1
LTC1408
1408fa
6 Channel, 14-Bit, 600ksps
Simultaneous Sampling ADC
with Shutdown
The LTC
®
1408 is a 14-bit, 600ksps ADC with six simulta-
neously sampled differential inputs. The device draws
only 5mA from a single 3V supply, and comes in a tiny 32
pin (5mm × 5mm) QFN package. A SLEEP shutdown
feature lowers power consumption to 6µW. The combina-
tion of low power and tiny package makes the LTC1408
suitable for portable applications.
The LTC1408 contains six separate differential inputs that
are sampled simultaneously on the rising edge of the
CONV signal. These six sampled inputs are then
converted at a rate of 100ksps per channel.
The 90dB common mode rejection allows users to
eliminate ground loops and common mode noise by
measuring signals differentially from the source.
The device converts 0V to 2.5V unipolar inputs differen-
tially, or ±1.25V bipolar inputs also differentially,
depending on the state of the BIP pin. Any analog input
may swing rail-to-rail as long as the differential input
range is maintained.
The conversion sequence can be abbreviated to convert
fewer than six channels, depending on the logic state of
the SEL2, SEL1 and SEL0 inputs.
The serial interface sends out the six conversion results in
96 clocks for compatibility with standard serial interfaces.
■
600ksps ADC with 6 Simultaneously Sampled
Differential Inputs
■
100ksps Throughput per Channel
■
76dB SINAD
■
Low Power Dissipation: 15mW
■
3V Single Supply Operation
■
2.5V Internal Bandgap Reference, Can be Overdriven
with External Reference
■
3-Wire Serial Interface
■
Internal Conversion Triggered by CONV
■
SLEEP (6µW) Shutdown Mode
■
NAP (3.3mW) Shutdown Mode
■
0V to 2.5V Unipolar, or ±1.25V Bipolar Differential
Input Range
■
90dB Common Mode Rejection
■
Tiny 32-Pin (5mm ××
××
× 5mm) QFN Package
■
Multiphase Power Measurement
■
Multiphase Motor Control
■
Data Acquisition Systems
■
Uninterruptable Power Supplies
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
BLOCK DIAGRA
W
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners. Protected by U.S. Patents
including 6084440, 6522187.
–
+
4
525
24
–
+
7 6912
13
1619 8
CH0
–
CH0
+
CH1
–
CH1
+
10
11
–
+
1415
CH2
–
CH2
+
CH3
–
CH3
+
–
+
17
18
–
+
2021
S AND H S AND H S AND H S AND H S AND H
CH4
–
CH4
+
CH5
–
CH5
+
MUX
V
REF
10µF
BIP SEL2 SEL1 SEL0
GND
2.5V
REFERENCE
600ksps
14-BIT ADC
14-BIT LATCH 5
14-BIT LATCH 4
14-BIT LATCH 3
14-BIT LATCH 2
14-BIT LATCH 1
14-BIT LATCH 0
10µF3V
V
CC
V
DD
2629232233 27 28
2
1SD0
0.1µF
3OV
DD
3V
32 SCK
31 DGND
OGND
30 CONV
THREE-
STATE
SERIAL
OUTPUT
PORT
TIMING
LOGIC
1408 TA01
–
+
S AND H
2
LTC1408
1408fa
A
U
G
W
A
W
U
W
ARBSOLUTEXI T
IS
(Notes 1, 2)
Supply Voltage (V
DD
, V
CC
, OV
DD
) .............................. 4V
Analog Input Voltage
(Note 3) ................................... – 0.3V to (V
DD
+ 0.3V)
Digital Input Voltage .................... – 0.3V to (V
DD
+ 0.3V)
Digital Output Voltage .................. – 0.3V to (V
DD
+ 0.3V)
Power Dissipation.............................................. 100mW
Operation Temperature Range
LTC1408C ............................................... 0°C to 70°C
LTC1408I............................................ –40°C to 85°C
Storage Temperature Range ................. –65°C to 125°C
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. With internal reference, VDD = 3V, VDD = VCC = 3V.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution (No Missing Codes) ●14 Bits
Integral Linearity Error (Note 5) ●–3 ±0.5 3 LSB
Offset Error (Note 4) ●–4.5 ±1 4.5 mV
Offset Match from CH0 to CH5 –3 ±0.5 3 mV
Range Error (Note 4) ●–12 ±212 mV
Range Match from CH0 to CH5 –5 ±15 mV
Range Tempco Internal Reference (Note 4) ±15 ppm/°C
External Reference ±1 ppm/°C
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. With internal reference, VDD = 3V, VDD = VCC = 3V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IN
Analog Differential Input Range (Notes 3, 8, 9) 2.7V ≤ V
DD
≤ 3.3V 0 to 2.5 V
V
CM
Analog Common Mode + Differential 0 to V
DD
V
Input Range (Note 8)
I
IN
Analog Input Leakage Current ●1µA
C
IN
Analog Input Capacitance 13 pF
t
ACQ
Sample-and-Hold Acquisition Time (Note 6) ●39 ns
t
AP
Sample-and-Hold Aperture Delay Time 1 ns
t
JITTER
Sample-and-Hold Aperture Delay Time Jitter 0.3 ps
t
SK
Channel to Channel Aperture Skew 200 ps
CMRR Analog Input Common Mode Rejection Ratio f
IN
= 100kHz, V
IN
= 0V to 3V –83 dB
f
IN
= 10MHz, V
IN
= 0V to 3V –67 dB
CO VERTER CHARACTERISTICS
U
A ALOG I PUT
UU
WU
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
QFN PART MARKING
T
JMAX
= 125°C, θ
JA
= 34°C/ W
EXPOSED PIN IS GND (PAD 33)
MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
16 15 14 13 12 11 10 9
25 26 27 28
TOP VIEW
QFN PACKAGE
32-PIN (5mm × 5mm) PLASTIC QFN
29 30 31 32
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17CH4+
CH4–
GND
CH5+
CH5–
GND
VREF
VCC
CH1–
CH1+
GND
CH0–
CH0+
OVDD
OGND
SDO
GND
CH3–
CH3+
GND
GND
CH2–
CH2+
GND
VDD
SEL2
SEL1
SEL0
BIP
CONV
DGND
SCK
33
LTC1408CUH
LTC1408IUH
1408
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
3
LTC1408
1408fa
I TER AL REFERE CE CHARACTERISTICS
UU U
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. With internal reference, VDD = 3V, VDD = VCC = 3V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
SINAD Signal-to-Noise Plus 100kHz Input Signal ●73 76 dB
Distortion Ratio 300kHz Input Signal 76 dB
100kHz Input Signal, External V
REF
= 3.3V, V
DD
≥ 3.3V 79 dB
300kHz Input Signal, External V
REF
= 3.3V, V
DD
≥ 3.3V 79 dB
THD Total Harmonic 100kHz First 5 Harmonics ●–80 –90 dB
Distortion 300kHz First 5 Harmonics –86 dB
SFDR Spurious Free 100kHz Input Signal 90 dB
Dynamic Range 300kHz Input Signal 86 dB
IMD Intermodulation 0.625V
P-P
, 833kHz into CH0+, 0.625V
P-P
, 841kHz into CH0–. –80 dB
Distortion Bipolar Mode. Also Applicable to Other Channels
Code-to-Code V
REF
= 2.5V (Note 17) 0.7 LSB
RMS
Transition Noise
Full Power Bandwidth V
IN
= 2.5V
P-P
, SDO = 11585LSB
P-P
(–3dBFS) (Note 15) 50 MHz
Full Linear Bandwidth S/(N + D) ≥ 68dB, Bipolar Differential Input 5 MHz
TA = 25°C. VDD = VCC = 3V.
