ADC12D040
Dual 12-Bit, 40 MSPS, 600 mW A/D Converter with
Internal/External Reference
General Description
The ADC12D040 is a dual, low power monolithic CMOS
analog-to-digital converter capable of converting analog in-
put signals into 12-bit digital words at 40 Megasamples per
second (Msps), minimum. This converter uses a differential,
pipeline architecture with digital error correction and an on-
chip sample-and-hold circuit to minimize die size and power
consumption while providing excellent dynamic perfor-
mance. Operating on a single 5V power supply, the
ADC12D040 achieves 10.9 effective bits at 10 MHz input
and consumes just 600 mW at 40 Msps, including the refer-
ence current. The Power Down feature reduces power con-
sumption to 75 mW.
The differential inputs provide a full scale differential input
swing equal to 2V
REF
with the possibility of a single-ended
input. Full use of the differential input is recommended for
optimum performance. The digital outputs for the two ADCs
are available on separate 12-bit buses with an output data
format choice of offset binary or 2’s complement.
For ease of interface, the digital output driver power pins of
the ADC12D040 can be connected to a separate supply
voltage in the range of 2.4V to the digital supply voltage,
making the outputs compatible with low voltage systems.
The ADC12D040’s speed, resolution and single supply op-
eration make it well suited for a variety of applications.
This device is available in the 64-lead TQFP package and
will operate over the industrial temperature range of −40˚C to
+85˚C. An evaluation board is available to facilitate the prod-
uct evaluation process
Features
nBinary or 2’s complement output format
nSingle supply operation
nInternal sample-and-hold
nOutputs 2.4V to 5V compatible
nPower down mode
nPin-compatible with ADC12DL066
nInternal/External Reference
Key Specifications
nSNR (f
IN
= 10 MHz) 68 dB (typ)
nENOB (f
IN
= 10 MHz) 10.9 bits (typ)
nSFDR (f
IN
= 10 MHz) 80 dB (typ)
nData Latency 6 Clock Cycles
nSupply Voltage +5V ±5%
nPower Consumption, Operating
— Operating 600 mW (typ)
— Power Down Mode 75 mW (typ)
Applications
nUltrasound and Imaging
nInstrumentation
nCommunications Receivers
nSonar/Radar
nxDSL
nCable Modems
Connection Diagram
20046001
TRI-STATE®is a registered trademark of National Semiconductor Corporation.
December 2005
ADC12D040 Dual 12-Bit, 40 MSPS, 600 mW A/D Converter with Internal/External Reference
© 2005 National Semiconductor Corporation DS200460 www.national.com
Ordering Information
Industrial (−40˚C ≤T
A
≤+85˚C) Package
ADC12D040CIVS 64 Pin TQFP
ADC12D040CIVSX 64 Pin TQFP Tape and Reel
ADC12D040EVAL Evaluation Board
Block Diagram
20046002
ADC12D040
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Pin Descriptions and Equivalent Circuits
Pin No. Symbol Equivalent Circuit Description
ANALOG I/O
15
2
V
IN
A+
V
IN
B+
Non-Inverting analog signal Inputs. With a 2.0V reference the
full-scale input signal level is 2.0 V
P-P
on each pin of the input
pair, centered on a common V
CM
.
16
1
V
IN
A−
V
IN
B−
Inverting analog signal Input. With a 2.0V reference the
full-scale input signal level is 2.0 V
P-P
on each pin of the input
pair, centered on a common V
CM
. These (-) input pins may be
connected to a common V
CM
for single-ended operation, but
a differential input signal is required for best performance.
7V
REF
Reference input. This pin should be bypassed to AGND with
a 0.1 µF monolithic capacitor when external reference is
used. V
REF
is 2.0V nominal and should be between 1.0V to
2.4V.
11 INT/EXT REF
V
REF
select pin. With a logic low at this pin the internal 2.0V
reference is selected. With a logic high on this pin an external
reference voltage must be applied to V
REF
input pin 7.
13
5
V
RP
A
V
RP
B
These pins are high impedance reference bypass pins only.
Connect a 0.1 µF capacitor from each of these pins to AGND.
DO NOT LOAD these pins.
14
4
V
RM
A
V
RM
B
12
6
V
RN
A
V
RN
B
DIGITAL I/O
60 CLK
Digital clock input. The range of frequencies for this input is
100 kHz to 55 MHz (typical) with guaranteed performance at
40 MHz. The input is sampled on the rising edge of this input.
22
41
OEA
OEB
OEA and OEB are the output enable pins that, when low,
enables their respective TRI-STATE®data output pins. When
either of these pins is high, the corresponding outputs are in a
high impedance state.
59 PD
PD is the Power Down input pin. When high, this input puts
the converter into the power down mode. When this pin is
low, the converter is in the active mode.
21 OF
Output Format pin. A logic low on this pin causes output data
to be in offset binary format. A logic high on this pin causes
the output data to be in 2’s complement format.
ADC12D040
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Pin Descriptions and Equivalent Circuits (Continued)
Pin No. Symbol Equivalent Circuit Description
24–29
34–39 DA0–DA11
Digital data output pins that make up the 12-bit conversion
results of their respective converters. DA0 and DB0 are the
LSBs, while DA11 and DB11 are the MSBs of the output
word. Output levels are TTL/CMOS compatible.
42–47
52–57 DB0–DB11
ANALOG POWER
9, 18, 19,
62, 63 V
A
Positive analog supply pins. These pins should be connected
to a quiet +5V source and bypassed to AGND with 0.1 µF
monolithic capacitors located within 1 cm of these power pins,
and with a 10 µF capacitor.
3, 8, 10,
17, 20, 61,
64
AGND The ground return for the analog supply.
DIGITAL POWER
33, 48 V
D
Positive digital supply pin. This pin should be connected to
the same quiet +5V source as is V
A
and be bypassed to
DGND with a 0.1 µF monolithic capacitor located within 1 cm
of the power pin and with a 10 µF capacitor.
32, 49 DGND The ground return for the digital supply.
30, 51 V
DR
Positive digital supply pins for the ADC12D040’s output
drivers. These pins should be connected to a voltage source
of +2.4V to +5V and bypassed to DR GND with a 0.1 µF
monolithic capacitor. If the supply for these pins are different
from the supply used for V
A
and V
D
, they should also be
bypassed with a 10 µF tantalum capacitor. V
DR
should never
exceed the voltage on V
D
. All bypass capacitors should be
located within 1 cm of the supply pin.
