AD AD9240

a
FEATURES
Monolithic 14-Bit, 10 MSPS A/D Converter
Low Power Dissipation: 285 mW
Single +5 V Supply
Integral Nonlinearity Error: 2.5 LSB
Differential Nonlinearity Error: 0.6 LSB
Input Referred Noise: 0.36 LSB
Complete: On-Chip Sample-and-Hold Amplifier and
Voltage Reference
Signal-to-Noise and Distortion Ratio: 77.5 dB
Spurious-Free Dynamic Range: 90 dB
Out-of-Range Indicator
Straight Binary Output Data
44-Lead MQFP
Complete 14-Bit, 10 MSPS
Monolithic A/D Converter
AD9240
FUNCTIONAL BLOCK DIAGRAM
AVDD
CLK
DRVDD
DVDD
SHA
VINA
MDAC1
GAIN = 16
VINB
CML
MDAC2
GAIN = 8
MDAC3
GAIN = 8
4
4
5
A/D
CAPT
A/D
5
A/D
4
BIAS
A/D
4
4
DIGITAL CORRECTION LOGIC
14
CAPB
VREF
OTR
OUTPUT BUFFERS
SENSE
MODE
SELECT
BIT 1
(MSB)
1V
AD9240
REFCOM
AVSS
BIT 14
(LSB)
DVSS
DRVSS
PRODUCT DESCRIPTION
PRODUCT HIGHLIGHTS
The AD9240 is a 10 MSPS, single supply, 14-bit analog-todigital converter (ADC). It combines a low cost, high speed
CMOS process and a novel architecture to achieve the resolution
and speed of existing hybrid implementations at a fraction of the
power consumption and cost. It is a complete, monolithic ADC
with an on-chip, high performance, low noise sample-and-hold
amplifier and programmable voltage reference. An external reference can also be chosen to suit the dc accuracy and temperature
drift requirements of the application. The device uses a multistage
differential pipelined architecture with digital output error correction logic to guarantee no missing codes over the full operating
temperature range.
The AD9240 offers a complete single-chip sampling 14-bit,
analog-to-digital conversion function in a 44-lead Metric Quad
Flatpack.
Low Power and Single Supply
The AD9240 consumes only 280 mW on a single +5 V power
supply.
Excellent DC Performance Over Temperature
The AD9240 provides no missing codes, and excellent temperature drift performance over the full operating temperature range.
Excellent AC Performance and Low Noise
The AD9240 provides nearly 13 ENOB performance and has an
input referred noise of 0.36 LSB rms.
Flexible Analog Input Range
The versatile onboard sample-and-hold (SHA) can be configured
for either single ended or differential inputs of varying input spans.
Flexible Digital Outputs
The digital outputs can be configured to interface with +3 V and
+5 V CMOS logic families.
The input of the AD9240 is highly flexible, allowing for easy
interfacing to imaging, communications, medical and dataacquisition systems. A truly differential input structure allows
for both single-ended and differential input interfaces of varying
input spans. The sample-and-hold amplifier (SHA) is equally
suited for multiplexed systems that switch full-scale voltage
levels in successive channels as well as sampling single-channel
inputs at frequencies up to and beyond the Nyquist rate. The
AD9240 also performs well in communication systems employing Direct-IF Down Conversion, since the SHA in the differential input mode can achieve excellent dynamic performance well
beyond its specified Nyquist frequency of 5 MHz.
A single clock input is used to control all internal conversion
cycles. The digital output data is presented in straight binary
output format. An out-of-range (OTR) signal indicates an
overflow condition which can be used with the most significant
bit to determine low or high overflow.
Excellent Undersampling Performance
The full power bandwidth and dynamic range of the AD9240
make it well suited for Direct-IF Down Conversion extending to
45 MHz.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998
AD9240–SPECIFICATIONS
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE = 10 MSPS, RBIAS = 2 k⍀, VREF = 2.5 V, VINB = 2.5 V,
MIN to TMAX unless otherwise noted)
DC SPECIFICATIONS T
Parameter
AD9240
Units
RESOLUTION
14
Bits min
MAX CONVERSION RATE
10
MHz min
INPUT REFERRED NOISE
VREF = 1 V
VREF = 2.5 V
0.9
0.36
LSB rms typ
LSB rms typ
INL1
DNL1
No Missing Codes
Zero Error (@ +25°C)
Gain Error (@ +25°C)2
Gain Error (@ +25°C)3
± 2.5
± 0.6
± 1.0
± 2.5
± 0.7
14
0.3
1.5
0.75
LSB typ
LSB typ
LSB max
LSB typ
LSB typ
Bits Guaranteed
% FSR max
% FSR max
% FSR max
TEMPERATURE DRIFT
Zero Error
Gain Error2
Gain Error3
3.0
20.0
5.0
ppm/°C typ
ppm/°C typ
ppm/°C typ
POWER SUPPLY REJECTION
0.1
% FSR max
2
5
0
AVDD
16
V p-p min
V p-p max
V min
V max
pF typ
INTERNAL VOLTAGE REFERENCE
Output Voltage (1 V Mode)
Output Voltage Tolerance (1 V Mode)
Output Voltage (2.5 V Mode)
Output Voltage Tolerance (2.5 V Mode)
Load Regulation4
1
± 14
2.5
± 35
5.0
Volts typ
mV max
Volts typ
mV max
mV max
REFERENCE INPUT RESISTANCE
5
kΩ typ
ACCURACY
Integral Nonlinearity (INL)
Differential Nonlinearity (DNL)
ANALOG INPUT
Input Span (with VREF = 1.0 V)
Input Span (with VREF = 2.5 V)
Input (VINA or VINB) Range
Input Capacitance
POWER SUPPLIES
Supply Voltages
AVDD
DVDD
DRVDD
Supply Current
IAVDD
IDRVDD
IDVDD
+5
+5
+5
V (± 5% AVDD Operating)
V (± 5% DVDD Operating)
V (± 5% DRVDD Operating)
50
1
15
mA max (46 mA typ)
mA max (0.1 mA typ)
mA max (11 mA typ)
POWER CONSUMPTION
330
mW max (285 mW typ)
NOTES
1
VREF = 1 V.
2
Including internal reference.
3
Excluding internal reference.
4
Load regulation with 1 mA load current (in addition to that required by the AD9240).
Specification subject to change without notice.
–2–
REV. A
AD9240
(AVDD = +5 V, DVDD= +5 V, DRVDD = +5 V, fSAMPLE = 10 MSPS, RBIAS = 2 k⍀, VREF = 2.5 V, AIN = –0.5 dBFS,
MIN to TMAX unless otherwise noted)
AC SPECIFICATIONS AC Coupled/Differential Input, T
Parameter
SIGNAL-TO-NOISE AND DISTORTION RATIO (S/N+D)
fINPUT = 500 kHz
fINPUT = 1.0 MHz
fINPUT = 5.0 MHz
EFFECTIVE NUMBER OF BITS (ENOB)
fINPUT = 500 kHz
fINPUT = 1.0 MHz
fINPUT = 5.0 MHz
SIGNAL-TO-NOISE RATIO (SNR)
fINPUT = 500 kHz
fINPUT = 1.0 MHz
fINPUT = 5.0 MHz
TOTAL HARMONIC DISTORTION (THD)
fINPUT = 500 kHz
fINPUT = 1.0 MHz
fINPUT = 5.0 MHz
SPURIOUS FREE DYNAMIC RANGE
fINPUT = 500 kHz
fINPUT = 1.0 MHz
fINPUT = 5.0 MHz
DYNAMIC PERFORMANCE
Full Power Bandwidth
Small Signal Bandwidth
Aperture Delay
Aperture Jitter
Acquisition to Full-Scale Step (0.0025%)
Overvoltage Recovery Time
AD9240
Units
75.0
77.5
77.5
75.0
dB min
dB typ
dB typ
dB typ
12.2
12.6
12.6
12.2
Bits min
Bits typ
Bits typ
Bits typ
76.0
78.5
78.5
78.5
dB min
dB typ
dB typ
dB typ
–78.0
–85.0
–85.0
–77.0
dB max
dB typ
dB typ
dB typ
90.0
90.0
80.0
dB typ
dB typ
dB typ
70
70
1
4
45
167
MHz typ
MHz typ
ns typ
ps rms typ
ns typ
ns typ
Specifications subject to change without notice.