PARAMETER CONDITIONS MIN TYP MAX UNITS
V
REF
Output Voltage I
OUT
= 0 2.5 V
V
REF
Output Tempco 15 ppm/°C
V
REF
Line Regulation V
DD
= 2.7V to 3.6V, V
REF
= 2.5V 600 µV/V
V
REF
Output Resistance Load Current = 0.5mA 0.2 Ω
V
REF
Settling Time 2ms
The ● denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. VDD = VCC = 3V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IH
High Level Input Voltage V
DD
= 3.3V ●2.4 V
V
IL
Low Level Input Voltage V
DD
= 2.7V ●0.6 V
I
IN
Digital Input Current V
IN
= 0V to V
DD
●±10 µA
C
IN
Digital Input Capacitance 5pF
V
OH
High Level Output Voltage V
DD
= 3V, I
OUT
= –200µA●2.5 2.9 V
V
OL
Low Level Output Voltage V
DD
= 2.7V, I
OUT
= 160µA 0.05 V
V
DD
= 2.7V, I
OUT
= 1.6mA ●0.4 V
I
OZ
Hi-Z Output Leakage D
OUT
V
OUT
= 0V and V
DD
●±10 µA
C
OZ
Hi-Z Output Capacitance D
OUT
1pF
I
SOURCE
Output Short-Circuit Source Current V
OUT
= 0V, V
DD
= 3V 20 mA
I
SINK
Output Short-Circuit Sink Current V
OUT
= V
DD
= 3V 15 mA
DY A IC ACCURACY
U
W
DIGITAL I PUTS A D DIGITAL OUTPUTS
UU
4
LTC1408
1408fa
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
f
SAMPLE(MAX)
Maximum Sampling Frequency per Channel ●100 kHz
(Conversion Rate)
t
THROUGHPUT
Minimum Sampling Period (Conversion + Acquisiton Period) ●667 ns
t
SCK
Clock Period (Note 16) ●100 10000 ns
t
CONV
Conversion Time (Notes 6, 17) 96 SCLK cycles
t
1
Minimum Positive or Negative SCLK Pulse Width (Note 6) 2 ns
t
2
CONV to SCK Setup Time (Notes 6, 10) 3 10000 ns
t
3
SCK Before CONV (Note 6) 0 ns
t
4
Minimum Positive or Negative CONV Pulse Width (Note 6) 4 ns
t
5
SCK to Sample Mode (Note 6) 4 ns
t
6
CONV to Hold Mode (Notes 6, 11) 1.2 ns
t
7
96th SCK↑ to CONV↑ Interval (Affects Acquisition Period) (Notes 6, 7, 13) 45 ns
t
8
Minimum Delay from SCK to Valid Bits 0 Through 11 (Notes 6, 12) 8 ns
t
9
SCK to Hi-Z at SDO (Notes 6, 12) 6 ns
t
10
Previous SDO Bit Remains Valid After SCK (Notes 6, 12) 2 ns
t
11
V
REF
Settling Time After Sleep-to-Wake Transition (Notes 6, 14) 2 ms
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. With internal reference, VDD = VCC= 3V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
DD
, V
CC
Supply Voltage 2.7 3.6 V
I
DD
+ I
CC
Supply Current Active Mode, f
SAMPLE
= 600ksps ●57 mA
Nap Mode ●1.1 1.9 mA
Sleep Mode 2.0 15 µA
PD Power Dissipation Active Mode with SCK, f
SAMPLE
= 600ksps 15 mW
POWER REQUIRE E TS
WU
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. VDD = 3V.
TI I G CHARACTERISTICS
UW
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliabilty and lifetime.
Note 2: All voltage values are with respect to ground GND.
Note 3: When these pins are taken below GND or above V
DD
, they will be
clamped by internal diodes. This product can handle input currents greater
than 100mA below GND or greater than V
DD
without latchup.
Note 4: Offset and range specifications apply for a single-ended CH0
+
–
CH5
+
input with CH0
–
– CH5
–
grounded and using the internal 2.5V
reference.
Note 5: Integral linearity is tested with an external 2.55V reference and is
defined as the deviation of a code from the straight line passing through
the actual endpoints of a transfer curve. The deviation is measured from
the center of quantization band. Linearity is tested for CH0 only.
Note 6: Guaranteed by design, not subject to test.
Note 7: Recommended operating conditions.
Note 8: The analog input range is defined for the voltage difference
between CHx
+
and CHx
–
, x = 0–5.
Note 9: The absolute voltage at CHx
+
and CHx
–
must be within this range.
Note 10: If less than 3ns is allowed, the output data will appear one clock
cycle later. It is best for CONV to rise half a clock before SCK, when
running the clock at rated speed.
Note 11: Not the same as aperture delay. Aperture delay (1ns) is the
difference between the 2.2ns delay through the sample-and-hold and the
1.2ns CONV to Hold mode delay.
Note 12: The rising edge of SCK is guaranteed to catch the data coming
out into a storage latch.
Note 13: The time period for acquiring the input signal is started by the
96th rising clock and it is ended by the rising edge of CONV.
Note 14: The internal reference settles in 2ms after it wakes up from Sleep
mode with one or more cycles at SCK and a 10µF capacitive load.
Note 15: The full power bandwidth is the frequency where the output code
swing drops by 3dB with a 2.5V
P-P
input sine wave.
