with differential input drive.
16 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
0.1 1 10 100 1000
RX (k:)
1
10
100
1000
|VOOS| (mV)
VX = 5V
VX = 3.3V
RS1
RS2
VIN
RL
100:
LMH6555
RX
VX
+
-
RS1
RS2
VIN
RL
100:
LMH6555
RX
VX
+
-
-20 -15 -10 -5 0 5 10 20
1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
2.80
2.90
|VOUT/VIN| (V/V)
RS2 - RS1 (:)
-100
-60
-20
20
60
100
VOOS (mV)
RS1 = 50:
DIFFERENTIAL DRIVE
GAIN
VOOS
15
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
Figure 30. Gain & Output Offset Voltage vs. Source Impedance Shift for Differential Input Drive
It is possible to manipulate output offset with little or no effect on source resistance balance, gain, and, cable
termination.
(a) (b)
Figure 31. Differential Output Shift Circuits
RX, shown in Figure 31(a) and Figure 31(b), injects current into the input to achieve the required output shift. For
a positive shift, positive current would need to be injected into the VIN+terminal (Figure 31(a)) and for a negative
shift, to the VIN−terminal (Figure 31(b)). Figure 32 shows the effect of RXon the output with VX= 3.3V or 5V, and
RS1 = RS2 = 50Ω.
Figure 32. LMH6555 Differential Output Shift Due to RXin Figure 31
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LMH6555
VOOS t (VIN_OFFSET x 1.89)
VOUT = -30 mV = (-1.89) -50 mV +
¨
¨
©
§
¨
¨
©
§
RX = 3.76 k:
Ÿ
RX
248
VOUT
VTH =-RF
2RS + 78 Ÿ
¨
¨
©
§
-50 mV + RX3.3V
¨
¨
©
§75
VOUT = -430:
(150 + 78):x
RS1
75:
RL
100:
LMH6555
RX
VS = 3.3V
VIN+
VIN-
RS2
75:
VOUT-
VOUT+
VIN
with -50 mV
OFFSET
RS1
RS2
RL
100:
LMH6555
VIN+
VIN-
VOUT-
VOUT+
75:
VTH # -50 mV + RX3.3V
RS2 || RX #75:
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
To shift the LMH6555 differential output negative by about 100 mV, referring to the plot in Figure 32, RXwould be
chosen to be around 3.9 kΩin the schematic of Figure 31(b) (using VX= VS= 3.3V).
In applications where VIN has a built-in non-zero offset voltage, or when RS1 and RS2 are not 50Ω, the Figure 32
plot cannot be used to estimate the required value for RX.
Consider the case of a more general offset correction application, shown in Figure 33(a), where RS1 = RS2 = 75Ω
and VIN has a built-in offset of −50 mV. It is necessary to shift the differential output offset voltage of the
LMH6555 to 0 mV. Figure 33(b) is the Thevenin equivalent of the circuit in Figure 33(a) assuming RX>> RS2.
(b)
(a)
Figure 33. Offset Correction Example (RS= 75Ω)
From the gain expression in Equation 44 (see Appendix) (but with opposite polarity because VTH is applied to
VIN−instead):
(20)
The expression derived for VOUT in Equation 20 can be set equal to zero to solve for RXresulting in RX= 4.95
kΩ. If the differential output offset voltage, VOOS, is also known, VOUT could be set to a value equal to –VOOS. For
example, if the VOOS for the particular LMH6555 is +30 mV, then the following nulls the differential output:
(21)
RX>> RS2 confirming the assumption made in the derivation. Note that Equation 21, which is derived based on
the configuration in Figure 31(b), will yield a real solution for RXif and only if:
For Figure 31(b) and with Rs= 75Ω
where
• VIN_OFFSET is the source offset shown as −50 mV in Figure 33(a) (22)
If Equation 22 were not satisfied, then Figure 31(a) offset correction, where RXis tied to the VIN+side, should be
employed instead.
Alternatively, replace the VXand RXcombination with a discrete current source or current sink. Because of a
current source’s high output impedance, there will be less gain imbalance. However, a current source might have
a relatively large output capacitance which could degrade high frequency performance.
INTERFACE DESIGN EXAMPLE
As shown in Figure 34 below, the LMH6555 can be used to interface an open collector output device (U1) to a
high speed ADC. In this application, the LMH6555 performs the task of amplifying and driving the 100Ω
differential input impedance of the ADC.