DIGITAL SPECIFICATIONS
(AVDD = +5 V, DVDD = +5 V, TMIN to TMAX unless otherwise noted)
Parameters
Symbol
AD9240
Units
CLOCK INPUT
High Level Input Voltage
Low Level Input Voltage
High Level Input Current (VIN = DVDD)
Low Level Input Current (VIN = 0 V)
Input Capacitance
VIH
VIL
IIH
IIL
CIN
+3.5
+1.0
± 10
± 10
5
V min
V max
µA max
µA max
pF typ
LOGIC OUTPUTS (with DRVDD = 5 V)
High Level Output Voltage (IOH = 50 µA)
High Level Output Voltage (IOH = 0.5 mA)
Low Level Output Voltage (IOL = 1.6 mA)
Low Level Output Voltage (IOL = 50 µA)
Output Capacitance
VOH
VOH
VOL
VOL
COUT
+4.5
+2.4
+0.4
+0.1
5
V min
V min
V max
V max
pF typ
LOGIC OUTPUTS (with DRVDD = 3 V)
High Level Output Voltage (IOH = 50 µA)
Low Level Output Voltage (IOL = 50 µA)
VOH
VOL
+2.4
+0.7
V min
V max
Specifications subject to change without notice.
REV. A
–3–
AD9240
SWITCHING SPECIFICATIONS (T
MIN to TMAX
Parameters
Symbol
AD9240
Units
tC
tCH
tCL
tOD
100
45
45
8
13
19
3
ns min
ns min
ns min
ns min
ns typ
ns max
Clock Cycles
1
Clock Period
CLOCK Pulsewidth High
CLOCK Pulsewidth Low
Output Delay
with AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, RBIAS = 2 k⍀, CL = 20 pF)
Pipeline Delay (Latency)
NOTES
1
The clock period may be extended to 1 ms without degradation in specified performance @ +25 °C.
Specifications subject to change without notice.
THERMAL CHARACTERISTICS
S1
tCH
Thermal Resistance
44-Lead MQFP
θJA = 53.2°C/W
θJC = 19°C/W
S4
tC
tCL
S3
INPUT
CLOCK
tOD
DATA
OUTPUT
ORDERING GUIDE
DATA 1
Model
Figure 1. Timing Diagram
AD9240AS
AD9240EB
ABSOLUTE MAXIMUM RATINGS*
–40oC to +85oC
44-Lead MQFP
S-44
Evaluation Board
Package
Option*
*S = Metric Quad Flatpack.
DVSS 1
AVSS 2
NC
CAPB
BIAS
CAPT
CML
NC
33 REFCOM
PIN 1
IDENTIFIER
32 VREF
DVDD 3
31 SENSE
AVDD 4
30 NC
DRVSS 5
AD9240
29 AVSS
DRVDD 6
TOP VIEW
(Not to Scale)
28 AVDD
CLK 7
27 NC
NC 8
26 NC
NC 9
25 OTR
NC 10
24 BIT 1 (MSB)
(LSB) BIT 14 11
°C
23 BIT 2
BIT 3
BIT 13
BIT 4
12 13 14 15 16 17 18 19 20 21 22
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
sections of this specification is not implied. Exposure to absolute maximum ratings
for extended periods may effect device reliability.
BIT 6
+300
44 43 42 41 40 39 38 37 36 35 34
BIT 5
–65
PIN CONFIGURATION
BIT 7
V
V
V
V
V
V
V
V
V
V
V
V
V
V
°C
°C
NC
+6.5
+6.5
+0.3
+6.5
+6.5
+0.3
+0.3
AVDD + 0.3
DRVDD + 0.3
AVDD + 0.3
AVDD + 0.3
AVDD + 0.3
AVDD + 0.3
AVDD + 0.3
+150
+150
BIT 8
–0.3
–0.3
–0.3
–6.5
–0.3
–0.3
–0.3
–0.3
–0.3
–0.3
–0.3
–0.3
–0.3
–0.3
VINA
AVSS
DVSS
DVSS
DVDD
DRVSS
AVSS
AVSS
AVSS
DRVSS
AVSS
AVSS
AVSS
AVSS
AVSS
BIT 9
Units
VINB
Max
BIT 11
Min
BIT 10
With
Respect
to
NC
AVDD
DVDD
AVSS
AVDD
DRVDD
DRVSS
REFCOM
CLK
Digital Outputs
VINA, VINB
VREF
SENSE
CAPB, CAPT
BIAS
Junction Temperature
Storage Temperature
Lead Temperature
(10 sec)
Package
Description
NC
Parameter
Temperature
Range
BIT 12
ANALOG
INPUT
S2
NC = NO CONNECT
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD9240 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. A
AD9240
PIN FUNCTION DESCRIPTIONS
Pin
Number
1
2, 29
3
4, 28
5
6
7
8–10
11
12–23
24
25
26, 27, 30
31
32
33
34, 38, 40,
43, 44
35
36
37
39
41
42
Name
Description
DVSS
AVSS
DVDD
AVDD
DRVSS
DRVDD
CLK
NC
BIT 14
BIT 13–BIT 2
BIT 1
OTR
NC
SENSE
VREF
REFCOM
Digital Ground
Analog Ground
+5 V Digital Supply
+5 V Analog Supply
Digital Output Driver Ground
Digital Output Driver Supply
Clock Input Pin
No Connect
Least Significant Data Bit (LSB)
Data Output Bits
Most Significant Data Bit (MSB)
Out of Range
No Connect
Reference Select
Reference I/O
Reference Common
NC
BIAS*
CAPB
CAPT
CML
VINA
VINB
No Connect
Power/Speed Programming
Noise Reduction Pin
Noise Reduction Pin
Common-Mode Level (Midsupply)
Analog Input Pin (+)
Analog Input Pin (–)
*See Speed/Power Programmability section.
OVERVOLTAGE RECOVERY TIME
Overvoltage recovery time is defined as that amount of time
required for the ADC to achieve a specified accuracy after an
overvoltage (50% greater than full-scale range), measured from
the time the overvoltage signal reenters the converter’s range.
TEMPERATURE DRIFT
The temperature drift for zero error and gain error specifies the
maximum change from the initial (+25°C) value to the value at
TMIN or TMAX.
POWER SUPPLY REJECTION
The specification shows the maximum change in full scale from
the value with the supply at the minimum limit to the value
with the supply at its maximum limit.
APERTURE JITTER
Aperture jitter is the variation in aperture delay for successive
samples and is manifested as noise on the input to the A/D.
APERTURE DELAY
Aperture delay is a measure of the sample-and-hold amplifier
(SHA) performance and is measured from the rising edge of the
clock input to when the input signal is held for conversion.
SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD)
RATIO
S/N+D is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc.
The value for S/N+D is expressed in decibels.
EFFECTIVE NUMBER OF BITS (ENOB)
DEFINITIONS OF SPECIFICATION
INTEGRAL NONLINEARITY (INL)
INL refers to the deviation of each individual code from a line
drawn from “negative full scale” through “positive full scale.”
The point used as “negative full scale” occurs 1/2 LSB before
the first code transition. “Positive full scale” is defined as a
level 1 1/2 LSB beyond the last code transition. The deviation
is measured from the middle of each particular code to the true
straight line.
DIFFERENTIAL NONLINEARITY (DNL, NO MISSING
CODES)
An ideal ADC exhibits code transitions that are exactly 1 LSB
apart. DNL is the deviation from this ideal value. Guaranteed
no missing codes to 14-bit resolution indicates that all 16384
codes, respectively, must be present over all operating ranges.
ZERO ERROR
The major carry transition should occur for an analog value
1/2 LSB below VINA = VINB. Zero error is defined as the
deviation of the actual transition from that point.
GAIN ERROR
The first code transition should occur at an analog value 1/2 LSB
above negative full scale. The last transition should occur at an
analog value 1 1/2 LSB below the nominal full scale. Gain error
is the deviation of the actual difference between first and last
code transitions and the ideal difference between first and last
code transitions.
REV. A
For a sine wave, SINAD can be expressed in terms of the number of bits. Using the following formula,
N = (SINAD – 1.76)/6.02
it is possible to get a measure of performance expressed as N,
the effective number of bits.