Note 16: Maximum clock period guarantees analog performance during
conversion. Output data can be read with an arbitrarily long clock period.
Note 17: The conversion process takes 16 clocks for each channel that is
enabled, up to 96 clocks for all 6 channels.
5
LTC1408
1408fa
SINAD, ENOBs vs Frequency
SFDR vs Input Frequency
VDD = 3V, TA = 25°C
THD, 2nd and 3rd
vs Input Frequency
TYPICAL PERFOR A CE CHARACTERISTICS
UW
98kHz Unipolar Sine Wave 4096
Point FFT Plot, 100 ksps
98kHz Bipolar Sine Wave 4096
Point FFT Plot, 100 ksps
SNR vs Input Frequency
THD, 2nd and 3rd
vs Input Frequency
FREQUENCY (MHz)
56
SINAD (dB)
ENOBS (bits)
59
62
65
68
71
74
77
1 10 100
1408 G01
53
9
9.5
10
10.5
11
11.5
12
12.5
8.5
0.1
UNIPOLAR SINGLE
ENDED
BIPOLAR
DIFFERENTIAL
FREQUENCY (MHz)
–98
THD (dB)
–92
–74
–80
–86
–68
–62
–56
–50
–44
1 10 100
1408 G02
–104
0.1
5 HARMONIC THD
UNIPOLAR SINGLE ENDED
2nd
3rd
FREQUENCY (MHz)
–98
THD (dB)
–92
–74
–80
–86
–68
–62
–56
–50
–44
1 10 100
1408 G03
–104
0.1
5 HARMONIC THD
BIPOLAR SINGLE ENDED
V
CM
= 1.5V
2nd
3rd
FREQUENCY (MHz)
50
SFDR (dB)
56
74
68
62
80
86
92
98
104
1 10 100
1408 G04
44
0.1
UNIPOLAR SINGLE ENDED
BIPOLAR DIFFERENTIAL
FREQUENCY (MHz)
65
SNR (dB)
66
69
68
67
70
71
72
75
74
73
76
1 10 100
1408 G05
64
0.1
UNIPOLAR
SINGLE ENDED
BIPOLAR
DIFFERENTIAL
FREQUENCY(kHz)
0
–120
OUTPUT MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
10 3020
1408 G06
40 50
FREQUENCY(kHz)
0
–120
OUTPUT MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
10 3020
1408 G07
40 50
591kHz Bipolar Differential Sine
Wave 4096 Point FFT Plot,
100 ksps
101kHz Unipolar Single Ended
Sine Wave 4096 Point FFT Plot,
625 ksps
FREQUENCY(kHz)
0
–120
OUTPUT MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
10 3020
1408 G08
40 50
FREQUENCY(kHz)
0
–120
OUTPUT MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
62.5 188125
1408 G09
250 313
6
LTC1408
1408fa
610kHz Unipolar Single Ended
Input Sine Wave 4096 Point FFT,
625ksps
833kHz into CH0 +, 841kHz into
CH0 –, 588ksps Bipolar Mode
4096 Point FFT
Full Scale Signal Response
Integral LinearIty End Point Fit
Differential Linearity for CH0 with
Internal 2.5V Reference,
Unipolar Mode
Integral Linearity End Point Fit for
CH0 with Internal 2.5V
Reference, Unipolar Mode
VDD = 3V, TA = 25°C
TYPICAL PERFOR A CE CHARACTERISTICS
UW
FREQUENCY(kHz)
0
–120
OUTPUT MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
62.5 188125
1408 G10
250 313
FREQUENCY(kHz)
0
–120
MAGNITUDE (dB)
–80
–90
–100
–110
–40
–60
–70
–50
0
–10
–20
–30
50 150100
1408 G11
200 250 300
1408 G12
OUTPUT CODE
0
–1.0
DIFFERENTIAL LINEARITY (LSB)
–0.8
–0.4
–0.2
0
1.0
0.4
4096 8192
–0.6
0.6
0.8
0.2
12288 16384
OUTPUT CODE
0
–4.0
INTEGRAL LINEARITY (LSB)
–3.2
–1.6
–0.8
0
4.0
1.6
4096 8192
1408 G13
–2.4
2.4
3.2
0.8
12288 16384
OUTPUT CODE
0
–1.0
DIFFERENTIAL LINEARITY (LSB)
–0.8
–0.4
–0.2
0
1.0
0.4
4096 8192
1408 G14
–0.6
0.6
0.8
0.2
12288 16384
Differential Linearity for CH0 with
Internal 2.5V Reference,
Differential Input in Bipolar Mode
OUTPUT CODE
0
–4.0
INTEGRAL LINEARITY (LSB)
–3.2
–1.6
–0.8
0
4.0
1.6
4096 8192
1408 G15
–2.4
2.4
3.2
0.8
12288 16384
FREQUENCY (MHz)
–24
–27
MAGNITUDE (dB)
–21
–12
–15
–18
–9
–6
–3
0
3
100 1000
1408 G16
–30
10
1408 G20
FREQUENCY (Hz)
–100
CMRR (dB)
–60
–20
–40
–80
0
10k 100k 1M 10M 100M 1G
–120
100 1k
1408 G21
FREQUENCY (Hz)
–100
PSSR (dB)
–60
–20
–40
–80
0
10k 100k 1M 10M 100M 1G
–120
100 1k
CROSSTALK vs Frequency
CMRR vs Frequency
7
LTC1408
1408fa
UU
U
PI FU CTIO S
SDO (Pin 1): Three-State Serial Data Output. Each set of
six output data words represent the six analog input
channels at the start of the previous conversion. Data for
CH0 comes out first and data for CH5 comes out last. Each
data word comes out MSB first.
OGND (Pin 2): Ground Return for SDO Currents. This pad
must always be within 300mV of the ground plane poten-
tial.
OV
DD
(Pin 3): Power Supply for the SDO Pin. OV
DD
must
be no more than 300mV higher than V
DD
and can be
brought to a lower voltage to interface to low voltage logic
families. The unloaded high state at SDO is at the potential
of OV
DD
.
CH0
+
(Pin 4): Non-Inverting Channel 0. CH0
+
operates
fully differentially with respect to CH0
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH0
–
(Pin 5): Inverting Channel 0. CH0
–
operates fully
differentially with respect to CH0
+
with a –2.5V to 0V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
GND (Pins 6, 9, 12, 13, 16, 19): Analog Grounds. These
ground pins must be tied directly to the solid ground plane
under the part. Analog signal currents flow through these
connections.