23, 31, 40,
50, 58 DR GND
The ground return for the digital supply for the ADC12D040’s
output drivers. These pins should be connected to the system
digital ground, but not be connected in close proximity to the
ADC12D040’s DGND or AGND pins. See Section 5 (Layout
and Grounding) for more details.
ADC12D040
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Absolute Maximum Ratings
(Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
A
,V
D
,V
DR
6.5V
V
DR
V
D
+ 0.3V
|V
A
–V
D
|≤100 mV
Voltage on Any Input or Output Pin −0.3V to (V
A
or V
D
+0.3V)
Input Current at Any Pin (Note 3) ±25 mA
Package Input Current (Note 3) ±50 mA
Package Dissipation at T
A
= 25˚C See (Note 4)
ESD Susceptibility
Human Body Model (Note 5) 2500V
Machine Model (Note 5) 250V
Soldering Temperature,
Infrared, 10 sec. (Note 6) 235˚C
Storage Temperature −65˚C to +150˚C
Operating Ratings (Notes 1, 2)
Operating Temperature −40˚C ≤T
A
≤+85˚C
Supply Voltage (V
A
,V
D
) +4.75V to +5.25V
Output Driver Supply (V
DR
) +2.35V to V
D
V
REF
Input 1.0V to 2.4V
CLK, PD, OE −0.5V to (V
D
+ 0.5V)
Analog Input Pins −0V to (V
A
− 0.5V)
Input Common Mode Voltage
(V
CM
)
V
REF
/2 to V
A
−V
REF
|AGND–DGND| ≤100mV
Package Thermal Resistance
Package θ
J-A
64-Lead TQFP 50˚C / W
Converter Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, V
A
V
D
+5V, V
DR
+3.0V, PD
= 0V, INT/EXT = V
D
,V
REF
= +2.0V, OEA, OEB = 0V, f
CLK
= 40 MHz, t
r
=t
f
= 3 ns, C
L
= 20 pF/pin. Boldface limits apply for
T
J
=T
MIN
to T
MAX
:all other limits T
J
= 25˚C (Notes 7, 8, 9)
Symbol Parameter Conditions Typical
(Note 10)
Limits
(Note 10)
Units
(Limits)
STATIC CONVERTER CHARACTERISTICS
Resolution with No Missing Codes 12 Bits (min)
INL Integral Non Linearity (Note 11) ±0.7 ±2.0 LSB (max)
DNL Differential Non Linearity ±0.4 ±1.0 LSB (max)
GE Gain Error Positive Error 0.51 +2.8/−1.9 %FS
Negative Error 0.68 +4/−2.7 %FS
TC GE Gain Error Tempco External Reference 15 ppm/oC
Internal Reference 100 ppm/oC
V
OFF
Offset Error (V
IN
+=V
IN
−) −0.1 ±1.2 %FS (max)
TC
V
OFF
Offset Error Tempco External Reference 3 ppm/oC
Internal Reference 3 ppm/oC
Under Range Output Code 0 0
Over Range Output Code 4095 4095
DYNAMIC CONVERTER CHARACTERISTICS
FPBW Full Power Bandwidth 0 dBFS Input, Output at −3 dB 100 MHz
SNR Signal-to-Noise Ratio f
IN
= 1 MHz, V
IN
= −0.5 dBFS 69 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS 68 66.5 dB (min)
SINAD Signal-to-Noise and Distortion f
IN
= 1 MHz, V
IN
= −0.5 dBFS 69 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS 68 65.6 dB (min)
ENOB Effective Number of Bits f
IN
= 1 MHz, V
IN
= −0.5 dBFS 11.1 Bits
f
IN
= 10 MHz, V
IN
= −0.5 dBFS 10.9 10.6 Bits (min)
THD Total Harmonic Distortion f
IN
= 1 MHz, V
IN
= −0.5 dBFS −80 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS −78 −69 dB (max)
H2 Second Harmonic f
IN
= 1 MHz, V
IN
= −0.5 dBFS −84 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS −80 −73 dB (max)
ADC12D040
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Converter Electrical Characteristics (Continued)
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, V
A
V
D
+5V, V
DR
+3.0V, PD
= 0V, INT/EXT = V
D
,V
REF
= +2.0V, OEA, OEB = 0V, f
CLK
= 40 MHz, t
r
=t
f
= 3 ns, C
L
= 20 pF/pin. Boldface limits apply for
T
J
=T
MIN
to T
MAX
:all other limits T
J
= 25˚C (Notes 7, 8, 9)
Symbol Parameter Conditions Typical
(Note 10)
Limits
(Note 10)
Units
(Limits)
H3 Third Harmonic f
IN
= 1 MHz, V
IN
= −0.5 dBFS −84 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS −82 −69.5 dB (max)
SFDR Spurious Free Dynamic Range f
IN
= 1 MHz, V
IN
= −0.5 dBFS 84 dB
f
IN
= 10 MHz, V
IN
= −0.5 dBFS 80 69.5 dB (min)
IMD Intermodulation Distortion f
IN
= 9.6 MHz and 10.2 MHz,
each = −6.0 dBFS −80 dBFS
INTER-CHANNEL CHARACTERISTICS
Channel — Channel Offset Match ±0.02 %FS
Channel — Channel Gain Error
Match ±0.05 %FS
Crosstalk 10 MHz Tested Channel. 15 MHz
Other Channel −80 dB
REFERENCE AND ANALOG INPUT CHARACTERISTICS
C
IN
V
IN
Input Capacitance (each pin to
GND)
V
IN
= 2.5 Vdc
+ 0.7 V
rms
(CLK LOW) 8 pF
(CLK HIGH) 7 pF
V
REF
Input Reference Voltage (Note 13) 2.00 1.0 V (min)
2.4 V (max)
R
REF
Reference Input Resistance 100 MΩ(min)
V
IN
Analog Input Voltage Range 0 V (min)
4 V (max)
DC and Logic Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, V
A
=V
D
= +5V, V
DR
=
+3.0V, PD = 0V, INT/EXT = V
D
,V
REF
= +2.0V, OEA, OEB = 0V, f
CLK
= 40 MHz, t
r
=t
f
= 3 ns, C
L
= 20 pF/pin. Boldface limits
apply for T
J
=T
MIN
to T
MAX
:all other limits T
J
= 25˚C (Notes 7, 8, 9)
Symbol Parameter Conditions Typical
(Note 10)
Limits
(Note 10)
Units
(Limits)
CLK, PD, OE DIGITAL INPUT CHARACTERISTICS
V
IN(1)
Logical “1” Input Voltage V
D
= 5.25V 2.0 V (min)
V
IN(0)
Logical “0” Input Voltage V
D
= 4.75V 1.0 V (max)
I
IN(1)
Logical “1” Input Current V
IN
= 5.0V 10 µA
I
IN(0)
Logical “0” Input Current V
IN
= 0V −10 µA
C
IN
Digital Input Capacitance 5 pF
D0–D11 DIGITAL OUTPUT CHARACTERISTICS
V
OUT(1)