Thus, an effective number of bits for a device for sine wave
inputs at a given input frequency can be calculated directly
from its measured SINAD.
TOTAL HARMONIC DISTORTION (THD)
THD is the ratio of the rms sum of the first six harmonic
components to the rms value of the measured input signal and
is expressed as a percentage or in decibels.
SIGNAL-TO-NOISE RATIO (SNR)
SNR is the ratio of the rms value of the measured input signal
to the rms sum of all other spectral components below the
Nyquist frequency, excluding the first six harmonics and dc.
The value for SNR is expressed in decibels.
SPURIOUS FREE DYNAMIC RANGE (SFDR)
SFDR is the difference in dB between the rms amplitude of the
input signal and the peak spurious signal.
TWO-TONE SFDR
The ratio of the rms value of either input tone to the rms value
of the peak spurious component. The peak spurious component
may or may not be an IMD product. Two-tone SFDR may be
reported in dBc (i.e., degrades as signal level is lowered), or in
dBFS (always related back to converter full scale).
–5–
AD9240
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE =
BIAS = 2 k⍀, TA = +25ⴗC, Differential Input)
Typical Differential AC Characterization Curves/Plots 10 MSPS, R
90
0
–40
85
–50
–20
–0.5dBFS
–6.0dBFS
70
65
AMPLITUDE – dB
75
–30
–60
THD – dB
SINAD – dB
80
–20.0dBFS
–20.0dBFS
–70
–6.0dBFS
–80
60
–0.5dBFS
–90
50
0.1
10
1
INPUT FREQUENCY – MHz
20
Figure 2. SINAD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
–50
–60
–70
0.1
10
1
INPUT FREQUENCY – MHz
20
Figure 3. THD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
–110
–120
9th
0
6th
5.0
FREQUENCY – MHz
1
–30
70
–6.0dBFS
AMPLITUDE – dB
–0.5dBFS
–60
THD – dB
75
65
–20.0dBFS
–70
–6.0dBFS
–80
–20.0dBFS
20
Figure 5. SINAD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
–100
0.1
–65
100
SFDR – dBc AND dBFS
110
5V SPAN
2V SPAN
–85
5V SPAN – dBc
2V SPAN – dBc
50
–95
30
Figure 8. THD vs. Sample Rate
(fIN = 5.0 MHz, AIN = –0.5 dBFS,
VCM = 2.5 V)
2V SPAN – dBFS
60
40
10
5V SPAN – dBFS
80
–90
1
SAMPLE RATE – MHz
20
90
70
20
–60
–75
2
–90
4
6
9 7
8
5
3
–135
1
10
INPUT FREQUENCY – MHz
Figure 6. THD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
–60
–75
–0.5dBFS
–50
–40
–30
–20
AIN – dB
–10
Figure 9. Single Tone SFDR
(fIN = 5.0 MHz, VCM = 2.5 V)
–6–
–150
0
0
5.0
FREQUENCY – MHz
Figure 7. Typical FFT, fIN = 5.0 MHz
(Input Span = 2 V, VCM = 2.5 V)
WORST CASE SPURIOUS – dBc AND dBFS
1
10
INPUT FREQUENCY – MHz
–60
–120
–90
55
–45
–105
60
–100
0.1
7th
–15
–50
–80
8th
0
–40
80
–70
5th
4th
Figure 4. Typical FFT, fIN = 1.0 MHz
(Input Span = 5 V, VCM = 2.5 V)
85
50
0.1
3rd
2nd
–80
–100
–100
90
SINAD – dB
–40
–90
55
THD – dB
1st
–10
110
105
5V SPAN – dBFS
100
95
2V SPAN – dBFS
90
85
80
75 5V SPAN – dBc
70
2V SPAN – dBc
65
60
–40 –35 –30 –25 –20 –15 –10
–5
0
INPUT POWER LEVEL (f1 = f2) – dBFS
Figure 10. Dual Tone SFDR
(f1 = 0.95 MHz, f2 = 1.04 MHz,
VCM = 2.5 V)
REV. A
AD9240
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE = 10 MSPS, RBIAS = 2 k⍀,
Other Characterization Curves/Plots T = +25ⴗC, Single-Ended Input)
A
3.0
1.0
2.5
0.8
2.0
13484335
0.6
1.5
0.4
0.5
0.0
–0.5
–1.0
0.0
–0.2
–0.4
–1.5
1414263
–0.6
–2.0
1482053
–0.8
–2.5
–3.0
0.2
HITS
DNL – LSB
INL – LSB
1.0
–1.0
0
CODE
0
16863
CODE
Figure 11. Typical INL
(Input Span = 5 V)
N–1
16383
–40
0
85
–10
–50
80
N+1
Figure 13. “Grounded-Input”
Histogram (Input Span = 5 V)
Figure 12. Typical DNL
(Input Span = 5 V)
90
N
CODE
–60
70
65
THD – dB
SINAD – dB
–0.5dBFS
–6.0dBFS
60
55
–20.0dBFS
–70
–80
–20.0dBFS
–90
45
1
10
INPUT FREQUENCY – MHz
–6.0dBFS
–30
–40
–50
–60
50
40
0.1
AMPLITUDE – dB
–20
75
20
Figure 14. SINAD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
–0.5dBFS
–100
0.1
–70
1
10
INPUT FREQUENCY – MHz
Figure 15. THD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
90
–80
20
1
10
FREQUENCY – MHz
100
Figure 16. CMR vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
0.01
–40
0.008
85
–50
0.006
–60
THD – dB
SINAD – dB
–0.5dBFS
75
–6.0dBFS
70
65
VREF ERROR – V
80
–0.5dBFS
–20dBFS
–70
–80
60
–20.0dBFS
–6.0dBFS
0
–0.002
–0.004
–0.008
10
1
INPUT FREQUENCY – MHz
20
Figure 17. SINAD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
REV. A
0.002
–0.006
–90
55
50
0.1
0.004
–100
0.1
1
10
INPUT FREQUENCY – MHz
20
Figure 18. THD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
–7–
–0.01
–60 –40 –20 0 20 40 60 80 100 120 140
TEMPERATURE – 8C
Figure 19. Typical Voltage Reference
Error vs. Temperature
AD9240
INTRODUCTION
80
The AD9240 uses a four-stage pipeline architecture with a
wideband input sample-and-hold amplifier (SHA) implemented
on a cost-effective CMOS process. Each stage of the pipeline,
excluding the last, consists of a low resolution flash A/D connected to a switched capacitor DAC and interstage residue
amplifier (MDAC). The residue amplifier amplifies the difference between the reconstructed DAC output and the flash input
for the next stage in the pipeline. One bit of redundancy is used
in each of the stages to facilitate digital correction of flash errors. The last stage simply consists of a flash A/D.
RBIAS =
4kV
SINAD – dB
60
50
RBIAS = 10kV
40
RBIAS = 20kV
30
20
RBIAS = 200kV
10
The pipeline architecture allows a greater throughput rate at the
expense of pipeline delay or latency. This means that while the
converter is capable of capturing a new input sample every clock
cycle, it actually takes three clock cycles for the conversion to be
fully processed and appear at the output. This latency is not a
concern in most applications. The digital output, together with
the out-of-range indicator (OTR), is latched into an output
buffer to drive the output pins. The output drivers can be configured to interface with +5 V or +3.3 V logic families.
0
1
10
20
CLOCK FREQUENCY – MHz
Figure 21. SINAD vs. Clock Frequency for Varying RBIAS
Values (VCM = 2.5 V, AIN = –0.5 dB, 5 V Span, fIN = fCLK/2)
400
350
POWER – mW
The AD9240 uses both edges of the clock in its internal timing
circuitry (see Figure 1 and specification page for exact timing
requirements). The A/D samples the analog input on the rising
edge of the clock input. During the clock low time (between the
falling edge and rising edge of the clock), the input SHA is in
the sample mode; during the clock high time it is in the hold
mode. System disturbances just prior to the rising edge of the
clock and/or excessive clock jitter may cause the input SHA to
acquire the wrong value, and should be minimized.