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Product Folder Links: LMH6555
ADC081000/
ADC081500
VCMO
RG1
RF1
LMH6555 VCM_REF
VOUT+
VOUT-
+
-
RL2
RF2
U1
RL1
VCC
RT2
50:
RT1
50:
RG2
RS1
RS2
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
VCM_REF buffer not shown
Figure 34. Differential Amplification and ADC Drive
For applications similar to the one shown in Figure 34, the following conditions should be maintained:
1. The LMH6555 differential output voltage has to comply with the ADC full scale voltage (800 mVPP in this
case).
2. The LMH6555 input Common Mode Voltage Range is observed. “CMVR”, as specified in Electrical
Characteristics, is to be between −0.3V and 2.0V for the specified CMRR.
3. U1 collector voltage swing must to be observed so that the U1 output transistors do not saturate. The
expected operating range of these output transistors is defined by the specifications and operating conditions
of U1.
Consider a numerical example (RLrefers to RL1 & RL2, RSrefers to RS1 & RS2).
Assume:
VCC = 10V, U1 peak-to-peak collector current (IPP) = 15 mAPP with 10 mA quiescent (IcQ), and minimum
operational U1 collector voltage = 6V.
Here are the series of steps to take in order to carry out this design:
a. Select the RLvalue which allows compliance with the U1 collector voltage (6V in this case) with 1V extra as
margin because of LMH6555 loading.
RL= [10 - (6+1)] V / (10+ 7.5) mA = 171Ω
Choose 169Ω, 1% resistors for RL
b. Find the value of RSto get the proper swing at the output (800 mVPP). To do so, convert the input stage into
its Norton equivalent as shown in Figure 35
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LMH6555
Ÿ
Q1
12.6 mA
Vx
RE
25:
RG
39:
RS
IcQ + IPP
U1
RL
VCC
LMH6555
RE
25:
RN
IN
Q1 12.6 mA
Vx
IN = 1
RL + RS + RG(VCC ± IcQ RL) IPP RL
COMMON
MODE DIFFERENTIAL
RN = RL + RS + RG
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
Figure 35. Norton Equivalent of the Input Circuitry Tied to Q1 within the LMH6555 in Figure 34
IN= IN(common mode) + IN(differential)
IN(common mode) = (VCC – IcQ * RL) / (RL+ RS+ RG)
IN(differential) = IPP * RL/ (RL+ RS+ RG) (23)
The entirety of the Norton source differential component will flow through the feedback resistors within the
LMH6555 and generate an output. Therefore:
IN(differential) * RF= 800 mVPP
→RS= (RL* IPP * RF/ 0.8) – RG– RL
where
• RF= 430Ω
• RG= 39Ω(RFand RGare internal LMH6555 resistances) (24)
So, in this case:
RS= (169 * 15 mAPP * 430/ 0.8) – 39 – 169 = 1154Ω
Choose 1.15 kΩ, 1% resistors for RS(25)
c. With RLand RSdefined, ensure that the U1 collector voltage(s) minimum is not violated due to the loading
effect of the LMH6555 through RS. Also, it is important to ensure that the LMH6555's CMVR is also not
violated.
The “Vx” node voltage within the LMH6555 (see Figure 35) would need to be calculated. Use the Common
Mode component of the Norton equivalent source from above, and write the KCL at the Vxnode as follows:
Vx/ RE+ Vx/ RN= 12.6 mA + IN(common mode); with RE= 25Ω
Vx/ RE+ Vx/ RN= 12.6 mA + (VCC – IcQ RL)/ (RL+ RS+ RG)
→Vx= 0.4595V (26)
With Vxcalculated, both the input voltage range (high and low) and the low end of the U1 collector voltage
(VC) can be derived to be within the acceptable range. If necessary, steps “a” through “c” would have to be
repeated to readjust these values.