Logical “1” Output Voltage I
OUT
= −0.5 mA V
DR
= 2.5V 2.3 V (min)
V
DR
=3V 2.7 V (min)
V
OUT(0)
Logical “0” Output Voltage I
OUT
= 1.6 mA, V
DR
=3V 0.4 V (max)
I
OZ
TRI-STATE Output Current V
OUT
= 2.5V or 5V 100 nA
V
OUT
= 0V −100 nA
+I
SC
Output Short Circuit Source
Current V
OUT
= 0V −20 mA
−I
SC
Output Short Circuit Sink Current V
OUT
=V
DR
20 mA
C
OUT
Digital Output Capacitance 5 pF
ADC12D040
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DC and Logic Electrical Characteristics (Continued)
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, V
A
=V
D
= +5V, V
DR
=
+3.0V, PD = 0V, INT/EXT = V
D
,V
REF
= +2.0V, OEA, OEB = 0V, f
CLK
= 40 MHz, t
r
=t
f
= 3 ns, C
L
= 20 pF/pin. Boldface limits
apply for T
J
=T
MIN
to T
MAX
:all other limits T
J
= 25˚C (Notes 7, 8, 9)
Symbol Parameter Conditions Typical
(Note 10)
Limits
(Note 10)
Units
(Limits)
POWER SUPPLY CHARACTERISTICS
I
A
Analog Supply Current PD Pin = DGND, V
REF
= 2.0V
PD Pin = V
DR
93
15
110 mA (max)
mA
I
D
Digital Supply Current PD Pin = DGND
PD Pin = V
DR
16
0
18 mA (max)
mA
I
DR
Digital Output Supply Current PD Pin = DGND, C
L
= 0 pF (Note 14)
PD Pin = V
DR
10.5
0
12 mA (max)
mA
Total Power Consumption PD Pin = DGND, C
L
= 0 pF (Note 15)
PD Pin = V
DR
600
75
700 mW
mW
PSRR1 Power Supply Rejection Rejection of Full-Scale Error with
V
A
= 4.75V vs. 5.25V 56 dB
AC Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, V
A
=V
D
= +5V, V
DR
=
+3.0V, PD = 0V, INT/EXT = V
D
,V
REF
= +2.0V, OEA, OEB = 0V, f
CLK
= 40 MHz, t
r
=t
f
= 3 ns, C
L
= 20 pF/pin. Boldface limits
apply for T
J
=T
MIN
to T
MAX
:all other limits T
J
= 25˚C (Notes 7, 8, 9, 12)
Symbol Parameter Conditions Typical
(Note 10)
Limits
(Note 10)
Units
(Limits)
f
CLK
1Maximum Clock Frequency 40 MHz (min)
f
CLK
2Minimum Clock Frequency 100 kHz
t
CH
Clock High Time 9 ns
t
CL
Clock Low Time 9 ns
t
CONV
Conversion Latency 6Clock
Cycles
t
OD
Data Output Delay after Rising
CLK Edge V
DR
= 3.0V 10 17.5 ns (max)
t
AD
Aperture Delay 1.2 ns
t
AJ
Aperture Jitter 2 ps rms
t
HOLD
Clock Edge to Data Transition 8 ns
t
DIS
Data outputs into TRI-STATE
Mode 4ns
t
EN
Data Outputs Active after
TRI-STATE 4ns
t
PD
Power Down Mode Exit Cycle 500 ns
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed
specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test
conditions.
Note 2: All voltages are measured with respect to GND = AGND = DGND DR GND = 0V, unless otherwise specified.
Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN <AGND, or VIN >VA), the current at that pin should be limited to 25 mA. The
50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 25 mA to two.
Note 4: The absolute maximum junction temperature (TJmax) for this device is 150˚C. The maximum allowable power dissipation is dictated by TJmax, the
junction-to-ambient thermal resistance (θJA), and the ambient temperature, (TA), and can be calculated using the formula PDMAX=(T
Jmax - TA)/θJA. The values
for maximum power dissipation listed above will be reached only when the device is operated in a severe fault condition (e.g. when input or output pins are driven
beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided.
Note 5: Human body model is 100 pF capacitor discharged through a 1.5 kΩresistor. Machine model is 220 pF discharged through 0Ω.
Note 6: The 235˚C reflow temperature refers to infrared reflow. For Vapor Phase Reflow (VPR), the following Conditions apply: Maintain the temperature at the top
of the package body above 183˚C for a minimum 60 seconds. The temperature measured on the package body must not exceed 220˚C. Only one excursion above
183˚C is allowed per reflow cycle.
Note 7: The inputs are protected as shown below. Input voltage magnitudes above VAor below GND will not damage this device, provided current is limited per
(Note 3). However, errors in the A/D conversion can occur if the input goes above VAor below GND by more than 100 mV. As an example, if VAis 4.75V, the full-scale
input voltage must be ≤4.85V to ensure accurate conversions.
ADC12D040
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AC Electrical Characteristics (Continued)
20046007
Note 8: To guarantee accuracy, it is required that |VA–VD|≤100 mV and separate bypass capacitors are used at each power supply pin.
Note 9: With the test condition for VREF = +2.0V (4VP-P differential input), the 12-bit LSB is 977 µV.
Note 10: Typical figures are at TA=T
J= 25˚C, and represent most likely parametric norms. Test limits are guaranteed to National’s AOQL (Average Outgoing Quality
Level).
Note 11: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive and negative
full-scale.
Note 12: Timing specifications are tested at TTL logic levels, VIL = 0.4V for a falling edge and VIH = 2.4V for a rising edge.
Note 13: Optimum performance will be obtained by keeping the reference input in the 1.8V to 2.4V range. The LM4051CIM3-ADJ (SOT23 package) is
recommended for this application.