300
R BIAS
R BIAS
250
150
100
2
The AD9240’s maximum conversion rate and associated power
dissipation can be set using the part’s BIAS pin. A simplified
diagram of the on-chip circuitry associated with the BIAS pin is
shown in Figure 20.
= 1.7kV
= 2kV
= 2.5kV
R BIAS
= 3.3kV
R BIAS
= 5kV
R BIAS
10kV
=
R BIAS
V
= 100k
R BIAS
200
Speed/Power Programmability
4
6
8
10
12
14
16
CLOCK FREQUENCY – MHz
18
20
Figure 22. Power Dissipation vs. Clock Frequency for
Varying RBIAS Values
ANALOG INPUT AND REFERENCE OVERVIEW
ADCBIAS
Figure 23, a simplified model of the AD9240, highlights the relationship between the analog inputs, VINA, VINB, and the reference voltage, VREF. Like the voltage applied to the top of
the resistor ladder in a flash A/D converter, the value VREF defines
the maximum input voltage to the A/D core. The minimum input
voltage to the A/D core is automatically defined to be –VREF.
BIAS
I FIXED
RBIAS =
2kV
70
RBIAS
AD9240
Figure 20.
The value of RBIAS can be varied over a limited range to set the
maximum sample rate and power dissipation of the AD9240. A
typical plot of S/(N+D) @ fIN = Nyquist vs. fCLK at varying
RBIAS is shown in Figure 21. A similar plot of power vs. fCLK
at varying RBIAS is shown in Figure 22. These plots indicate
typical performance vs. RBIAS. Note that all other plots and
specifications in this data sheet reflect performance at a fixed
RBIAS = 2 kΩ.
VINA
AD9240
VCORE
VINB
+VREF
A/D
CORE
14
–VREF
Figure 23. Equivalent Functional Input Circuit
–8–
REV. A
AD9240
The addition of a differential input structure gives the user an
additional level of flexibility that is not possible with traditional
flash converters. The input stage allows the user to easily configure the inputs for either single-ended operation or differential
operation. The A/D’s input structure allows the dc offset of the
input signal to be varied independently of the input span of the
converter. Specifically, the input to the A/D core is the difference of the voltages applied at the VINA and VINB input pins.
Therefore, the equation,
VCORE = VINA – VINB
The input SHA of the AD9240 is optimized to meet the performance requirements for some of the most demanding communication, imaging, and data acquisition applications while
maintaining low power dissipation. Figure 25 is a graph of the
full-power bandwidth of the AD9240, typically 60 MHz. Note
that the small signal bandwidth is the same as the full-power
bandwidth. The settling time response to a full-scale stepped
input is shown in Figure 26 and is typically less than 40 ns to
0.0025%. The low input referred noise of 0.36 LSB’s rms is
displayed via a grounded histogram and is shown in Figure 13.
(1)
1
defines the output of the differential input stage and provides
the input to the A/D core.
0
–1
The voltage, VCORE , must satisfy the condition,
–2
AMPLITUDE – dB
–VREF ≤ VCORE ≤ VREF
(2)
where VREF is the voltage at the VREF pin.
While an infinite combination of VINA and VINB inputs exist
that satisfy Equation 2, there is an additional limitation placed
on the inputs by the power supply voltages of the AD9240. The
power supplies bound the valid operating range for VINA and
VINB. The condition,
–3
–4
–5
–6
–7
–8
–9
–10
AVSS – 0.3 V < VINA < AVDD + 0.3 V
1
(3)
10
FREQUENCY – MHz
100
Figure 25. Full-Power Bandwidth
AVSS – 0.3 V < VINB < AVDD + 0.3 V
16000
where AVSS is nominally 0 V and AVDD is nominally +5 V,
defines this requirement. Thus, the range of valid inputs for
VINA and VINB is any combination that satisfies both Equations 2 and 3.
12000
CODE
For additional information showing the relationship between
VINA, VINB, VREF and the digital output of the AD9240, see
Table IV.
Refer to Table I and Table II for a summary of the various
analog input and reference configurations.
8000
4000
ANALOG INPUT OPERATION
Figure 24 shows the equivalent analog input of the AD9240
which consists of a differential sample-and-hold amplifier (SHA).
The differential input structure of the SHA is highly flexible,
allowing the devices to be easily configured for either a differential or single-ended input. The dc offset, or common-mode
voltage, of the input(s) can be set to accommodate either singlesupply or dual supply systems. Note also that the analog inputs,
VINA and VINB, are interchangeable with the exception that
reversing the inputs to the VINA and VINB pins results in a
polarity inversion.
CH
VINA
VINB
CPIN+
CPAR
QS2
QS1
CS
QS1
QH1 CS
CPIN–
CPAR
QS2
0
0
20
30
40
50
SETTLING TIME – ns
60
70
80
Figure 26. Settling Time
The SHA’s optimum distortion performance for a differential or
single-ended input is achieved under the following two conditions: (1) the common-mode voltage is centered around midsupply (i.e., AVDD/2 or approximately 2.5 V) and (2) the input
signal voltage span of the SHA is set at its lowest (i.e., 2 V input
span). This is due to the sampling switches, QS1, being CMOS
switches whose RON resistance is very low but has some signal
dependency which causes frequency dependent ac distortion
while the SHA is in the track mode. The RON resistance of a
CMOS switch is typically lowest at its midsupply but increases
symmetrically as the input signal approaches either AVDD or
AVSS. A lower input signal voltage span centered at midsupply
reduces the degree of RON modulation.
CH
Figure 24. Simplified Input Circuit
REV. A
10
–9–
AD9240
VCC
Figure 27 compares the AD9240’s THD vs. frequency performance for a 2 V input span with a common-mode voltage of
1 V and 2.5 V. Note the difference in the amount of degradation in THD performance as the input frequency increases.
Similarly, note how the THD performance at lower frequencies
becomes less sensitive to the common-mode voltage. As the
input frequency approaches dc, the distortion will be dominated by static nonlinearities such as INL and DNL. It is
important to note that these dc static nonlinearities are independent of any RON modulation.
VINA
RS*
VINB
VEE
VREF
10mF
REFCOM
*OPTIONAL SERIES RESISTOR
Figure 28. Series Resistor Isolates Switched-Capacitor
SHA Input from Op Amp. Matching Resistors Improve
SNR Performance
The optimum size of this resistor is dependent on several factors, which include the AD9240 sampling rate, the selected op
amp and the particular application. In most applications, a
30 Ω to 50 Ω resistor is sufficient; however, some applications
may require a larger resistor value to reduce the noise bandwidth or possibly limit the fault current in an overvoltage
condition. Other applications may require a larger resistor value
as part of an antialiasing filter. In any case, since the THD
performance is dependent on the series resistance and the above
mentioned factors, optimizing this resistor value for a given
application is encouraged.
–60
THD – dB
0.1mF
SENSE
–50
–70
VCM = 1.0V
–80
VCM = 2.5V
–90
0.1
AD9240
RS*
1
FREQUENCY – MHz
10
20
Figure 27. THD vs. Frequency for VCM = 2.5 V and 1.0 V
(AIN = –0.5 dB, Input Span = 2.0 V p-p)
Due to the high degree of symmetry within the SHA topology, a
significant improvement in distortion performance for differential input signals with frequencies up to and beyond Nyquist can
be realized. This inherent symmetry provides excellent cancellation of both common-mode distortion and noise. Also, the
required input signal voltage span is reduced a factor of two
which further reduces the degree of RON modulation and its
effects on distortion.
The optimum noise and dc linearity performance for either
differential or single-ended inputs is achieved with the largest
input signal voltage span (i.e., 5 V input span) and matched
input impedance for VINA and VINB. Note that only a slight
degradation in dc linearity performance exists between the
2 V and 5 V input span as specified in the AD9240 DC
SPECIFICATIONS.
Referring to Figure 24, the differential SHA is implemented
using a switched-capacitor topology. Hence, its input impedance and its subsequent effects on the input drive source should
be understood to maximize the converter’s performance. The
combination of the pin capacitance, CPIN, parasitic capacitance
CPAR, and the sampling capacitance, CS, is typically less than
16 pF. When the SHA goes into track mode, the input source
must charge or discharge the voltage stored on CS to the new
input voltage. This action of charging and discharging CS which
is approximately 4 pF, averaged over a period of time and for a
given sampling frequency, FS, makes the input impedance appear to have a benign resistive component (i.e., 83 kΩ at FS =
10 MSPS). However, if this action is analyzed within a sampling period (i.e., T = <1/FS), the input impedance is dynamic
due to the instantaneous requirement of charging and discharging CS. A series resistor inserted between the input drive source
and the SHA input as shown in Figure 28 provides effective
isolation.