VC= VXRL/ RN+ IN(RS+ RG) (27)
whitespace
IN_High = 7.05 mA, IN_Low = 5.19 mA (based on the values derived)
→VC_High = 0.4595 * 169 / 1358 + 7.05 mA (1150 + 39) = 8.44V
→VC_Low = 0.4595 * 169 / 1358 + 5.19 mA (1150 + 39) = 6.22V (28)
whitespace
20 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
ADC081000/
ADC081500
RL
80.6:
1%
U1
VCC
RS
523:
1%
VIN+
VIN-
RS
523:
1%
10 mA + 15 mAPP
RL
80.6:
1% VOUT = 800 mVPP
1.13V to 1.20V
10V
7.6V to 8.7V LMH6555
ADC081000/
ADC081500
RL1
169:
1%
U1
VCC
RS2
1.15 k:
1%
VIN+
VIN-
RS1
1.15 k:
1%
10 mA + 15 mAPP
RL2
169:
1% VOUT = 800 mVPP
0.65V to 0.72V
10V
LMH6555
6.22V to 8.44V
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
VIN = VX(RN– RG) / RN+ INRG
→VIN_High = 0.4595 * (1358- 39) / 1358 + 7.05 mA * 39 = 0.721V
→VIN_Low = 0.4595 * (1358- 39) / 1358 + 5.19 mA * 39 = 0.649V (29)
whitespace
Figure 36 shows the complete solution using the values derived above, with the node voltages marked on the
schematic for reference.
Figure 36. Implementation #1 of Figure 34
Design Example
It is important to note that the matching of the resistors on either input side of the LMH6555 (RS1 to RS2 and RL1
to RL2) is very important for output offset voltage and gain balance. This is particularly true with values of RS
higher than the nominal 50Ω. Therefore, in this example, 1% or better resistor values are specified.
If the U1 collector voltage turns out to be too low due to the loading of the LMH6555, lower RL. Lower values of
RLresult in lower RSwhich in turn increases the LMH6555's VI_CM because of increased pull up action towards
VCC. The upper limit on VI_CM is 2V. Figure 37 shows the 2nd implementation of this same application with
lowered values of RLand RS. Notice that the lower end of U1’s collector voltage and the upper end of LMH6555’s
VI_CM have both increased compared to the 1st implementation.
Figure 37. Implementation #2 of Figure 34 Design Example
An alternative would be to AC couple the LMH6555 inputs. With this approach, the design steps would be very
similar to the ones outlined except that there would be no common mode interaction between the LMH6555 and
U1 and this results in fewer design constraints:
Vx/ RE= 12.6 mA →Vx= 0.3150V (30)
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LMH6555
ADC081000/
ADC081500
RL1
80.6:
1%
U1
VCC
RS2
523:
1%
VIN+
VIN-
RS1
523:
1%
10 mA + 15 mAPP
RL2
80.6:
1%
VOUT = 800 mVPP
0.278V to 0.352V
10V
8.67V to 9.72V LMH6555
CS2
0.01 PF
CS1
0.01 PF
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
For the component values shown in Figure 37 use:
1. VC_High = VCC – RL(IcQ + IPP / 2 - IN(differential) /2)
VC_Low = VCC – RL(IcQ - IPP / 2 + IN(differential) /2) (31)
whitespace
IN(differential) = IPP * RL/ (RL+ RS+ RG) = 1.88 mA (based on the values used.)
→VC_High = 10 – 80.6 (10 + 15 / 2 −1.88 /2) mA = 8.67V
→VC_Low = 10 – 80.6 (10 −15 / 2 + 1.88 /2) mA = 9.72V (32)
whitespace
VIN = VX± RG. IN(differential) /2
→VIN_High = 0.3150 + 39 * 1.88 mA /2 = 0.3517V
→VIN_Low = 0.3150 - 39 * 1.88 mA /2 = 0.2783V (33)
Figure 38 shows the AC coupled implementation of the Figure 37 schematic along with the node voltages
marked to demonstrate the reduced VI_CM of the LMH6555 and the increase in the U1 collector voltage
minimum.
Figure 38. AC Coupled Version of Figure 37
Note that the lower cut-off frequency is:
f_cut-off = 1 / (πReqCS) where Req = RS1+ RS2 + RIN_DIFF where RIN_DIFF ≈78Ω(34)
So, for the component values shown (CS= 0.01 μF and RS1 = RS2 = 523Ω):
f_cut-off = 28.2 kHz (35)
22 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
RS1
50:
RS2
50:
6
4
1
3
MINI CIRCUITS
TYPE
TCI-1-13M
1.6 VPP
VIN
4.7 nF
4.7 nF
TO ADC
VIN+
TO ADC
VIN-
800 mVPP
VIN+
ADC081000/
ADC081500
VCMO SPI
RG1
RG2
RF1
RF2
LMH6555 VCM_REF
VOUT+
VOUT = 0.8 VPP
VIN-
VIN a
+
-
RS1
50:
RS2
50:
RT2
50:
RT1
50:
340 mVPP
-
+
OPT LMV321
3.3V
OPT
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
DATA ACQUISITION APPLICATIONS
Figure 39 shows the LMH6555 used as the differential driver to the Texas Instruments ADC081500 running at
1.5G samples/second.