Note 14: IDR is the current consumed by the switching of the output drivers and is primarily determined by load capacitance on the output pins, the supply voltage,
VDR, and the rate at which the outputs are switching (which is signal dependent). IDR=VDR(C0xf
0+C
1xf
1+....C11 xf
11) where VDR is the output driver power supply
voltage, Cnis total capacitance on the output pin, and fnis the average frequency at which that pin is toggling.
Note 15: Excludes IDR. See note 14.
ADC12D040
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Specification Definitions
APERTURE DELAY is the time after the rising edge of the
clock to when the input signal is acquired or held for conver-
sion.
APERTURE JITTER (APERTURE UNCERTAINTY) is the
variation in aperture delay from sample to sample. Aperture
jitter manifests itself as noise in the output.
CLOCK DUTY CYCLE is the ratio of the time during one
cycle that a repetitive digital waveform is high to the total
time of one period. The specification here refers to the ADC
clock input signal.
COMMON MODE VOLTAGE (V
CM
)is the d.c. potential
present at both signal inputs to the ADC.
CONVERSION LATENCY See PIPELINE DELAY.
CROSSTALK is coupling of energy from one channel into
the other channel.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of
the maximum deviation from the ideal step size of 1 LSB.
EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE
BITS) is another method of specifying Signal-to-Noise and
Distortion or SINAD. ENOB is defined as (SINAD - 1.76) /
6.02 and says that the converter is equivalent to a perfect
ADC of this (ENOB) number of bits.
FULL POWER BANDWIDTH is a measure of the frequency
at which the reconstructed output fundamental drops 3 dB
below its low frequency value for a full scale input.
GAIN ERROR is the deviation from the ideal slope of the
transfer function. It can be calculated as:
Gain Error = Positive Full Scale Error − Negative Full-
Scale Error
Gain Error can also be separated into Positive Gain Error
and Negative Gain Error, which are.
PGE = Positive Full-Scale Error − Offset Error
NGE = Offset Error − Negative Full-Scale Error
GAIN ERROR MATCHING is the difference in gain errors
between the two converters divided by the average gain of
the converters.
INTEGRAL NON LINEARITY (INL) is a measure of the
deviation of each individual code from a line drawn from
negative full scale (
1
⁄
2
LSB below the first code transition)
through positive full scale (
1
⁄
2
LSB above the last code
transition). The deviation of any given code from this straight
line is measured from the center of that code value.
INTERMODULATION DISTORTION (IMD) is the creation of
additional spectral components as a result of two sinusoidal
frequencies being applied to the ADC input at the same time.
It is defined as the ratio of the power in the intermodulation
products to the total power in the original frequencies. IMD is
usually expressed in dBFS.
LSB (LEAST SIGNIFICANT BIT) is the bit that has the
smallest value or weight of all bits. This value is V
REF
/2
n
,
where “n” is the ADC resolution in bits, which is 12 in the
case of the ADC12D040.
MISSING CODES are those output codes that will never
appear at the ADC outputs. The ADC12D040 is guaranteed
not to have any missing codes.
MSB (MOST SIGNIFICANT BIT) is the bit that has the
largest value or weight. Its value is one half of full scale.
NEGATIVE FULL SCALE ERROR is the difference between
the actual first code transition and its ideal value of
1
⁄
2
LSB
above negative full scale.
OFFSET ERROR is the difference between the two input
voltages (V
IN
+–V
IN
−) required to cause a transition from
code 2047 to 2048.
OUTPUT DELAY is the time delay after the rising edge of
the clock before the data update is presented at the output
pins.
OVER RANGE RECOVERY TIME is the time required after
V
IN
goes from a specified voltage out of the normal input
range to a specified voltage within the normal input range
and the converter makes a conversion with its rated accu-
racy.
PIPELINE DELAY (LATENCY) is the number of clock cycles
between initiation of conversion and when that data is pre-
sented to the output driver stage. Data for any given sample
is available at the output pins the Pipeline Delay plus the
Output Delay after the sample is taken. New data is available
at every clock cycle, but the data lags the conversion by the
pipeline delay.
POSITIVE FULL SCALE ERROR is the difference between
the actual last code transition and its ideal value of 1
1
⁄
2
LSB
below positive full scale.
POWER SUPPLY REJECTION RATIO (PSRR) is a mea-
sure of how well the ADC rejects a change in the power
supply voltage. For the ADC12D040, PSRR1 is the ratio of
the change in Full-Scale Error that results from a change in
the d.c. power supply voltage, expressed in dB. PSRR2 is a
measure of how well an a.c. signal riding upon the power
supply is rejected at the output.
SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in
dB, of the rms value of the input signal to the rms value of the
sum of all other spectral components below one-half the
sampling frequency, not including harmonics or d.c.
SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD)
Is the ratio, expressed in dB, of the rms value of the input
signal to the rms value of all of the other spectral compo-
nents below half the clock frequency, including harmonics
but excluding d.c.
SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-
ence, expressed in dB, between the rms values of the input
signal and the peak spurious signal, where a spurious signal
is any signal present in the output spectrum that is not
present at the input.
TOTAL HARMONIC DISTORTION (THD) is the ratio, ex-
pressed in dB, of the rms total of the first seven harmonic
levels at the output to the level of the fundamental at the
output. THD is calculated as
where f
1
is the RMS power of the fundamental (output)
frequency and f
2
through f
10
are the RMS power of the first
9 harmonic frequencies in the output spectrum.
– Second Harmonic Distortion (2ND HARM) is the differ-
ence expressed in dB, between the RMS power in the input
frequency at the output and the power in its 2nd harmonic
level at the output.
– Third Harmonic Distortion (3RD HARM) is the differ-
ence, expressed in dB, between the RMS power in the input
frequency at the output and the power in its 3rd harmonic
level at the output.