A slight improvement in SNR performance and dc offset
performance is achieved by matching the input resistance connected to VINA and VINB. The degree of improvement is dependent on the resistor value and the sampling rate. For series
resistor values greater than 100 Ω, the use of a matching resistor is encouraged.
The noise or small-signal bandwidth of the AD9240 is the same
as its full-power bandwidth. For noise sensitive applications, the
excessive bandwidth may be detrimental and the addition of a
series resistor and/or shunt capacitor can help limit the wideband noise at the A/D’s input by forming a low-pass filter. Note,
however, that the combination of this series resistance with the
equivalent input capacitance of the AD9240 should be evaluated for those time-domain applications that are sensitive to the
input signal’s absolute settling time. In applications where harmonic distortion is not a primary concern, the series resistance
may be selected in combination with the SHA’s nominal 16 pF
of input capacitance to set the filter’s 3 dB cutoff frequency.
A better method of reducing the noise bandwidth, while possibly establishing a real pole for an antialiasing filter, is to add
some additional shunt capacitance between the input (i.e.,
VINA and/or VINB) and analog ground. Since this additional
shunt capacitance combines with the equivalent input capacitance of the AD9240, a lower series resistance can be selected to
establish the filter’s cutoff frequency while not degrading the
distortion performance of the device. The shunt capacitance
also acts as a charge reservoir, sinking or sourcing the additional
charge required by the hold capacitor, CH, further reducing
current transients seen at the op amp’s output.
The effect of this increased capacitive load on the op amp driving the AD9240 should be evaluated. To optimize performance
when noise is the primary consideration, increase the shunt
capacitance as much as the transient response of the input signal
will allow. Increasing the capacitance too much may adversely
affect the op amp’s settling time, frequency response and distortion performance.
–10–
REV. A
AD9240
Table I. Analog Input Configuration Summary
Input
Connection
Single-Ended
Single-Ended
Differential
1
Input
Coupling Span (V)
DC
2
AC
AC or
DC
Input Range (V)
VINA1
VINB1
0 to 2
1
Figure
#
32, 33
2 × VREF
0 to
2 × VREF
VREF
32, 33
Same as above but with improved noise performance due to
increase in dynamic range. Headroom/settling time requirements
of ± 5 V op amp should be evaluated.
5
0 to 5
2.5
32, 33
Optimum noise performance, excellent THD performance. Requires
op amp with VCC > +5 V due to insufficient headroom @ 5 V.
2 × VREF
2.5 – VREF
to
2.5 + VREF
2.5
39
Optimum THD performance with VREF = 1, noise performance
improves while THD performance degrades as VREF increases
to 2.5 V. Single supply operation (i.e., +5 V) for many op amps.
2 or
2 × VREF
0 to 1 or
1 or VREF
0 to 2 × VREF
34
Suboptimum ac performance due to input common-mode level
not biased at optimum midsupply level (i.e., 2.5 V).
5
0 to 5
2.5
34
Optimum noise performance, excellent THD performance.
2 × VREF
2.5 – VREF
to
2.5 + VREF
2.5
35
Flexible input range, Optimum THD performance with VREF = 1.
Noise performance improves while THD performance degrades as
VREF increases to 2.5 V.
2
2 to 3
3 to 2
29–31
Optimum full-scale THD and SFDR performance well beyond the
A/Ds Nyquist frequency.
2 × VREF
2.5 – VREF/2
to
2.5 + VREF/2
2.5 + VREF/2
to
2.5 – VREF/2
29–31
Same as 2 V to 3 V input range with the exception that full-scale
THD and SFDR performance can be traded off for better noise
performance.
5
1.25 to 3.75
3.75 to 1.25
29–31
Widest dynamic range (i.e., ENOBs) due to optimum noise
performance.
Comments
Best for stepped input response applications, suboptimum THD
and noise performance, requires ± 5 V op amp.
VINA and VINB can be interchanged if signal inversion is required.
Table II. Reference Configuration Summary
Reference
Operating Mode
Input Span (VINA–VINB)
(V p-p)
Required VREF (V)
Connect
To
INTERNAL
INTERNAL
INTERNAL
2
5
2 ≤ SPAN ≤ 5 AND
SPAN = 2 × VREF
1
2.5
1 ≤ VREF ≤ 2.5 AND
VREF = (1 + R1/R2)
SENSE
SENSE
R1
R2
VREF
REFCOM
VREF AND SENSE
SENSE AND REFCOM
EXTERNAL
(NONDYNAMIC)
2 ≤ SPAN ≤ 5
1 ≤ VREF ≤ 2.5
SENSE
VREF
AVDD
EXT. REF.
EXTERNAL
(DYNAMIC)
2 ≤ SPAN ≤ 5
CAPT and CAPB
Externally Driven
SENSE
VREF
EXT. REF. 1
EXT. REF. 2
AVDD
REFCOM
CAPT
CAPB
REV. A
–11–
AD9240
REFERENCE OPERATION
The AD9240 contains an onboard bandgap reference that provides a pin-strappable option to generate either a 1 V or 2.5 V
output. With the addition of two external resistors, the user can
generate reference voltages other than 1 V and 2.5 V. Another
alternative is to use an external reference for designs requiring
enhanced accuracy and/or drift performance. See Table II for a
summary of the pin-strapping options for the AD9240 reference
configurations.
Figure 29 shows a simplified model of the internal voltage
reference of the AD9240. A pin-strappable reference amplifier buffers a 1 V fixed reference. The output from the reference amplifier, A1, appears on the VREF pin. The voltage on
the VREF pin determines the full-scale input span of the A/D.
This input span equals,
The actual reference voltages used by the internal circuitry of
the AD9240 appear on the CAPT and CAPB pins. For proper
operation when using the internal or an external reference, it is
necessary to add a capacitor network to decouple these pins.
Figure 30 shows the recommended decoupling network. This
capacitive network performs the following three functions: (1)
along with the reference amplifier, A2, it provides a low source
impedance over a large frequency range to drive the A/D internal circuitry, (2) it provides the necessary compensation for A2
and (3) it bandlimits the noise contribution from the reference.
The turn-on time of the reference voltage appearing between
CAPT and CAPB is approximately 15 ms and should be evaluated in any power-down mode of operation.
0.1mF
CAPT
Full-Scale Input Span = 2 × VREF
AD9240
The voltage appearing at the VREF pin as well as the state of
the internal reference amplifier, A1, are determined by the voltage appearing at the SENSE pin. The logic circuitry contains
two comparators which monitor the voltage at the SENSE pin.
The comparator with the lowest set point (approximately 0.3 V)
controls the position of the switch within the feedback path of
A1. If the SENSE pin is tied to REFCOM, the switch is connected to the internal resistor network thus providing a VREF of
2.5 V. If the SENSE pin is tied to the VREF pin via a short or
resistor, the switch is connected to the SENSE pin. A short will
provide a VREF of 1.0 V while an external resistor network will
provide an alternative VREF between 1.0 V and 2.5 V.
The second comparator controls internal circuitry that will
disable the reference amplifier if the SENSE pin is tied AVDD.
Disabling the reference amplifier allows the VREF pin to be
driven by an external voltage reference.
0.1mF
10mF
CAPB
0.1mF
Figure 30. Recommended CAPT/CAPB Decoupling Network
The A/D’s input span may be varied dynamically by changing
the differential reference voltage appearing across CAPT and
CAPB symmetrically around 2.5 V (i.e., midsupply). To change
the reference at speeds beyond the capabilities of A2, it will be
necessary to drive CAPT and CAPB with two high speed, low
noise amplifiers. In this case, both internal amplifiers (i.e., A1
and A2) must be disabled by connecting SENSE to AVDD and
VREF to REFCOM and the capacitive decoupling network
removed. The external voltages applied to CAPT and CAPB
must be 2.5 V + Input Span/4 and 2.5 V – Input Span/4, respectively, where the input span can be varied between 2 V and 5 V.