Figure 39. Schematic of the LMH6555 Interfaced to the ADC081500
In the schematic of Figure 39, the LMH6555 converts a single ended input into a differential output for direct
interface to the ADC's 100Ωdifferential input. An alternative approach to using the LMH6555 for this purpose,
would have been to use a balun transformer, as shown in Figure 40.
Figure 40. Single Ended to Differential Conversion
(AC only) with a Balun Transformer
In the circuit of Figure 40, the ADC will see a 100Ωdifferential driver which will swing the required 800 mVPP
when VIN is 1.6 VPP. The source (VIN) will see an overall impedance of 200Ωfor the frequency range that the
transformer is specified to operate. Note that with this scheme, the signal to the ADC must be AC coupled,
because of the transformer’s minimum operating frequency which would prevent DC coupling. For the
transformer specified, the lower operating frequency is around 4.5 MHz and the input high pass filter’s −3 dB
bandwidth is around 340 kHz for the values shown (or (1/πREQC)Hz where REQ = 200Ω).
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 23
Product Folder Links: LMH6555
VIN+
ADC081000/
ADC081500
VCMO SPI
RG1
RG2
RF1
RF2
LMH6555 VCM_REF
VOUT = 0.8 VPP
VIN-
VIN a
+
-
RS1
50:
RS2
50:
RT2
50:
RT1
50:
340 mVPP
-
+
OPT LMV321
3.3V
OPT
CO
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
Table 1 compares the LMH6555 solution (Figure 39) vs. that of the balun transformer coupling (Figure 40) for
various categories.
Table 1. ADC Input Coupling Schemes Compared
Preferred Solution
Category LMH6555 Balun Transformer
Lower Power Consumption ✓
Lower Distortion ✓
Wider Dynamic Range ✓
DC Coupling & Broadband Applications ✓
Highest Gain & Phase Balance ✓
Input/ Output Broadband Impedance Matching (Highest Return Loss) ✓
Additional Gain ✓
ADC Input Protection against Overdrive ✓
Highest SNR ✓
Ability to Control Gain Flatness ✓
(see below)
GAIN FLATNESS
In applications where the full 1.2 GHz bandwidth of the LMH6555 is not necessary, it is possible to improve the
gain flatness frequency at the expense of bandwidth. Figure 41 shows COplaced across the LMH6555 output
terminals to reduce the frequency response gain peaking and thereby to increase the ±0.5 dB gain flatness
frequency.
Figure 41. Increasing ±0.5 dB Gain Flatness using External Output Capacitance, CO
Figure 42,Figure 43, and and Figure 44 show the FFT analysis results with the setup shown in Figure 39.
24 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
675
-110
-100
-80
-60
-40
-20
0
-90
-70
-50
-30
-10
10
685 695 705 715 725 735 745 755 765
H9
694.174 MHz
H5
719.083 MHz H3
731.538 MHz
H7
706.628 MHz
Fundamental
743.993 MHz
FREQUENCY (MHz)
(dBc)
-110 5 15 25 35 45 55 65 75
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
H2
12.455 MHz
H4
24.91 MHz
H6
37.364 MHz
H8
49.819 MHz
H10
62.274 MHz
FREQUENCY (MHz)
(dBc)
0
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
100 200 300 400 500 600 700
FREQUENCY (MHz)
(dBc)
Fundamental
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
Figure 42. LMH6555 FFT Result When Used as the Differential Driver to ADC081500
Figure 43. LMH6555 FFT Result When Used as the Differential Driver to ADC081500
(Lower Fs/2 Region Magnified)
Figure 44. LMH6555 FFT Result When Used as the Differential Driver to ADC081500
(Upper Fs/2 Region Magnified)
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 25
Product Folder Links: LMH6555
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
Figure 42,Figure 43, and Figure 44 information summary:
• Fundamental Test Frequency 744 MHz
• LMH6555 Output 0.8 VPP
• Sampling Rate: 1.5G samples/second
• 2nd Harmonic: −59 dBc @ ∼12 MHz or |1.5 GHz*1– 744 MHz*2|
• 3rd Harmonic: −57 dBc @ ∼732 MHz or |1.5 GHz*1- 744 MHz *3|
• 4th Harmonic −71 dBc @ ∼24 MHz or |1.5 GHz*2 – 744 MHz *4|
• 5th Harmonic −68 dBc @ ∼720 MHz or |1.5 GHz*2- 744 MHz*5|
• 6th Harmonic −68 dBc @ ∼36 MHz or |1.5 GHz*3- 744 MHz*6|
• THD −51.8 dBc
• SNR 43.4 dB
• Spurious Free Dynamic
• Range (SFDR): 57 dB
• SINAD 42.8 dB
• ENOB 6.8 bits
The LMH6555 is capable of driving a variety of Texas Instruments Analog to Digital Converters. This is shown in
Table 2, which offers a complete list of possible signal path ADC+ Amplifier combinations. The use of the
LMH6555 to drive an ADC is determined by the application and the desired sampling process (Nyquist operation,
sub-sampling or over-sampling). See application note AN-236 (SNAA079) for more details on the sampling
processes and application note AN-1393 (SNOA461) for details on “Using High Speed Differential Amplifiers to
Drive ADCs”. For more information regarding a particular ADC, refer to the particular ADC datasheet for details.