ADC12D040
www.national.com9
Timing Diagram
20046009
Output Timing
Transfer Characteristic
20046010
FIGURE 1. Transfer Characteristic
ADC12D040
www.national.com 10
Typical Performance Characteristics V
A
=V
D
= 5V, V
DR
= 3V, f
CLK
= 40 MHz, f
IN
= 10 MHz unless
otherwise stated
DNL INL
20046036 20046037
INL & DNL vs. Supply Voltage DNL & INL vs. Clock Frequency
20046038 20046044
DNL & INL vs. Clock Duty Cycle DNL & INL vs. Reference Voltage
20046047 20046050
ADC12D040
www.national.com11
Typical Performance Characteristics V
A
=V
D
= 5V, V
DR
= 3V, f
CLK
= 40 MHz, f
IN
= 10 MHz unless
otherwise stated (Continued)
INL & DNL vs. Temperature SNR, SINAD, SFDR vs. Supply Voltage
20046039 20046040
SNR, SINAD, SFDR vs. Clock Frequency SNR, SINAD, SFDR vs. Clock Duty Cycle
20046045 20046048
SNR, SINAD, SFDR vs. Input Frequency SNR, SINAD, SFDR vs. Reference Voltage
20046042 20046051
ADC12D040
www.national.com 12
Typical Performance Characteristics V
A
=V
D
= 5V, V
DR
= 3V, f
CLK
= 40 MHz, f
IN
= 10 MHz unless
otherwise stated (Continued)
SNR, SINAD, SFDR vs. Temperature Distortion vs. Supply Voltage
20046058 20046041
Distortion vs. Clock Frequency Distortion vs. Clock Duty Cycle
20046046 20046049
Distortion vs. Input Frequency Distortion vs. Reference Voltage
20046043 20046052
ADC12D040
www.national.com13
Typical Performance Characteristics V
A
=V
D
= 5V, V
DR
= 3V, f
CLK
= 40 MHz, f
IN
= 10 MHz unless
otherwise stated (Continued)
Distortion vs. Temperature Power Consumption vs. Reference Voltage
20046059 20046053
Power Consumption vs. Temperature
Spectral Response @Fin = 9.95 MHz,
F
CLK
=40MHz
20046054 20046055
IMD Response Fin = 9.6 MHz, 10.2 MHz,
F
CLK
=40MHz
Crosstalk Response Fin = 9.95 MHz,
F
CROSSTALK
= 15 MHz, F
CLK
=40MHz
20046056
20046057
ADC12D040
www.national.com 14
Functional Description
Operating on a single +5V supply, the ADC12D040 uses a
pipeline architecture and has error correction circuitry to help
ensure maximum performance. The differential analog input
signal is digitized to 12 bits and the reference input is buff-
ered to ease the task of driving that pin.
The output word rate is the same as the clock frequency,
which can be between 100 ksps (typical) and 40 Msps with
fully specified performance at 40 Msps. The analog input
voltage for both channels is acquired at the rising edge of the
clock and the digital data for a given sample is delayed by
the pipeline for 6 clock cycles. A choice of Offset Binary or
Two’s Complement output format is selected with the OF pin.
A logic high on the power down (PD) pin reduces the con-
verter power consumption to 75 mW.
Applications Information
1.0 OPERATING CONDITIONS
We recommend that the following conditions be observed for
operation of the ADC12D040:
4.75V ≤V
A
≤5.25V
V
D
=V
A
2.35V ≤V
DR
≤V
D
V
REF
/2 ≤V
CM
≤V
A
-V
REF
100 kHz ≤f
CLK
≤40 MHz
1.0V ≤V
REF
≤2.4V
1.1 Analog Inputs
The ADC12D040 has two analog signal inputs, V
IN
+ and
V
IN
−. These two pins form a differential input pair. There is
one reference input pin, V
REF
.
The analog input circuitry contains an input boost circuit that
provides improved linearity over a wide range of analog input
voltages. To prevent an on-chip over voltage condition that
could impair device reliability, the input signal should never
exceed the voltage described as
V
A
-V
REF
/2.
1.2 Reference Pins
The ADC12D040 is designed to operate with a 2.0V refer-
ence, but performs well with reference voltages in the range
of 1.0V to 2.4V. Lower reference voltages will decrease the
signal-to-noise ratio (SNR) of the ADC12D040. Increasing
the reference voltage (and the input signal swing) beyond
2.4V may degrade THD for a full-scale input especially at
higher input frequencies. It is important that all grounds
associated with the reference voltage and the input signal
make connection to the analog ground plane at a single point
in that plane to minimize the effects of noise currents in the
ground path.
The ADC12040 will perform well with reference voltages up
to 2.4V for full-scale input frequencies up to 10 MHz. How-
ever, more headroom is needed as the input frequency
increases, so the maximum reference voltage (and input
swing) will decrease for higher full-scale input frequencies.
The six Reference Bypass Pins (V
RP
A, V
RM
A, V
RN
A, V
RP
B,
V
RM
B and V
RN
B) are made available for bypass purposes.
These pins should each be bypassed to ground with a 0.1 µF
capacitor. Smaller capacitor values will allow faster recovery
from the power down mode, but may result in degraded
noise performance. DO NOT LOAD these pins. Loading any
of these pins may result in performance degradation.
The nominal voltages for the reference bypass pins are as
follows:
V
RM
A=V
RM
B=V
A
/2
V
RP
A=V
RP
B=V
RM
+V
REF
/2
V
RN
A=V
RN
B=V
RM
−V
REF
/2
The V
RN
pins may be used as a common mode voltage
source (V
CM
) for the analog input pins as long as no d.c.
current is drawn from it. However, because the voltages at
these pins are half that of the V
A
supply pin, using these pins
for a common mode source will result in reduced input
headroom (the difference between the V
A
supply voltage
and the peak signal voltage at either analog input) and the
possibility of reduced THD and SFDR performance. For this
reason, it is recommended that V
A
always exceed V
REF
by at
least 2 Volts. For high input frequencies it may be necessary
to increase this headroom to maintain THD and SFDR per-
formance.
1.3 Signal Inputs
The signal inputs are V
IN
+ and V
IN
−. The input signal, V
IN
,is
defined as
V
IN
=(V
IN
+) – (V
IN
−)
Figure 2 shows the expected input signal range.
Note that the common mode input voltage range is 1V to 3V
with a nominal value of V
A
/2. The input signals should re-
main between ground and 4V.
The Peaks of the individual input signals (V
IN
+ and V
IN
−)
should each never exceed the voltage described as
V
IN
+, V
IN
−=(V
REF
/2+V
CM
)≤4V (differential)
to maintain THD and SINAD performance.
The ADC12D040 performs best with a differential input with
each input centered around a common V
CM
. The peak-to-
peak voltage swing at both V
IN
+ and V
IN
− should not exceed
the value of the reference voltage or the output data will be
clipped.
The two input signals should be exactly 180˚ out of phase
from each other and of the same amplitude. For single
frequency inputs, angular errors result in a reduction of the
effective full scale input. For a complex waveform, however,
angular errors will result in distortion.