Note that those samples within the pipeline A/D during any
reference transition will be corrupted and should be discarded.
AD9240
TO
A/D
5kV
CAPT
5kV
A2
5kV
CAPB
5kV
DISABLE
A2
LOGIC
VREF
A1
1V
7.5kV
SENSE
DISABLE
A1
LOGIC
5kV
REFCOM
Figure 29. Equivalent Reference Circuit
–12–
REV. A
AD9240
DRIVING THE ANALOG INPUTS
The AD9240 has a highly flexible input structure allowing it to
interface with single-ended or differential input interface circuitry. The applications shown in sections Driving the Analog
Inputs and Reference Configurations, along with the information presented in the Input and Reference Overview section of
this data sheet, give examples of both single-ended and differential operation. Refer to Tables I and II for a list of the different
possible input and reference configurations and their associated
figures in the data sheet.
The optimum mode of operation, analog input range and associated interface circuitry will be determined by the particular
applications performance requirements as well as power supply
options. For example, a dc coupled single-ended input may be
appropriate for many data acquisition and imaging applications.
Also, many communication applications which require a dc
coupled input for proper demodulation can take advantage of
the excellent single-ended distortion performance of the AD9240.
The input span should be configured such that the system’s
performance objectives and the headroom requirements of the
driving op amp are simultaneously met.
Alternatively, the differential mode of operation provides the
best THD and SFDR performance over a wide frequency range.
A transformer coupled differential input should be considered
for the most demanding spectral-based applications which allow
ac coupling (e.g., Direct IF to Digital Conversion). The dccoupled differential mode of operation also provides an enhancement in distortion and noise performance at higher input spans.
Furthermore, it allows the AD9240 to be configured for a 5 V
span using op amps specified for +5 V or ± 5 V operation.
Single-ended operation requires that VINA be ac or dc coupled
to the input signal source while VINB of the AD9240 be biased
to the appropriate voltage corresponding to a midscale code
transition. Note that signal inversion may be easily accomplished by transposing VINA and VINB.
Differential operation requires that VINA and VINB be simultaneously driven with two equal signals that are in and out of
phase versions of the input signal. Differential operation of the
AD9240 offers the following benefits: (1) Signal swings are
smaller and therefore linearity requirements placed on the input
signal source may be easier to achieve, (2) Signal swings are
smaller and therefore may allow the use of op amps which
may otherwise have been constrained by headroom limitations,
(3) Differential operation minimizes even-order harmonic products and (4) Differential operation offers noise immunity based
on the device’s common-mode rejection as shown in Figure 16.
As is typical of most CMOS devices, exceeding the supply limits
will turn on internal parasitic diodes resulting in transient currents within the device. Figure 31 shows a simple means of
clamping a dc coupled input with the addition of two series
resistors and two diodes. Note that a larger series resistor could
be used to limit the fault current through D1 and D2 but should be
evaluated since it can cause a degradation in overall performance.
REV. A
AVDD
VCC
RS1
30V
INTRODUCTION
D2
1N4148
RS2
20V
AD9240
D1
1N4148
VEE
Figure 31. Simple Clamping Circuit
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for
differential operation, there is often a need to perform a
single-ended-to-differential conversion. A single-ended-todifferential conversion can be realized with an RF transformer
or a dual op amp differential driver. The optimum method
depends on whether the application requires the input signal to
be ac or dc coupled to AD9240.
AC Coupling via an RF Transformer
An RF transformer with a center tap can be used to generate
differential inputs for the AD9240. It provides all of the benefits
of operating the ADC in the differential mode while contributing no additional noise and minimal distortion. As a result, an
RF transformer is recommended in high frequency applications, especially undersampling, in which the performance of
a dual op amp differential driver may not be adequate. An RF
transformer has the added benefit of providing electrical isolation between the signal source and the ADC. However, since the
lower cutoff frequency of most RF transformers is nominally a
few 100 kHz, a dual op amp differential driver may be more suitable in ac-coupling applications, where the spectral content of the
input signal falls below the cutoff frequency of a suitable RF
transformer.
Figure 32 is a suggested transformer circuit using a MiniCircuits RF transformer, model #T4-6T, which has an impedance ratio of four (turns ratio of 2). The 1:4 impedance ratio
requires the 200 Ω secondary termination for optimum power
transfer and VSWR. The centertap of the transformer provides
a convenient means of level-shifting the input signal to a desired common-mode voltage. Optimum performance can be
realized when the centertap is tied to CML of the AD9240
which is the common-mode bias level of the internal SHA.
VINA
50V
CML
200V
0.1mF
AD9240
VINB
MINI-CIRCUITS
T4-6T
Figure 32. Transformer Coupled Input
Transformers with other turns ratios may also be selected to
optimize the performance of a given application. For example, a
given input signal source or amplifier may realize an improvement in distortion performance at reduced output power levels
and signal swings. Hence, selecting a transformer with a higher
impedance ratio (i.e., Mini-Circuits T16-6T with a 1:16 impedance ratio) effectively “steps up” the signal level, further reducing the driving requirements of the signal source.
–13–
AD9240
AC Coupling with Op Amps
390V
As previously stated, a dual op amp differential driver may be
more suitable in applications in which the spectral content of the
input signal falls below the cutoff frequency of a suitable RF
transformer and/or the cost of an RF transformer and a low
distortion driver for the transformer is prohibitive.
The ac-coupled differential driver shown in Figure 33 is best
suited for ± 5 V systems in which the input signal is ground
referenced. In this case, VCM will be 0 V. This driver circuit can
achieve performance similar to an RF transformer over the
AD9240’s full Nyquist bandwidth of 5 MHz. However, unlike
the RF transformer, the lower cutoff frequency can be arbitrarily
set low by adjusting the RC time constant formed by CC and
RB/2. CN, in combination with RS, can be used to limit the contribution of op amp-generated noise at higher frequencies. Low
cost, high performance dual op amps operating from ± 5 V such
as the AD8056 and AD8058, are excellent choices for this application and are capable of maintaining 78 dB SNR and 83 dB
THD at 1 MHz (5 V span). An optional resistor RO can be
added to U1B to achieve a similar group delay as U1A, potentially
improving overall distortion performance. A resistor divider network formed by RB centers the inputs of the AD9240 around
AVDD/2 to achieve its optimum distortion performance.
VCM
VCC
U1A
RB
5kV
RS
CC
0.1mF
RB
5kV
RB
5kV
U1B
RS
CC
0.1mF
220V
AD9631
VCML–VIN
33V
VINA
VIN
390V
390V
390V
AVDD
220V
390V
0.1mF
AD9240
VCML+VIN
AD9631
33V
VINB
390V
2.5kV
390V
0.1mF
100V
CML
0.1mF
1mF
OP113
Figure 34. Differential Driver with Level-Shifting
Single Supply DC-Coupled Driver
The circuit of Figure 33 can be easily modified for a single
supply, dc-coupled application. This is done by biasing VCM to
AVDD/2, the normal common-mode level in a single supply
system. Since the outputs of the op amps are centered at
AVDD/2, the ac coupling network of CC and RB can be removed.
With this done, the differential driving pair can now be run from
a single supply.
The AD9240 can be configured for single-ended operation
using dc or ac coupling. In either case, the input of the A/D
must be driven from an operational amplifier that will not degrade the A/D’s performance. Because the A/D operates from a
single supply, it will be necessary to level-shift ground-based
bipolar signals to comply with its input requirements. Both dc
and ac coupling provide this necessary function, but each
method results in different interface issues which may influence
the system design and performance.
VCC
RO
390V
SINGLE-ENDED MODE OF OPERATION
CN
SOURCE
AVDD
0.1mF
RB
5kV
Figure 33. AC Coupling of Op Amps
DC Coupling with Op Amps
The dc-coupled differential driver in Figure 34 is best suited for
± 5 V systems in which the input signal is ground referenced and
optimum distortion performance is desired. This driver circuit
provides the ability to level-shift the input signal to within the
common-mode range of the AD9240. The two op amps are
configured as matched differential amps with the input signal
applied to opposing inputs to provide the differential output.