Table 2. Differential Input ADC’s Compatible with the LMH6555 Driver
ADC Part Number Resolution (bits) Single/Dual Speed (MSPS)
ADC08D500 8 S 500
ADC081000 8 S 1000
ADC08D1000 8 D 1000
ADC08D1020 8 D 1000
ADC081500 8 S 1500
ADC08D1500 8 D 1500
ADC08D1520 8 D 1500
ADC083000 8 S 3000
ADC08B3000 8 S 3000
EXPOSED PAD WQFN PACKAGE
The LMH6555 is in a thermally enhanced package. The exposed pad (device bottom) is connected to the GND
pins. It is recommended, but not necessary, that the exposed pad be connected to the supply ground plane. The
thermal dissipation of the device is largely dependent on the connection of this pad. The exposed pad should be
attached to as much copper on the circuit board as possible, preferably external copper. However, it is very
important to maintain good high speed layout practices when designing a system board.
Here is a link to more information on the Texas Instruments 16-pin WQFN package:
http://www.ti.com/packaging
26 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
V+
12.6 mA
Q1
Vx
RE1
25:
RG1
39:
VIN+
V+
12.6 mA
Q2
Vy
RE2
25:
RG2
39:
VIN-
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
EVALUATION BOARD
Texas Instruments suggests the following evaluation board as a guide for high frequency layout and as an aid in
device testing and characterization.
Device Package Evaluation Board Ordering ID
LMH6555 16-Pin WQFN LMH6555EVAL
The evaluation board can be ordered when a device sample request is placed with Texas Instruments.
Appendix
Here is a more detailed analysis of the LMH6555, including the derivation of the expressions used throughout
APPLICATION INFORMATION.
INPUT STAGE
Because of the input stage cross-coupling, if the instantaneous values of the input node voltages (VIN+and VIN−)
and current values are required, use the circuit of Figure 45 as the equivalent input stage for each input (VIN+and
VIN−).
Figure 45. Equivalent Input Stage
Using this simplified circuit, one can assume a constant collector current, to simplify the analysis. This is a valid
approximation as the large open loop gain of the device will keep the two collector currents relatively constant.
First derive Q1 and Q2 emitter voltages. From there, derive the voltages at VIN+and VIN−.
With the component values shown, it is possible to analyze the input circuits of Figure 45 in order to determine
Q1 and Q2 emitter voltages. This will result in a first order estimate of Q1 and Q2 emitter voltages. Since Q1 and
Q2 emitters are cross-coupled, the voltages derived would have to be equal. With the action of the common
mode amplifier, “ACM”, shown in Figure 24, these two emitters will be equalized. So, one other iteration can be
performed whereby both emitters are set to be equal to the average of the 1st derived emitter voltages. Using this
new emitter voltage, one could recalculate VIN+and VIN−voltages. The values derived in this fashion will be
within ±10% of the measured values.
Single Ended Input Analysis
Here is an actual example to further clarify the procedure.