CH1
+
(Pin 7): Non-Inverting Channel 1. CH1
+
operates
fully differentially with respect to CH1
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH1
–
(Pin 8): Inverting Channel 1. CH1
–
operates fully
differentially with respect to CH1
+
with a –2.5V to 0V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH2
+
(Pin 10): Non-Inverting Channel 2. CH2
+
operates
fully differentially with respect to CH2
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH2
–
(Pin 11): Inverting Channel 2. CH2
–
operates fully
differentially with respect to CH2
+
with a –2.5V to 0V, or
±1.25V differential swing and a 0V to V
DD
absolute
input range.
VDD = 3V, TA = 25°C
TYPICAL PERFOR A CE CHARACTERISTICS
UW
TIME (ns)
–5
OUTPUT CODE
6144
8192
10240
10 20
1408 G22
4096
2048
005 15
12288
14336
16384
25
CH0 THRU CH5
RISING
CH0 THRU CH5
FALLING
1408 G23
FREQUENCY (Hz)
–100
CMRR (dB)
–60
–20
–40
–80
0
10k 100k 1M 10M 100M 1G
–120
100 1k
Simultaneous Step at all 6 Channels
from 25Ω Source Sampling
Frequency = 625ksps
Input Frequency = 625.0381MHz PSRR vs Frequency
8
LTC1408
1408fa
CH3
+
(Pin 14): Non-Inverting Channel 3. CH3
+
operates
fully differentially with respect to CH3
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH3
–
(Pin 15): Inverting Channel 3. CH3
–
operates fully
differentially with respect to CH3
+
with a –2.5V to 0V, or
±1.25V differential swing and a 0V to V
DD
absolute
input range.
CH4
+
(Pin 17): Non-Inverting Channel 4. CH4
+
operates
fully differentially with respect to CH4
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute input
range.
CH4
–
(Pin 18): Inverting Channel 4. CH4
–
operates fully
differentially with respect to CH4
+
with a –2.5V to 0V, or
±1.25V differential swing and a 0V to V
DD
absolute input
range.
CH5
+
(Pin 20): Non-Inverting Channel 5. CH5
+
operates
fully differentially with respect to CH5
–
with a 0V to 2.5V,
or ±1.25V differential swing and a 0V to V
DD
absolute input
range.
CH5
–
(Pin 21): Inverting Channel 5. CH5
–
operates fully
differentially with respect to CH5
+
with a –2.5V to 0V, or
±1.25V differential swing and a 0V to V
DD
absolute input
range.
GND (PIN 22): Analog Ground for Reference. Analog
ground must be tied directly to the solid ground plane
under the part. Analog signal currents flow through this
connection. The 10µF reference bypass capacitor should
be returned to this pad.
V
REF
(Pin 23): 2.5V Internal Reference. Bypass to GND
and a solid analog ground plane with a 10µF ceramic
capacitor (or 10µF tantalum in parallel with 0.1µF ce-
ramic). Can be overdriven by an external reference voltage
between 2.55% and V
DD
, V
CC
.
V
CC
(Pin 24): 3V Positive Analog Supply. This pin supplies
3V to the analog section. Bypass to the solid analog
ground plane with a 10µF ceramic capacitor (or 10µF
tantalum) in parallel with 0.1µF ceramic. Care should be
taken to place the 0.1µF bypass capacitor as close to
Pin 24 as possible. Pin 24 must be tied to Pin 25.
V
DD
(Pin 25): 3V Positive Digital Supply. This pin supplies
3V to the logic section. Bypass to DGND pin and solid
analog ground plane with a 10µF ceramic capacitor (or
10µF tantalum in parallel with 0.1µF ceramic). Keep in
mind that internal digital output signal currents flow
through this pin. Care should be taken to place the 0.1µF
bypass capacitor as close to Pin 25 as possible. Pin 25
must be tied to Pin 24.
SEL2 (Pin 26): Most significant bit controlling the
number of channels being converted. In combination with
SEL1 and SEL0, 000 selects just the first channel (CH0) for
conversion. Incrementing SELx selects additional
channels(CH0–CH5) for conversion. 101, 110 or 111
select all 6 channels for conversion. Must be kept in a fixed
state during conversion and during the subsequent con-
version to read data.
SEL1 (Pin 27): Middle significance bit controlling the
number of channels being converted. In combination with
SEL0 and SEL2, 000 selects just the first channel (CH0) for
conversion. Incrementing SELx selects additional
channels for conversion. 101, 110 or 111 select all 6
channels (CH0–CH5) for conversion. Must be kept in a
fixed state during conversion and during the subsequent
conversion to read data.
SEL0 (Pin 28): Least significant bit controlling the
number of channels being converted. In combination with
SEL1 and SEL2, 000 selects just the first channel (CH0) for
conversion. Incrementing SELx selects additional
channels for conversion. 101, 110 or 111 select all 6
channels (CH0–CH5) for conversion. Must be kept in a
fixed state during conversion and during the subsequent
conversion to read data.
BIP (Pin 29): Bipolar/Unipolar Mode. The input differen-
tial range is 0V – 2.5V when BIP is LOW, and it is ±1.25
when BIP is HIGH. Must be kept in fixed state during
conversion and during subsequent conversion to read
data. When changing BIP between conversions the full
acquisition time must be allowed before starting the next
conversion. The output data is in 2’s complement
format for bipolar mode and straight binary format for
unipolar mode.
UU
U
PI FU CTIO S
9
LTC1408
1408fa
BLOCK DIAGRA
W
2
OGND
1
SD0
3
OV
DD
3V
–
+
4
5
24
23
S & H
–
+
7
6
9
12 13
16
19
8
S & H
GND
EXPOSED PAD V
REF
10µF
CH0
–
CH0
+
CH1
–
CH1
+
–
+
10
11
S & H
–
+
14
15
S & H
CH2
–
CH2
+
CH3
–
CH3
+
–
+
17
18
S & H
–
+
20
21
S & H
CH4
–
CH4
+
CH5
–
CH5
+
10µF
0.1µF
LTC1408
DGND
32 SCK
30 CONV
SEL2 SEL1 SEL0
THREE-
STATE
SERIAL
OUTPUT
PORT
MUX
2.5V
REFERENCE
TIMING
LOGIC
V
CC
25
3V
V
DD
1408 BD
600ksps
14-BIT ADC
14-BIT LATCH 5
14-BIT LATCH 4
14-BIT LATCH 3
14-BIT LATCH 2
14-BIT LATCH 1
14-BIT LATCH 0
26 27
BIP
29 28 31
2233
0.1µF
CONV (Pin 30): Convert Start. Holds the six analog input
signals and starts the conversion on the rising edge. Two
pulses with SCK in fixed high or fixed low state starts Nap
mode. Four or more pulses with SCK in fixed high or fixed
low state starts Sleep mode.