The common-mode offset voltage is applied to the noninverting
resistor network, which provides the proper level shifting. The
AD9631 is given as the amplifier of choice in this application
due to its superior distortion performance for relatively large
output swings and wide bandwidth. If cost or space are factors,
the AD8056 dual op amp will save on both, but at the cost of
slightly increased distortion with large signal levels. Figure 34
also illustrates the use of protection diodes, which are used to
protect the AD9240 from any fault condition in which the op
amps outputs inadvertently go above VDD or below GND.
DC COUPLING AND INTERFACE ISSUES
Many applications require the analog input signal to be dc
coupled to the AD9240. An operational amplifier can be configured to rescale and level-shift the input signal so it is compatible with the selected input range of the A/D. The input range
to the A/D should be selected on the basis of system performance objectives as well as the analog power supply availability
since this will place certain constraints on the op amp selection.
Many of the new high performance op amps are specified for
only ± 5 V operation and have limited input/output swing capabilities. Hence, the selected input range of the AD9240 should
be sensitive to the headroom requirements of the particular op
amp to prevent clipping of the signal. Also, since the output of a
dual supply amplifier can swing below –0.3 V, clamping its
output should be considered in some applications.
In some applications, it may be advantageous to use an op amp
specified for single supply +5 V operation since it will inherently
limit its output swing to within the power supply rails. Rail-torail output amplifiers such as the AD8041 allow the AD9240 to
be configured with larger input spans which improves the noise
performance.
–14–
REV. A
AD9240
If the application requires the largest single-ended input range
(i.e., 0 V to 5 V) of the AD9240, the op amp will require larger
supplies to drive it. Various high speed amplifiers in the Op
Amp Selection Guide of this data sheet can be selected to
accommodate a wide range of supply options. Once again,
clamping the output of the amplifier should be considered for
these applications. Alternatively, a single-ended to differential
op amp driver circuit using the AD8042 could be used to
achieve the 5 V input span while operating from a single +5 V
supply as discussed in the previous section.
500V*
+VCC
0.1mF
+VREF
–VREF
0.1mF
Figure 36. Single-Ended Input With DC-Coupled Level-Shift
AC COUPLING AND INTERFACE ISSUES
For applications where ac coupling is appropriate, the op amp’s
output can be easily level-shifted to the common-mode voltage,
VCM, of the AD9240 via a coupling capacitor. This has the
advantage of allowing the op amps common-mode level to be
symmetrically biased to its midsupply level (i.e., (VCC + VEE)/
2). Op amps that operate symmetrically with respect to their
power supplies typically provide the best ac performance as well
as greatest input/output span. Hence, various high speed/
performance amplifiers that are restricted to +5 V/–5 V operation and/or specified for +5 V single-supply operation can be
easily configured for the 5 V or 2 V input span of the AD9240,
respectively. The best ac distortion performance is achieved
when the A/D is configured for a 2 V input span and commonmode voltage of 2.5 V. Note that differential transformer
coupling, which is another form of ac coupling, should be
considered for optimum ac performance.
Simple AC Interface
Figure 37 shows a typical example of an ac-coupled, singleended configuration. The bias voltage shifts the bipolar,
ground-referenced input signal to approximately VREF. The
value for C1 and C2 will depend on the size of the resistor, R.
The capacitors, C1 and C2, are typically a 0.1 µF ceramic and
10 µF tantalum capacitor in parallel to achieve a low cutoff
frequency while maintaining a low impedance over a wide frequency range. The combination of the capacitor and the resistor
form a high-pass filter with a high-pass –3 dB frequency determined by the equation,
AD9240
VINB
10mF
VREF
0.1mF
SENSE
Figure 35. Single-Ended AD9240 Op Amp Drive Circuit
f–3 dB = 1/(2 × π × R × (C1 + C2))
Op Amp with DC Level-Shifting
Figure 36 shows a dc-coupled level-shifting circuit employing an
op amp, A1, to sum the input signal with the desired dc offset.
Configuring the op amp in the inverting mode with the given
resistor values results in an ac signal gain of –1. If the signal
inversion is undesirable, interchange the VINA and VINB connections to reestablish the original signal polarity. The dc voltage at VREF sets the common-mode voltage of the AD9240. For
example, when VREF = 2.5 V, the output level from the op amp
will also be centered around 2.5 V. The use of ratio matched,
thin-film resistor networks will minimize gain and offset errors.
An optional pull-up resistor, RP, may also be used to reduce the
output load on VREF to ±1 mA.
REV. A
VINB
*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D
**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE
VINA
2.5V
AD9240
NC
RS
RS
–V
500V*
VREF
+V
RS
VINA
500V*
AVDD
In the simplest case, the input signal to the AD9240 will already
be biased at levels in accordance with the selected input range.
It is simply necessary to provide an adequately low source impedance for the VINA and VINB analog input pins of the A/D.
Figure 35 shows the recommended configuration for a singleended drive using an op amp. In this case, the op amp is shown
in a noninverting unity gain configuration driving the VINA pin.
The internal reference drives the VINB pin. Note that the addition of a small series resistor of 30 Ω to 50 Ω connected to VINA
and VINB will be beneficial in nearly all cases. Refer to the
Analog Input Operation section for a discussion on resistor
selection. Figure 35 shows the proper connection for a 0 V to
5 V input range. Alternative single ended input ranges of 0 V to
2 × VREF can also be realized with the proper configuration of
VREF (refer to the section, Using the Internal Reference).
U1
RS
RP**
Simple Op Amp Buffer
0V
NC
500V*
A1
Two dc coupled op amp circuits using a noninverting and
inverting topology are discussed below. Although not shown,
the noninverting and inverting topologies can be easily configured as part of an antialiasing filter by using a Sallen-Key or
Multiple-Feedback topology, respectively. An additional R-C
network can be inserted between the op amp’s output and the
AD9240 input to provide a real pole.
5V
0VDC
C1
+5V
+VREF
0V
–VREF
RS
C2
VIN
AD9240
VINA
R
RS
VINB
–5V
VREF
C2
C1
SENSE
Figure 37. AC-Coupled Input
The low impedance VREF voltage source biases both the VINB
input and provides the bias voltage for the VINA input. Figure
37 shows the VREF configured for 2.5 V. Thus the input range
of the A/D is 0 V to 5 V. Other input ranges could be selected
by changing VREF but the A/D’s distortion performance will
–15–
AD9240
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
AD9631:
220 MHz Unity GBW, 16 ns Settling to 0.01%,
±5 V Supplies
Best Applications: Best AC Specs, Low Noise,
AC-Coupled
Limits: Usable Input/Output Range, Power
Consumption
AD8047:
130 MHz Unity GBW, 30 ns Settling to 0.01%,
± 5 V Supplies
Best Applications: Good AC Specs, Low Noise,
AC-Coupled
Limits: THD > 5 MHz, Usable Input Range
AD8042:
Dual AD8041
Best Applications: Differential and/or Low Impedance Input Drivers
Limits: Noise with 2 V Input Range
Alternative AC Interface
Figure 38 shows a flexible ac-coupled circuit which can be configured for different input spans. Since the common-mode
voltage of VINA and VINB are biased to midsupply independent of VREF, VREF can be pin-strapped or reconfigured to
achieve input spans between 2 V and 5 V p-p. The AD9240’s
CMRR along with the symmetrical coupling R-C networks will
reject both power supply variations and noise. The resistors, R,
establish the common-mode voltage. They may have a high value
(e.g., 5 kΩ) to minimize power consumption and establish a low
cutoff frequency. The capacitors, C1 and C2, are typically a
0.1 µF ceramic and 10 µF tantalum capacitor in parallel to
achieve a low cutoff frequency while maintaining a low impedance over a wide frequency range. RS isolates the buffer amplifier from the A/D input. The optimum performance is achieved
when VINA and VINB are driven via symmetrical networks.