Consider the case where the LMH6555 is used as a single ended to differential converter shown in Figure 46.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 27
Product Folder Links: LMH6555
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
VOLTAGE (V)
TIME
VIN
VIN+
VIN-
150 mVPP @
138 mV DC
18.6 mVPP @
138 mV DC
Vx + Vy
2=0.279 + 0.246
2= 0.262V
0.213 + 0.246
2= 0.229V
VIN+ = ± 0.15V ± 50 89
VIN+ = 0.213V
63.2 mV ;VIN- = 0.262V
0.229V
50
89
VIN- = 0.147V
0.129V
±0.15V - 0.262V
0.229V
x
=Emitter
Voltage
Swing
Vx
25 +89 = 12.6 mA ŸVx = 0.279V
0.213V
Vy
25 +Vy
89 = 12.6 mA ŸVy = 0.246V
Vx
±
0.15
VIN+
VIN-
RS1
50:
RS2
50:
VIN
0.3 VPP
RL
100:
VOUT-
VOUT+
LMH6555
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
Figure 46. Single Ended Input Drive
The first task would be to derive the internal transistor emitter voltages based on the schematic of Figure 45
(assuming that there is no interaction between the stages.) Here is the derivation of VXand Vy:
(36)
VXvaries with VIN+(0.213V with negative VIN swing and 0.279V with positive.) The values derived above assume
that the two halves of the input circuit do not interact with each other. They do through the common mode
amplifier and the input stage cross-coupling. Vxand Vyare equal to the average of Vywith either end of the
swing of VX. This is calculated below along with the derivation of VIN+and VIN−based on this new average
emitter voltage (the average of VXand Vy.)
(37)
With 0.3 VPP VIN, VIN+experiences 150 mVPP (213 mV - 63.2 mV) of swing and VIN−will swing by about 18.6
mVPP in the process (147 mV – 129 mV). The input voltages are shown in Figure 47.
Figure 47. Input Voltages for Figure 46 Schematic
28 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
Vx + Vy
2=262.4 mV + 229.5 mV
2= 245.9 mV =
VIN+ = ±75 mV ± 50 89
±75 mV ± 245.9 mV
Emitter
Voltage
VIN+ = 171.0 mV
105.3 mV and by symmetry: VIN- = 105.3 mV
171.0 mV
Vx
25 +Vx 89 = 12.6 mA ŸVx = 262.4 mV
229.5 mV
±0.075
VIN+ =50
50 + 39 0.246 ŸVIN+ = VIN- = 0.138V
Vx
25 +Vx
50 + 39 = 12.6 mA ŸVx = Vy = 0.246V
x
RS1
50:
LMH6555
VOUT-
VOUT+
RL
100:
V1
V2RS2
50:
VIN-
VIN+
RIN = 'VIN+
'IIN+=150 mV
(-1.26 + 4.26) mA = 50:
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
Using the calculated swing on VIN+with known VIN, one can estimate the input impedance, RIN as follows:
(38)
Differential Input Analysis
Assume that the LMH6555 is used as a differential amplifier with a transformer with its Center Tap at ground as
shown in Figure 48:
Assuming transformer secondary, VIN, of 300 mVPP
Figure 48. Differential Input Drive
The input voltages (VIN+and VIN−) can be derived using the technique explained previously. Assuming no
transformer output and referring to the schematic of Figure 45:
(39)
The peak VIN+and VIN−voltages can be determined using the transformer output voltage. Assuming there is 0.3
VPP of signal across the transformer secondary, ½ of that, or 0.15 VPP (±75 mV peak), would appear at each
input side (V1or V2in Figure 48). Here is the derivation of the LMH6555 input terminal’s peak voltages.
(40)
When V1swings positive, V2will go negative by the same value, and vice versa. Therefore, the values derived
above for Vxcan be used to determine the average emitter voltage, as described earlier:
(41)
With the transformer voltage of 0.3 VPP, each input (VIN+and VIN−) swings from 105.3 mV to 171.0 mV or about
65.7 mVPP. The input voltages are shown in Figure 49.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 29
Product Folder Links: LMH6555
V+
RT2
50:
RL
100:
VOUT-
VOUT+
RT1
50:
V+
RIN_DIFF
RIN_DIFF + 100 =0.131 VPP
0.3 VPP
ŸRIN_DIFF = 78:
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
VOLTAGE (V)
TIME
V1
VIN+
VIN- 65.7 mVPP @
138 mV DC
V2
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
Figure 49. Input Voltages for Figure 48 Schematic
Knowing the device input terminal voltages, one can estimate the differential input impedance as follows:
(42)
This is comparable to RIN_DIFF found in Electrical Characteristics.