DGND (Pin 31): Digital Ground. This ground pin must be
tied directly to the solid ground plane. Digital input signal
currents flow through this pin.
SCK (Pin 32): External Clock Input. Advances the conver-
sion process and sequences the output data at SD0 (Pin1)
on the rising edge. One or more pulses wake from sleep
or nap power saving modes. 16 clock cycles are needed
for each of the channels that are activated by SELx (Pins
26, 27, 28), up to a total of 96 clock cycles needed to
convert and read out all 6 channels.
EXPOSED PAD (Pin 33): GND. Must be tied directly to the
solid ground plane.
UU
U
PI FU CTIO S
10
LTC1408
1408fa
LTC1408 Timing Diagram
TI I G DIAGRA S
WUW
11
LTC1408
1408fa
Nap Mode and Sleep Mode Waveforms
SCK to SDO Delay
t
8
t
10
SCK
SDO
1408 TD03
V
IH
V
OH
V
OL
t
9
SCK
SDO Hi-Z
V
IH
SCK
CONV
NAP
SLEEP
VREF
t1
t11
t1
NOTE: NAP AND SLEEP ARE INTERNAL SIGNALS
1408 TD02
TI I G DIAGRA S
WUW
12
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
SELECT NUMBER OF CONVERTED CHANNELS
(SEL2, SEL1, SEL0)
These three control pins select the number of channels
being converted. 000 selects only the first channel (CH0)
for conversion. Incrementing SELx selects additional chan-
nels for conversion, up to 6 channels. 101, 110 or 111
select all 6 channels for conversion. These pins must be
kept in a fixed state during conversion and during the
subsequent conversion to read data. When changing
modes between conversions, keep in mind that the output
data of a particular channel will remain unchanged until
after that channel is converted again. For example: convert
a sequence of 4 channels (CH0, CH1, CH2, CH3) with
SELx = 011, then, after these channels are converted
change SELx to 001 to convert just CH0 and CH1. See
Table 1. During the conversion of the first set of two
channels you will be able to read the data from the same
two channels converted as part of the previous group of
4 channels. Later, you could convert 4 or more channels
to read back the unread CH2 and CH3 data that was
converted in the first set of 4 channels. These pins are
often hardwired to enable the right number of channels for
a particular application.
BIPOLAR/UNIPOLAR MODE
The input voltage range for each of the CHx input differen-
tial pairs is UNIPOLAR 0V – 2.5V when BIP is LOW, and
BIPOLAR ±1.25V when BIP is HIGH. This pin must be kept
in fixed state during conversion and during subsequent
conversion to read data. When changing BIP between
conversions the full acquisition time must be allowed
before starting the next conversion. After changing modes
from BIPOLAR to UNIPOLAR, or from UNIPOLAR to
BIPOLAR, you can still read the first set of channels in the
new mode, by inverting the MSB to read these channels in
the mode that they were converted in.
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1408 are easy to
drive. The inputs may be driven differentially or as a single-
ended input (i.e., the CH0
–
input is grounded). All twelve
analog inputs of all six differential analog input pairs, CH0
+
with CH0
–
, CH1
+
with CH1
–
, CH2
+
with CH2
–
, CH3
+
with
CH3
–
, CH4
+
with CH4
–
and CH5
+
with CH5
–
, are sampled
at the same instant. Any unwanted signal that is common
to both inputs of each input pair will be reduced by the
common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charg-
ing the sample-and-hold capacitors at the end of conver-
sion. During conversion, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuit is low, then the LTC1408 inputs can be
driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifier must be used. The
main requirement is that the amplifier driving the analog
input(s) must settle after the small current spike before the
next conversion starts (the time allowed for settling must
be at least 39ns for full throughput rate). Also keep in
mind, while choosing an input amplifier, the amount of
noise and harmonic distortion added by the amplifier.
Table 1. Conversion Sequence Control
(“acquire” represents simultaneous sampling of all channels; CHx represents conversion of channels)
SEL2 SEL1 SEL0 CHANNEL ACQUISITION AND CONVERSION SEQUENCE
0 0 0 acquire, CH0, acquire, CH0...
0 0 1 acquire, CH0, CH1, acquire, CH0, CH1...
0 1 0 acquire, CH0, CH1, CH2, acquire, CH0, CH1, CH2...
0 1 1 acquire, CH0, CH1, CH2, CH3, acquire, CH0, CH1, CH2, CH3...
1 0 0 acquire, CH0, CH1, CH2, CH3, CH4, acquire, CH0,CH1,CH2, CH3, CH4...
1 0 1 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
1 1 0 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
1 1 1 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
13
LTC1408
1408fa
LTC1566-1: Low Noise 2.3MHz Continuous Time
Lowpass Filter.
LT
®
1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 500µV offset and
520ns settling to 0.5LSB for a 4V swing. THD and noise
are –93dB to 40kHz and below 1LSB to 320kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for AC
applications (to 1/3 Nyquist) where rail-to-rail perfor-
mance is desired. Quad version is available as LT1631.
LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 1.5mV offset and
400ns settling to 0.5LSB for a 4V swing. It is suitable for
applications with a single 5V supply. THD and noise are
–93dB to 40kHz and below 1LSB to 800kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for AC
applications where rail-to-rail performance is desired.
Quad version is available as LT1633.
LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/ampli-
fier, 8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc distortion at
5MHz, unity gain stable, rail-to-rail in and out,
10mA/amplifier, 3.5nV/√Hz.
LT1810: 180MHz GBWP, –90dBc distortion at 5MHz,
unity gain stable, rail-to-rail in and out, 15mA/amplifier,
16nV/√Hz.
LT1818/LT1819: 400MHz, 2500V/µs, 9mA, Single/Dual
Voltage Mode Operational Amplifier.
LT6200: 165MHz GBWP, –85dBc distortion at 1MHz,
unity gain stable, rail-to-rail in and out, 15mA/amplifier,
0.95nV/√Hz.
LT6203: 100MHz GBWP, –80dBc distortion at 1MHz,
unity gain stable, rail-to-rail in and out, 3mA/amplifier,
1.9nV/√Hz.
LT6600: Amplifier/Filter Differential In/Out with 10MHz
Cutoff.