For single frequency sine waves with angular errors of less
than 45˚ (π/4) between the two inputs, the full scale error in
LSB can be described as approximately
E
FS
=2
(n-1)
*(1-cos(dev))=2048*(1-cos(dev) )
20046011
FIGURE 2. Expected Input Signal Range
ADC12D040
www.national.com15
Applications Information (Continued)
Where dev is the angular difference between the two signals
having a 180˚ relative phase relationship to each other (see
Figure 3). Drive the analog inputs with a source impedance
less than 100Ω.
TABLE 1. Input to Output Relationship – Differential
Input
V
IN
+V
IN
−Binary Output 2’s Complement
Output
V
CM
−
V
REF
/2
V
CM
+
V
REF
/2 0000 0000 0000 1000 0000 0000
V
CM
−
V
REF/4
V
CM
+
V
REF
/4 0100 0000 0000 1100 0000 0000
V
CM
V
CM
1000 0000 0000 0000 0000 0000
V
CM
+
V
REF
/4
V
CM
−
V
REF
/4 1100 0000 0000 0100 0000 0000
V
CM
+
V
REF
/2
V
CM
−
V
REF
/2 1111 1111 1111 0111 1111 1111
TABLE 2. Input to Output Relationship – Single-Ended
Input
V
IN
+V
IN
− Binary Output 2’s Complement
Output
V
CM
−
V
REF
V
CM
0000 0000 0000 1000 0000 0000
V
CM
−
V
REF
/2 V
CM
0100 0000 0000 1100 0000 0000
V
CM
V
CM
1000 0000 0000 0000 0000 0000
V
CM
+
V
REF
/2 V
CM
1100 0000 0000 0100 0000 0000
V
CM
+
V
REF
V
CM
1111 1111 1111 0111 1111 1111
1.3.1 Single-Ended Operation
Single-ended performance is lower than with differential in-
put signals. For this reason, single-ended operation is not
recommended. However, if single ended-operation is re-
quired and the resulting performance degradation is accept-
able, one of the analog inputs should be connected to the
d.c. mid point voltage of the driven input. The peak-to-peak
differential input signal should be twice the reference voltage
to maximize SNR and SINAD performance (Figure 2b).
For example, set V
REF
to 1.0V, bias V
IN
− to 2.5V and drive
V
IN
+ with a signal range of 1.5V to 3.5V.
Because very large input signal swings can degrade distor-
tion performance, better performance with a single-ended
input can be obtained by reducing the reference voltage
when maintaining a full-range output. Table 1 and Table 2
indicate the input to output relationship of the ADC12D040.
1.3.2 Driving the Analog Input
The V
IN
+ and the V
IN
− inputs of the ADC12D040 consist of
an analog switch followed by a switched-capacitor amplifier.
The capacitance seen at the analog input pins changes with
the clock level, appearing as 8 pF when the clock is low, and
7 pF when the clock is high.
As the internal sampling switch opens and closes, current
pulses occur at the analog input pins, resulting in voltage
spikes at the signal input pins. As a driving amplifier attempts
to counteract these voltage spikes, a damped oscillation
may appear at the ADC analog inputs. The best amplifiers
for driving the ADC12D040 input pins must be able to react
to these spikes and settle before the switch opens and
another sample is taken. The LMH6702 LMH6628 and the
LMH6622, LMH6655 are good amplifiers for driving the
ADC12D040.
To help isolate the pulses at the ADC input from the amplifier
output, use RCs at the inputs, as can be seen in Figure 4
and Figure 5. These components should be placed close to
the ADC inputs because the input pins of the ADC is the
most sensitive part of the system and this is the last oppor-
tunity to filter that input.
For Nyquist applications the RC pole should be at the ADC
sample rate. The ADC input capacitance in the sample mode
should be considered when setting the RC pole. Setting the
pole in this manner will provide best SNR performance.
To obtain best SINAD and ENOB performance, reduce the
RC time constant until SNR and THD are numerically equal
to each other. To obtain best distortion and SFDR perfor-
mance, eliminate the RC altogether.
For undersampling applications, RC pole should be set at
about 1.5 to 2 times the maximum input frequency to main-
tain a linear delay response.
Note that the ADC12DL040 is not designed to operate with
single-ended inputs. However, doing so is possible if the
degraded performance is acceptable. See Section 1.3.1.
Figure 4 shows a narrow band application with a transformer
used to convert single-ended input signals to differential.
Figure 5 shows the use of a fully differential amplifier for
single-ended to differential conversion.
20046012
FIGURE 3. Angular Errors Between the Two Input
Signals Will Reduce the Output Level or Cause
Distortion
ADC12D040
www.national.com 16
Applications Information (Continued)
1.3.3 Input Common Mode Voltage
The input common mode voltage, V
CM
, should be of a value
such that the peak excursions of the analog signal does not
go more negative than ground or more positive than 1.0
Volts below the V
A
supply voltage. The nominal V
CM
should
generally be about V
REF
/2. V
RB
A and V
RB
B can be used as
V
CM
sources as long as no d.c. current is drawn from these
pins.
2.0 DIGITAL INPUTS
Digital TTL/CMOS compatible inputs consist of CLK, OEA,
OEB, OF, INT/EXT REF, and PD.
2.1 CLK
The CLK signal controls the timing of the sampling process.
Drive the clock input with a stable, low jitter clock signal in
the range of 100 kHz to 55 MHz with rise and fall times of
less than 3ns. The trace carrying the clock signal should be
as short as possible and should not cross any other signal
line, analog or digital, not even at 90˚.
If the CLK is interrupted, or its frequency too low, the charge
on internal capacitors can dissipate to the point where the
accuracy of the output data will degrade. This is what limits
the lowest sample
20046013
FIGURE 4. Application Circuit using Transformer or Differential Op-Amp Drive Circuit
20046014
FIGURE 5. Differential Drive Circuit using a fully differential amplifier.
ADC12D040
www.national.com17
Applications Information (Continued)
The ADC clock line should be considered to be a transmis-
sion line and be series terminated at the source end to match
the source impedance with the characteristic impedance of
the clock line. It generally is not necessary to terminate the
far (ADC) end of the clock line, but if a single clock source is
driving more than one device (a condition that is generally
not recommended), far end termination may be needed. Far
end termination is a series RC with the resistor being the
same as the characteristic impedance of the clock line. The
capacitor should have a minimum value of
20046060
where t
PD
is the propagation time in ns/unit length, "L" is the
length of the line and Z
O
is the characteristic impedance of
the line. The units of t
PD
and "L" should be consistent with
each other. The typical board of FR-4 material has a t
PD
of
about 150 ps/inch, or about 60 ps/cm.