OUTPUT STAGE AND GAIN ANALYSIS
Differential gain is determined by the differential current flow through the feedback resistors RF1 and RF2 as
shown in Figure 24. Current through RF1 (or RF2) sets the VOUT−(or VOUT+) swing. The nominal value of these
resistors is close to 430Ω.
The LMH6555 output stage consists of two bipolar common emitter amplifiers with built in output resistances, RT1
and RT2, of 50Ω, as shown in Figure 50.
Figure 50. Output Stage Including External Load RL
With an output differential load, RL, of 100Ω, half the differential swing between the output emitters appears at
the LMH6555 output terminals as VOUT.
30 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
AV_DIFF = VOUT50
RS + 50
=VOUT/VIN
50/100 = 2 VOUT/VIN = 4.83 V/V
= 13.7 dB
VIN x
AV_DIFF = VOUT
VIN 100:
2RS + 100
=VOUT/VIN
100/200 = 2 VOUT/VIN = 4.83 V/V
= 13.7 dB
x
for RS = 50: ŸVOUT
VIN =430
178 = 2.42 V/V
RS1
LMH6555 RL
100:
RS2
+
-
VIN+
-VOUT-
+
VOUT = 2RS + RIN_DIFF
VOUT
VIN =RF
2RS + 78:=430:
2RS + 78:
VIN xRF
LMH6555
www.ti.com
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
With good matching between the input source impedances, RS1 and RS2 shown in Figure 46 and Figure 48, it is
possible to infer the gain and output swing by inspection. The differential input impedance of the LMH6555,
RIN_DIFF, is close to 78Ω.
In differential input drive applications, there is a balanced swing across the input terminals of the LMH6555, VIN+
and VIN−. So, by using the RIN_DIFF value, one determines the differential current flow through the input terminals
and from that the output swing and gain.
(43)
For the special case where RS1 = RS2 = RS= 50Ωwe have:
(44)
The following is the expression for the Insertion Gain, AV_DIFF:
(45)
The expressions above apply equally to the single ended input drive case as well, as long as RS1 = RS2 = 50Ω.
For the case of the single ended input drive:
(46)
This is comparable to AV_DIFF found in Electrical Characteristics.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Links: LMH6555
LMH6555
SNOSAJ1D –NOVEMBER 2006–REVISED MARCH 2013
www.ti.com
REVISION HISTORY
Changes from Revision C (March 2013) to Revision D Page
• Changed layout of National Data Sheet to TI format .......................................................................................................... 31
32 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6555
PACKAGE OPTION ADDENDUM
www.ti.com 11-Apr-2013
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead/Ball Finish MSL Peak Temp
(3)
Op Temp (°C) Top-Side Markings
(4)
Samples
LMH6555SQ/NOPB ACTIVE WQFN RGH 16 1000 Green (RoHS
& no Sb/Br) CU SN Level-3-260C-168 HR -40 to 85 L6555SQ
LMH6555SQE/NOPB ACTIVE WQFN RGH 16 250 Green (RoHS
& no Sb/Br) CU SN Level-3-260C-168 HR -40 to 85 L6555SQ
LMH6555SQX/NOPB ACTIVE WQFN RGH 16 4500 Green (RoHS
& no Sb/Br) CU SN Level-3-260C-168 HR -40 to 85 L6555SQ
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4) Multiple Top-Side Markings will be inside parentheses. Only one Top-Side Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a
continuation of the previous line and the two combined represent the entire Top-Side Marking for that device.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
LMH6555SQ/NOPB WQFN RGH 16 1000 178.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1
LMH6555SQE/NOPB WQFN RGH 16 250 178.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1
LMH6555SQX/NOPB WQFN RGH 16 4500 330.0 12.4 4.3 4.3 1.3 8.0 12.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 26-Mar-2013
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LMH6555SQ/NOPB WQFN RGH 16 1000 213.0 191.0 55.0
LMH6555SQE/NOPB WQFN RGH 16 250 213.0 191.0 55.0
LMH6555SQX/NOPB WQFN RGH 16 4500 367.0 367.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 26-Mar-2013
Pack Materials-Page 2
MECHANICAL DATA
RGH0016A
www.ti.com
SQA16A (Rev A)
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