CHOOSING AN INPUT AMPLIFIER
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifier from charging
the sampling capacitor, choose an amplifier that has a low
output impedance (< 100Ω) at the closed-loop bandwidth
frequency. For example, if an amplifier is used in a gain of
1 and has a unity-gain bandwidth of 50MHz, then the
output impedance at 50MHz must be less than 100Ω. The
second requirement is that the closed-loop bandwidth
must be greater than 40MHz to ensure adequate small-
signal settling for full throughput rate. If slower op amps
are used, more time for settling can be provided by
increasing the time between conversions. The best choice
for an op amp to drive the LTC1408 depends on the
application. Generally, applications fall into two catego-
ries: AC applications where dynamic specifications are
most critical and time domain applications where DC
accuracy and settling time are most critical. The following
list is a summary of the op amps that are suitable for
driving the LTC1408. (More detailed information is avail-
able in the Linear Technology Databooks and on the
website at www.linear.com.)
APPLICATIO S I FOR ATIO
WUUU
LinearView is a trademark of Linear Technology Corporation.
14
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
INPUT FILTERING AND SOURCE IMPEDANCE
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1408 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 50MHz. Any noise
or distortion products that are present at the analog inputs
will be summed over this entire bandwidth. Noisy input
circuitry should be filtered prior to the analog inputs to
minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For example, Figure 1 shows a 47pF
capacitor from CHO
+
to ground and a 51Ω source resistor
to limit the net input bandwidth to 30MHz. The 47pF ca-
pacitor also acts as a charge reservoir for the input sample-
and-hold and isolates the ADC input from sampling-glitch
sensitive circuitry. High quality capacitors and resistors
should be used since these components can add distor-
tion. NPO and silvermica type dielectric capacitors have
excellent linearity. Carbon surface mount resistors can
generate distortion from self heating and from damage
that may occur during soldering. Metal film surface mount
resistors are much less susceptible to both problems.
When high amplitude unwanted signals are close in
frequency to the desired signal frequency a multiple pole
filter is required.
High external source resistance, combined with 13pF of
input capacitance, will reduce the rated 50MHz input band-
width and increase acquisition time beyond 39ns.
INPUT RANGE
The analog inputs of the LTC1408 may be driven fully
differentially with a single supply. Either input may swing
up to V
CC
, provided the differential swing is no greater than
2.5V with BIP (Pin 29) Low, or ±1.25V with (BIP Pin 29)
High. The 0V to 2.5V range is also ideally suited for single-
ended input use with single supply applications. The
common mode range of the inputs extend from ground to
the supply voltage V
CC
. If the difference between the CH
+
and CH
–
at any input pair exceeds 2.5V (unipolar) or 1.25V
(bipolar), the output code will stay fixed at positive full-
scale, and if this difference goes below 0V (unipolar) or –
1.25V (bipolar), the output code will stay fixed at negative
full-scale.
INTERNAL REFERENCE
The LTC1408 has an on-chip, temperature compensated,
bandgap reference that is factory trimmed near 2.5V to
obtain a precise 2.5V input span. The reference amplifier
output VREF, (Pin 23) must be bypassed with a capacitor
to ground. The reference amplifier is stable with capaci-
tors of 1µF or greater. For the best noise performance, a
10µF ceramic or a 10µF tantalum in parallel with a 0.1µF
ceramic is recommended. The VREF pin can be overdriven
with an external reference as shown in Figure 2. The volt-
age of the external reference must be higher than the 2.5V
of the open-drain P-channel output of the internal refer-
ence. The recommended range for an external reference
is 2.55V to VDD. An external reference at 2.55V will see a
DC quiescent load of 0.75mA and as much as 3mA
Figure 1. RC Input Filter
LTC1408
CH0
+
CH0
–
V
REF
GND
1408 F01
1
2
11
3
10µF
47pF*
51Ω*
CH1
+
CH1
–
4
5
47pF*
*TIGHT TOLERANCE REQUIRED TO AVOID
APERTURE SKEW DEGRADATION
51Ω*
ANALOG
INPUT
ANALOG
INPUT
Figure 2. External Reference
LTC1408
V
REF
GND
1408 F02
23
22
10µF
3V REF
15
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
Figure 4 shows the ideal input/output characteristics for
the LTC1408 in unipolar mode (BIP = Low). The code
transitions occur midway between successive integer LSB
values (i.e., 0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The
output code is straight binary with 1LSB = 2.5V/16384 =
153µV for the LTC1408. The LTC1408 has 0.7 LSB RMS
of Gaussian white noise.
during conversion.
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a unipolar voltage span
that equals the difference between the voltage at the
reference buffer output V
REF
(Pin 23) and the voltage at
ground. The differential input range of the ADC is 0V to
2.5V when using the internal reference. The internal ADC
is referenced to these two nodes. This relationship also
holds true with an external reference.
DIFFERENTIAL INPUTS
The ADC will always convert the difference of CH
+
minus
CH
–
, independent of the common mode voltage at any pair
of inputs. The common mode rejection holds up at high
frequencies (see Figure 3.) The only requirement is that
both inputs not go below ground or exceed V
DD
.
Figure 3. CMRR vs Frequency
1408 G20
FREQUENCY (Hz)
–100
CMRR (dB)
–60
–20
–40
–80
0
10k 100k 1M 10M 100M 1G
–120
100 1k
Integral nonlinearity errors (INL) and differential nonlin-
earity errors (DNL) are largely independent of the common
mode voltage. However, the offset error will vary. DC
CMRR is typically better than –90dB.
Figure 5 shows the ideal input/output characteristics for
the LTC1408 in bipolar mode (BIP = High). The code
transitions occur midway between successive integer LSB
values (i.e., 0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The
output code is 2’s complement with 1LSB = 2.5V/16384 =
153µV for the LTC1408. The LTC1408 has 0.7 LSB RMS
of Gaussian white noise.
Figure 5. LTC1408 Transfer Characteristic
in Bipolar Mode (BIP = High)
INPUT VOLTAGE (V)
2'S COMPLEMENT OUTPUT CODE
1408 F05
011...111
011...110
011...101
100...000
100...001
100...010
FS – 1LSB–FS
Figure 4. LTC1408 Transfer Characteristic
in Unipolar Mode (BIP = Low)
INPUT VOLTAGE (V)
STRAIGHT BINARY OUTPUT CODE
1408 F04
111...111
111...110
111...101
000...000
000...001
000...010
FS – 1LSB0
16
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
POWER-DOWN MODES
Upon power-up, the LTC1408 is initialized to the active
state and is ready for conversion. The Nap and Sleep mode
waveforms show the power down modes for the LTC1408.