The far end termination should be near but beyond the ADC
clock pin as seen from the clock source.
The duty cycle of the clock signal can affect the performance
of any A/D Converter. Because achieving a precise duty
cycle is difficult, the ADC12040 is designed to maintain
performance over a range of duty cycles. While it is specified
and performance is guaranteed with a 50% clock duty cycle,
performance is typically maintained over a clock duty cycle
range of 40% to 60%.
Take care to maintain a constant clock line impedance
throughout the length of the line. Refer to Application Note
AN-905 for information on setting characteristic impedance.
2.2 OEA, OEB
The OEA and OEB pin, when high, put the output pins of
their respective converters into a high impedance state.
When either of these pins is low the corresponding outputs
are in the active state. The ADC12D040 will continue to
convert whether these pins are high or low, but the output
can not be read while the pin is high.
Since ADC noise increases with increased output capaci-
tance at the digital output pins, do not use the TRI-STATE
outputs of the ADC12L066 to drive a bus. Rather, each
output pin should be located close to and drive a single
digital input pin. To further reduce ADC noise, a 100 Ω
resistor in series with each ADC digital output pin, located
close to their respective pins, should be added to the circuit.
2.3 The PD Pin
The PD pin, when high, holds the ADC12D040 in a power-
down mode to conserve power when the converter is not
being used. The power consumption in this state is 75 mW
with a 40 MHz clock and 40mW if the clock is stopped when
PD is high. The output data pins are undefined in the power
down mode and the data in the pipeline is corrupted while in
the power down mode.
The Power Down Mode Exit Cycle time is determined by the
value of the capacitors on pins 4, 5, 6, 12, 13 and 14. These
capacitors loose their charge in the Power Down mode and
must be recharged by on-chip circuitry before conversions
can be accurate. Smaller capacitor values allow faster re-
covery from the power down mode, but can result in a
reduction in SNR, SINAD and ENOB performance.
2.4 The OF Pin
The output data format is offset binary when the OF pin is at
a logic low or 2’s complement when the OF pin is at a logic
high. While the sense of this pin may be changed "on the fly,"
doing this is not recommended as the output data could be
erroneous for a few clock cycles after this change is made.
2.5 The INT/EXT REF Pin
The INT/EXT REF pin determines whether the internal ref-
erence or an external reference voltage is used. With this pin
at a logic low, the internal 2.0V reference is in use. With this
pin at a logic high an external reference must be applied to
the V
REF
pin, which should then be bypassed to ground.
There is no need to bypass the V
REF
pin when the internal
reference is used. There is no access to the internal refer-
ence voltage, but its value is approximately equal to V
RP
−
V
RN
. See Section 1.2
3.0 DATA OUTPUT PINS
The ADC12D040 has 24 TTL/CMOS compatible Data Out-
put pins. Valid data is present at these outputs while the OE
and PD pins are low. While the t
OD
time provides information
about output timing, t
OD
will change with a change of clock
frequency. At the rated 40 MHz clock rate, the data transition
is about 6 to 10 ns after the rise of the clock and about 4 to
10 ns before the fall of the clock (depending upon V
DR
), so
either clock edge may be used to capture data, depending
upon the data setup time of the circuit accepting the data.
Also, circuit board layout will affect relative delays of the
clock and data, so it is important to consider these relative
delays when designing the digital interface. At sample fre-
quencies below 40 MHz, there is a longer time between data
transition and the fall of the clock, so that the falling edge of
the clock is generally the best edge to use for output data
capture at low sample rates.
Be very careful when driving a high capacitance bus. The
more capacitance the output drivers must charge for each
conversion, the more instantaneous digital current flows
through V
DR
and DR GND. These large charging current
spikes can cause on-chip ground noise and couple into the
analog circuitry, degrading dynamic performance. Adequate
bypassing, limiting output capacitance and careful attention
to the ground plane will reduce this problem. Additionally,
bus capacitance beyond the specified 20 pF/pin will cause
t
OD
to increase, making it difficult to properly latch the ADC
output data. The result could be an apparent reduction in
dynamic performance.
To minimize noise due to output switching, minimize the load
currents at the digital outputs. This can be done by connect-
ing buffers (74AC541, for example) between the ADC out-
puts and any other circuitry. Only one driven input should be
connected to each output pin. Additionally, inserting series
resistors of about 100Ωat the digital outputs, close to the
ADC pins, will isolate the outputs from trace and other circuit
capacitances and limit the output currents, which could oth-
erwise result in performance degradation. See Figure 4.
Note that, although the ADC12D040 has Tri-State outputs,
these outputs should not be used to drive a bus and the
charging and discharging of large capacitances can degrade
SNR performance. Each output pin should drive only one pin
of a receiving device and the interconnecting lines should be
as short as practical.
ADC12D040
www.national.com 18
Applications Information (Continued)
4.0 POWER SUPPLY CONSIDERATIONS
The power supply pins should be bypassed with a 10 µF
capacitor and with a 0.1 µF ceramic chip capacitor within a
centimeter of each power pin. Leadless chip capacitors are
preferred because they have low series inductance.
As is the case with all high-speed converters, the
ADC12D040 is sensitive to power supply noise. Accordingly,
the noise on the analog supply pin should be kept below 100
mV
P-P
.
No pin should ever have a voltage on it that is in excess of
the supply voltages, not even on a transient basis. Be espe-
cially careful of this during turn on and turn off of power.
The V
DR
pin provides power for the output drivers and may
be operated from a supply in the range of 2.35V to V
D
(nominal 5V). This can simplify interfacing to low voltage
devices and systems. Note, however, that t
OD
increases with
reduced V
DR
.DO NOT operate the V
DR
pin at a voltage
higher than V
D
.
5.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals are es-
sential to ensure accurate conversion. Maintaining separate
analog and digital areas of the board, with the ADC12D040
between these areas, is required to achieve specified per-
formance.
The ground return for the data outputs (DR GND) carries the
ground current for the output drivers. The output current can
exhibit high transients that could add noise to the conversion
process. To prevent this from happening, the DR GND pins
should NOT be connected to system ground in close prox-
imity to any of the ADC12D040’s other ground pins.
Capacitive coupling between the typically noisy digital cir-
cuitry and the sensitive analog circuitry can lead to poor
performance. The solution is to keep the analog circuitry
separated from the digital circuitry, and to keep the clock line
as short as possible.