The SCK and CONV inputs control the power down modes
(see Timing Diagrams). Two rising edges at CONV, with-
out any intervening rising edges at SCK, put the LTC1408
in Nap mode and the power drain drops from 15mW to
3.3mW. The internal reference remains powered in Nap
mode. One or more rising edges at SCK wake up the
LTC1408 for service very quickly and CONV can start an
accurate conversion within a clock cycle. Four rising
edges at CONV, without any intervening rising edges at
SCK, put the LTC1408 in Sleep mode and the power drain
drops from 15mW to 10µW. One or more rising edges at
SCK wake up the LTC1408 for operation. The internal
reference (V
REF
) takes 2ms to slew and settle with a 10µF
load. Using sleep mode more frequently compromises the
accuracy of the output data. Note that for slower conver-
sion rates, the Nap and Sleep modes can be used for
substantial reductions in power consumption.
DIGITAL INTERFACE
The LTC1408 has a 3-wire SPI (Serial Peripheral Interface)
interface. The SCK and CONV inputs and SDO output
implement this interface. The SCK and CONV inputs
accept swings from 3V logic and are TTL compatible, if the
logic swing does not exceed V
DD
. A detailed description of
the three serial port signals follows:
Conversion Start Input (CONV)
The rising edge of CONV starts a conversion, but subse-
quent rising edges at CONV are ignored by the LTC1408
until the following 96 SCK rising edges have occurred. The
duty cycle of CONV can be arbitrarily chosen to be used as
a frame sync signal for the processor serial port. A simple
approach to generate CONV is to create a pulse that is one
SCK wide to drive the LTC1408 and then buffer this signal
to drive the frame sync input of the processor serial port.
It is good practice to drive the LTC1408 CONV input first
to avoid digital noise interference during the sample-to-
hold transition triggered by CONV at the start of conver-
sion. It is also good practice to keep the width of the low
portion of the CONV signal greater than 15ns to avoid
introducing glitches in the front end of the ADC just before
the sample-and-hold goes into Hold mode at the rising
edge of CONV.
17
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
Minimizing Jitter on the CONV Input
In high speed applications where high amplitude sinewaves
above 100kHz are sampled, the CONV signal must have as
little jitter as possible (10ps or less). The square wave
output of a common crystal clock module usually meets
this requirement easily. The challenge is to generate a
CONV signal from this crystal clock without jitter corrup-
tion from other digital circuits in the system. A clock
divider and any gates in the signal path from the crystal
clock to the CONV input should not share the same
integrated circuit with other parts of the system. The SCK
and CONV inputs should be driven first, with digital buffers
used to drive the serial port interface. Also note that the
master clock in the DSP may already be corrupted with
jitter, even if it comes directly from the DSP crystal.
Another problem with high speed processor clocks is that
they often use a low cost, low speed crystal (i.e., 10MHz)
to generate a fast, but jittery, phase-locked-loop system
clock (i.e., 40MHz). The jitter in these PLL-generated high
speed clocks can be several nanoseconds. Note that if you
choose to use the frame sync signal generated by the DSP
port, this signal will have the same jitter of the DSP’s
master clock.
The Typical Application Figure on page 20 shows a circuit
for level-shifting and squaring the output from an RF
signal generator or other low-jitter source. A single D-type
flip flop is used to generate the CONV signal to the
LTC1408. Re-timing the master clock signal eliminates
clock jitter introduced by the controlling device (DSP,
FPGA, etc.) Both the inverter and flip flop must be treated
as analog components and should be powered from a
clean analog supply.
Serial Clock Input (SCK)
The rising edge of SCK advances the conversion process
and also udpates each bit in the SDO data stream. After
CONV rises, the third rising edge of SCK sends out up to
six sets of 14 data bits, with the MSB sent first. A simple
approach is to generate SCK to drive the LTC1408 first and
then buffer this signal with the appropriate number of
inverters to drive the serial clock input of the processor
serial port. Use the falling edge of the clock to latch data
from the Serial Data Output (SDO) into your processor
serial port. The 14-bit Serial Data will be received right
justified, in six 16-bit words with 96 or more clocks per
frame sync. If fewer than 6 channels are selected by
SEL0–SEL2 for conversion, then 16 clocks are needed per
channel to convert the analog inputs and read out the
resulting data after the next convert pulse. It is good
practice to drive the LTC1408 SCK input first to avoid
digital noise interference during the internal bit compari-
son decision by the internal high speed comparator.
Unlike the CONV input, the SCK input is not sensitive to
jitter because the input signal is already sampled and held
constant.
Serial Data Output (SDO)
Upon power-up, the SDO output is automatically reset
to the high impedance state. The SDO output remains
in high impedance until a new conversion is started.
SDO sends out up to six sets of 14 bits in the output data
stream after the third rising edge of SCK after the start
of conversion with the rising edge of CONV. The six or
fewer 14-bit words are separated by two clock cycles in
high impedance mode. Please note the delay specifica-
tion from SCK to a valid SDO. SDO is always guaranteed
to be valid by the next rising edge of SCK. The 16 – 96-
bit output data stream is compatible with the 16-bit or
32-bit serial port of most processors.
18
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
1408 F06
3
30
32
1
2
3-WIRE SERIAL
INTERFACE LINK
OV
DD
CONV
SCK
LTC1408
SDO
V
CC
BFSR
BCLKR
TMS320C54x
BDR
OGND
31
DGND
CONV
0V TO 3V LOGIC SWING
CLK
5V3V
B13 B12
Figure 7. DSP Serial Interface to TMS320C54x
BOARD LAYOUT AND BYPASSING
Wire wrap boards are not recommended for high resolu-
tion and/or high speed A/D converters. To obtain the best
performance from the LTC1408, a printed circuit board
with ground plane is required. Layout for the printed circuit
board should ensure that digital and analog signal lines are
separated as much as possible. In particular, care should
be taken not to run any digital track alongside an analog
signal track. If optimum phase match between the inputs
is desired, the length of the twelve input wires of the six
input channels should be kept matched. But each pair of
input wires to the six input channels should be kept
separated by a ground trace to avoid high frequency
crosstalk between channels.