The effects of the noise generated from the ADC output
switching can be minimized through the use of 100Ωresis-
tors in series with each data output line. Locate these resis-
tors as close to the ADC output pins as possible.
20046016
FIGURE 6. Example of a Suitable Layout
ADC12D040
www.national.com19
Applications Information (Continued)
Since digital switching transients are composed largely of
high frequency components, total ground plane copper
weight will have little effect upon the logic-generated noise.
This is because of the skin effect. Total surface area is more
important than is total ground plane volume.
Generally, analog and digital lines should cross each other at
90˚ to avoid crosstalk. To maximize accuracy in high speed,
high resolution systems, however, avoid crossing analog and
digital lines altogether. It is important to keep clock lines as
short as possible and isolated from ALL other lines, including
other digital lines. Even the generally accepted 90˚ crossing
should be avoided with the clock line as even a little coupling
can cause problems at high frequencies. This is because
other lines can introduce jitter into the clock line, which can
lead to degradation of SNR. Also, the high speed clock can
introduce noise into the analog chain.
Best performance at high frequencies and at high resolution
is obtained with a straight signal path. That is, the signal path
through all components should form a straight line wherever
possible.
Be especially careful with the layout of inductors. Mutual
inductance can change the characteristics of the circuit in
which they are used. Inductors should not be placed side by
side, even with just a small part of their bodies beside each
other.
The analog input should be isolated from noisy signal traces
to avoid coupling of spurious signals into the input. Any
external component (e.g., a filter capacitor) connected be-
tween the converter’s input pins and ground or to the refer-
ence input pin and ground should be connected to a very
clean point in the analog ground plane.
Figure 6 gives an example of a suitable layout. All analog
circuitry (input amplifiers, filters, reference components, etc.)
should be placed in the analog area of the board. All digital
circuitry and I/O lines should be placed in the digital area of
the board. The ADC12DL040 should be between these two
areas. Furthermore, all components in the reference circuitry
and the input signal chain that are connected to ground
should be connected together with short traces and enter the
analog ground plane at a single, quiet point. All ground
connections should have a low inductance path to ground.
6.0 DYNAMIC PERFORMANCE
To achieve the best dynamic performance, the clock source
driving the CLK input must be free of jitter. Isolate the ADC
clock from any digital circuitry with buffers, as with the clock
tree shown in Figure 7. The gates used in the clock tree must
be capable of operating at frequencies much higher than
those used if added jitter is to be prevented.
Best performance will be obtained with a differential input
drive, compared with a single-ended drive, as discussed in
Sections 1.3.1 and 1.3.2.
As mentioned in Section 5.0, it is good practice to keep the
ADC clock line as short as possible and to keep it well away
from any other signals. Other signals can introduce jitter into
the clock signal, which can lead to reduced SNR perfor-
mance, and the clock can introduce noise into other lines.
Even lines with 90˚ crossings have capacitive coupling, so
try to avoid even these 90˚ crossings of the clock line.
7.0 COMMON APPLICATION PITFALLS
Driving the inputs (analog or digital) beyond the power
supply rails. For proper operation, all inputs should not go
more than 100 mV beyond the supply rails (more than
100 mV below the ground pins or 100 mV above the supply
pins). Exceeding these limits on even a transient basis may
cause faulty or erratic operation. It is not uncommon for high
speed digital components (e.g., 74F devices) to exhibit over-
shoot or undershoot that goes above the power supply or
below ground. A resistor of about 47Ωto 100Ωin series with
any offending digital input, close to the signal source, will
eliminate the problem.
Do not allow input voltages to exceed the supply voltage,
even on a transient basis. Not even during power up or
power down.
Be careful not to overdrive the inputs of the ADC12D040 with
a device that is powered from supplies outside the range of
the ADC12D040 supply. Such practice may lead to conver-
sion inaccuracies and even to device damage.
Attempting to drive a high capacitance digital data bus.
The more capacitance the output drivers must charge for
each conversion, the more instantaneous digital current
flows through V
DR
and DR GND. These large charging cur-
rent spikes can couple into the analog circuitry, degrading
dynamic performance. Adequate bypassing and maintaining
separate analog and digital areas on the pc board will reduce
this problem.
Additionally, bus capacitance beyond the specified 20 pF/pin
will cause t
OD
to increase, making it difficult to properly latch
the ADC output data. The result could, again, be an apparent
reduction in dynamic performance.
The digital data outputs should be buffered (with 74AC541,
for example). Dynamic performance can also be improved
by adding series resistors at each digital output, close to the
ADC12D040, which reduces the energy coupled back into
the converter output pins by limiting the output current. A
reasonable value for these resistors is 100Ω.
Using an inadequate amplifier to drive the analog input.
As explained in Section 1.3, the capacitance seen at the
input alternates between 8 pF and 7 pF, depending upon the
phase of the clock. This dynamic load is more difficult to
drive than is a fixed capacitance.
If the amplifier exhibits overshoot, ringing, or any evidence of
instability, even at a very low level, it will degrade perfor-
mance. A small series resistor at each amplifier output and a
capacitor across the analog inputs (as shown in Figure 5) will
20046017
FIGURE 7. Isolating the ADC Clock from other Circuitry
with a Clock Tree
ADC12D040
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Applications Information (Continued)
improve performance. The LMH6702 and the LMH6628
have been successfully used to drive the analog inputs of the
ADC12D040.
Also, it is important that the signals at the two inputs have
exactly the same amplitude and be exactly 180oout of phase
with each other. Board layout, especially equality of the
length of the two traces to the input pins, will affect the
effective phase between these two signals. Remember that
an operational amplifier operated in the non-inverting con-
figuration will exhibit more time delay than will the same
device operating in the inverting configuration.
Operating with the reference pins outside of the speci-
fied range. As mentioned in Section 1.2, V
REF
should be in
the range of
1.0V ≤V
REF
≤2.4V
Operating outside of these limits could lead to performance
degradation.
Using a clock source with excessive jitter, using exces-
sively long clock signal trace, or having other signals
coupled to the clock signal trace. This will cause the
sampling interval to vary, causing excessive output noise
and a reduction in SNR and SINAD performance.
ADC12D040
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Physical Dimensions inches (millimeters) unless otherwise noted
64-Lead TQFP Package
Ordering Number ADC12D040CIVS
NS Package Number VECO64A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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ADC12D040 Dual 12-Bit, 40 MSPS, 600 mW A/D Converter with Internal/External Reference