High quality tantalum and ceramic bypass capacitors should
be used at the V
CC
, V
DD
and V
REF
pins as shown in the Block
Diagram on the first page of this data sheet. For optimum
performance, a 10µF surface mount tantalum capacitor
with a 0.1µF ceramic is recommended for the V
CC
, V
DD
and
V
REF
pins. Alternatively, 10µF ceramic chip capacitors such
as X5R or X7R may be used. The capacitors must be lo-
cated as close to the pins as possible. The traces connect-
ing the pins and the bypass capacitors must be kept short
and should be made as wide as possible. The V
CC
and V
DD
bypass capacitor returns to the ground plane and the V
REF
bypass capacitor returns to the Pin 22. Care should be
taken to place the 0.1µF V
CC
and V
DD
bypass capacitor as
close to Pins 24 and 25 as possible.
Figure 6. Recommended Layout
Figure 6 shows the recommended system ground connec-
tions. All analog circuitry grounds should be terminated at
the LTC1408 Exposed Pad. The ground return from the
LTC1408 to the power supply should be low impedance for
noise-free operation. The Exposed Pad of the 32-pin QFN
package is also internally tied to the ground pads. The
Exposed Pad should be soldered on the PC board to reduce
ground connection inductance. All ground pins (GND,
DGND, OGND) must be connected directly to the same
ground plane under the LTC1408.
V
DD
BYPASS,
0.1µF, 0402
OV
DD
BYPASS,
0.1µF, 0402
V
REF
BYPASS,
10µF, 0805
V
CC
BYPASS,
0.1µF, 0402 AND
10µF, 0805
HARDWARE INTERFACE TO TMS320C54x
The LTC1408 is a serial output ADC whose interface has
been designed for high speed buffered serial ports in fast
digital signal processors (DSPs). Figure 7 shows an
example of this interface using a TMS320C54X.
The buffered serial port in the TMS320C54x has direct
access to a 2kB segment of memory. The ADC’s serial data
can be collected in two alternating 1kB segments, in real
time, at the full 600ksps conversion rate of the LTC1408.
The DSP assembly code sets frame sync mode at the BFSR
pin to accept an external positive going pulse and the serial
clock at the BCLKR pin to accept an external positive edge
clock. Buffers near the LTC1408 may be added to drive
long tracks to the DSP to prevent corruption of the signal
to LTC1408. This configuration is adequate to traverse a
typical system board, but source resistors at the buffer
outputs and termination resistors at the DSP, may be
needed to match the characteristic impedance of very long
transmission lines. If you need to terminate the SDO
transmission line, buffer it first with one or two 74ACxx
gates. The TTL threshold inputs of the DSP port respond
properly to the 3V swing used with the LTC1408.
19
LTC1408
1408fa
U
PACKAGE DESCRIPTIO
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
5.00 ± 0.10
(4 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE
M0-220 VARIATION WHHD-(X) (TO BE APPROVED)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 6)
0.40 ± 0.10
31
1
2
32
BOTTOM VIEW—EXPOSED PAD
3.45 ± 0.10
(4-SIDES)
0.75 ± 0.05 R = 0.115
TYP
0.25 ± 0.05
(UH32) QFN 1004
0.50 BSC
0.200 REF
0.00 – 0.05
0.70 ±0.05
3.45 ±0.05
(4 SIDES)
4.10 ±0.05
5.50 ±0.05
0.25 ± 0.05
PACKAGE OUTLINE
0.50 BSC
RECOMMENDED SOLDER PAD LAYOUT
PIN 1 NOTCH R = 0.30 TYP
OR 0.35 × 45° CHAMFER
UH Package
32-Lead Plastic QFN (5mm × 5mm)
(Reference LTC DWG # 05-08-1693)
20
LTC1408
1408fa
RELATED PARTS
© LINEAR TECHNOLOGY CORPORATION 2006
LT 0606 • PRINTED IN THE USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
PART NUMBER DESCRIPTION COMMENTS
ADCs
LTC1402 12-Bit, 2.2Msps Serial ADC 5V or ±5V Supply, 4.096V or ±2.5V Span
LTC1403/LTC1403A 12-/14-Bit, 2.8Msps Serial ADC 3V, 15mW, Unipolar Inputs, MSOP Package
LTC1403-1/LTC1403A-1 12-/14-Bit, 2.8Msps Serial ADC 3V, 15mW, Bipolar Inputs, MSOP Package
LTC1405 12-Bit, 5Msps Parallel ADC 5V, Selectable Spans, 115mW
LTC1407/LTC1407A 12-/14-Bit, 3Msps Simultaneous Sampling ADC 3V, 14mW, 2-Channel Unipolar Input Range
LTC1407-1/LTC1407A-1 12-/14-Bit, 3Msps Simultaneous Sampling ADC 3V, 14mW, 2-Channel Bipolar Input Range
LTC1411 14-Bit, 2.5Msps Parallel ADC 5V, Selectable Spans, 80dB SINAD
LTC1412 12-Bit, 3Msps Parallel ADC ±5V Supply, ±2.5V Span, 72dB SINAD
LTC1420 12-Bit, 10Msps Parallel ADC 5V, Selectable Spans, 72dB SINAD
LTC1608 16-Bit, 500ksps Parallel ADC ±5V Supply, ±2.5V Span, 90dB SINAD
LTC1609 16-Bit, 250ksps Serial ADC 5V Configurable Bipolar/Unipolar Inputs
LTC1864/LTC1865 16-Bit, 250ksps 1-/2-Channel Serial ADCs 5V or 3V (L-Version), Micropower, MSOP Package
LTC1864L/LTC1865L
DACs
LTC1592 16-Bit, Serial SoftSpanTM I
OUT
DAC ±1LSB INL/DNL, Software Selectable Spans
LTC1666/LTC1667 12-/14-/16-Bit, 50Msps DAC 87dB SFDR, 20ns Settling Time
LTC1668
References
LT1460-2.5 Micropower Series Voltage Reference 0.10% Initial Accuracy, 10ppm Drift
LT1461-2.5 Precision Voltage Reference 0.04% Initial Accuracy, 3ppm Drift
LT1790-2.5 Micropower Series Reference in SOT-23 0.05% Initial Accuracy, 10ppm Drift
SoftSpan is a trademark of Linear Technology Corporation.
U
TYPICAL APPLICATIO
PRE
VCC
1k
1k50Ω
VCC
NL17SZ74 CONVERT ENABLE
NC7SVU04P5X
MASTER CLOCK
0.1µF
TO LTC1408
CONV
CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
DQ
Q
CLR
1408 TA02
Clock Squaring/Level Shifting Circuit Allows Testing with RF Sine Generator.
Convert Re-Timing Flip-Flop Preserves Low-Jitter Clock Timing