The high pass f –3 dB point can be approximated by the equation,
f–3 dB = 1/(2 × π × R/2 × (C1 + C2))
+5V
+5V
R
C1
VIN
AD9240
RS
VINA
C2
R
–5V
RS
R
+5V
C2
For the purpose of simplicity, the figures associated with this
section on internal and external reference operation do not
show recommended matching series resistors for VINA and
VINB. Please refer to section Driving the Analog Inputs, Introduction, for a discussion of this topic. The figures do not show
the decoupling network associated with the CAPT and CAPB
pins. Please refer to the Reference Operation section for a discussion of the internal reference circuitry and the recommended
decoupling network shown in Figure 30.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2 ⴛ VREF Range
VINB
R
REFERENCE CONFIGURATIONS
C1
Figure 39 shows how to connect the AD9240 for a 0 V to 2 V or
0 V to 5 V input range via pin strapping the SENSE pin. An
intermediate input range of 0 to 2 × VREF can be established
using the resistor programmable configuration in Figure 41 and
connecting VREF to VINB.
Figure 38. AC-Coupled Input-Flexible Input Span,
VCM = 2.5 V
OP AMP SELECTION GUIDE
Op amp selection for the AD9240 is highly dependent on a
particular application. In general, the performance requirements
of any given application can be characterized by either time
domain or frequency domain parameters. In either case, one
should carefully select an op amp that preserves the performance of the A/D. This task becomes challenging when one
considers the AD9240’s high performance capabilities coupled
with other external system level requirements such as power
consumption and cost.
The ability to select the optimal op amp may be further complicated by limited power supply availability and/or limited acceptable supplies for a desired op amp. Newer, high performance op
amps typically have input and output range limitations in accordance with their lower supply voltages. As a result, some op
amps will be more appropriate in systems where ac-coupling is
allowable. When dc-coupling is required, op amps without
headroom constraints such as rail-to-rail op amps or ones where
larger supplies can be used should be considered. The following
section describes some op amps currently available from Analog
Devices. The system designer is always encouraged to contact
the factory or local sales office to be updated on Analog Devices’ latest amplifier product offerings. Highlights of the areas
where the op amps excel and where they may limit the performance of the AD9240 are also included.
2xVREF
VINA
0V
VINB
10mF
0.1mF
VREF
SHORT FOR 0 TO 2V
INPUT SPAN
AD9240
SENSE
SHORT FOR 0 TO 5V
INPUT SPAN
REFCOM
Figure 39. Internal Reference (2 V p-p Input Span,
VCM = 1 V, or 5 V p-p Input Span, VCM = 2.5 V)
In either case, both the common-mode voltage and input span
are directly dependent on the value of VREF. More specifically,
the common-mode voltage is equal to VREF while the input
span is equal to 2 × VREF. Thus, the valid input range extends
from 0 to 2 × VREF. When VINA is ≤ 0 V, the digital output
will be 0000 Hex; when VINA is ≥ 2 × VREF, the digital output
will be 3FFF Hex.
Shorting the VREF pin directly to the SENSE pin places the
internal reference amplifier in unity-gain mode and the resultant VREF output is 1 V. The valid input range is, therefore, 0 V
to 2 V. Shorting the SENSE pin directly to the REFCOM pin
configures the internal reference amplifier for a gain of 2.5 and
–16–
REV. A
AD9240
the resultant VREF output is 2.5 V. The valid input range thus
becomes 0 V to 5 V. The VREF pin should be bypassed to the
REFCOM pin with a 10 µF tantalum capacitor in parallel with a
low-inductance 0.1 µF ceramic capacitor.
Single-Ended or Differential Input, V CM = 2.5 V
the example shown, the valid input signal range for VINA is 1 V
to 4 V since VINB is set to an external, low impedance 2.5 V
source. The VREF pin should be bypassed to the REFCOM pin
with a 10 µF tantalum capacitor in parallel with a low inductance 0.1 µF ceramic capacitor.
Figure 37 shows the single-ended configuration that gives the
best SINAD performance. To optimize dynamic specifications,
center the common-mode voltage of the analog input at
approximately by 2.5 V by connecting VINB to VREF, a lowimpedance 2.5 V source. As described above, shorting the
SENSE pin directly to the REFCOM pin results in a 2.5 V
reference voltage and a 5 V p-p input span. The valid range
for input signals is 0 V to 5 V. The VREF pin should be bypassed to the REFCOM pin with a 10 µF tantalum capacitor in
parallel with a low inductance 0.1 µF ceramic capacitor.
This reference configuration could also be used for a differential
input in which VINA and VINB are driven via a transformer as
shown in Figure 32. In this case, the common-mode voltage,
VCM, is set at midsupply by connecting the transformer’s center
tap to CML of the AD9240. VREF can be configured for 1 V
or 2.5 V by connecting SENSE to either VREF or REFCOM
respectively. Note that the valid input range for each of the
differential inputs is one half of the single-ended input and thus
becomes VCM – VREF/2 to VCM + VREF/2.
5V
VINA
0V
VINB
2.5V
4V
VINA
1V
1.5V
VREF
0.1mF
R1
2.5kV
C1
0.1mF
AD9240
SENSE
R2
5kV
REFCOM
Figure 41. Resistor Programmable Reference (3 V p-p
Input Span, VCM = 2.5 V)
USING AN EXTERNAL REFERENCE
Using an external reference may enhance the dc performance of
the AD9240 by improving drift and accuracy. Figures 42
through 44 show examples of how to use an external reference
with the A/D. Table III is a list of suitable voltage references
from Analog Devices. To use an external reference, the user
must disable the internal reference amplifier and drive the
VREF pin. Connecting the SENSE pin to AVDD disables the
internal reference amplifier.
AD9240
Table III. Suitable Voltage References
SENSE
REFCOM
Figure 40. Internal Reference—5 V p-p Input Span,
VCM = 2.5 V
Resistor Programmable Reference
Figure 41 shows an example of how to generate a reference
voltage other than 1 V or 2.5 V with the addition of two external
resistors and a bypass capacitor. Use the equation,
VREF = 1 V × (1 + R1/R2),
to determine appropriate values for R1 and R2. These resistors
should be in the 2 kΩ to 100 kΩ range. For the example
shown, R1 equals 2.5 kΩ and R2 equals 5 kΩ. From the equation above, the resultant reference voltage on the VREF pin is
1.5 V. This sets the input span to be 3 V p-p. To assure stability, place a 0.1 µF ceramic capacitor in parallel with R1.
Internal
REF191
Internal
REF192
AD780
Output
Voltage
Drift
(ppm/ⴗC)
Initial
Accuracy
% (max)
Operating
Current
(␮A)
1.00
2.048
2.50
2.50
2.50
26
5–25
26
5–25
3–7
1.4
0.1–0.5
1.4
0.08–0.4
0.04–0.2
N/A
45
N/A
45
1000
The AD9240 contains an internal reference buffer, A2 (see
Figure 29), that simplifies the drive requirements of an external
reference. The external reference must be able to drive a ≈5 kΩ
(± 20%) load. Note that the bandwidth of the reference buffer is
deliberately left small to minimize the reference noise contribution. As a result, it is not possible to change the reference voltage rapidly in this mode without the removal of the CAPT/
CAPB Decoupling Network, and driving these pins directly.
The common-mode voltage can be set to VREF by connecting
VINB to VREF to provide an input span of 0 to 2 × VREF.
Alternatively, the common-mode voltage can be set to 2.5 V
by connecting VINB to a low impedance 2.5 V source. For
REV. A
0.1mF
10mF
VREF
10mF
VINB
2.5V
–17–
AD9240
Variable Input Span with V CM = 2.5 V
3.75V
VINA
Figure 42 shows an example of the AD9240 configured for an
input span of 2 × VREF centered at 2.5 V. An external 2.5 V
reference drives the VINB pin thus setting the common-mode
voltage at 2.5 V. The input span can be independently set by a
voltage divider consisting of R1 and R2, which generates the
VREF signal. A1 buffers this resistor network and drives VREF.
Choose this op amp based on accuracy requirements. It is
essential that a minimum of a 10 µF capacitor in parallel with a
0.1 µF low inductance ceramic capacitor decouple the reference
output to ground.
2.5V+VREF
2.5V
2.5V–VREF
2.5V
REF
+5V
0.1mF
22mF
0.1mF
+5V
0.1mF
1kV
VINB
10mF
0.1mF
2N2222
7.5kV
1/2
OP282
1kV
AD9240
316V
1.225V
+5V
AD1580
10mF
0.1mF
VREF
SENSE
+5V
VINB
DIGITAL INPUTS AND OUTPUTS
Digital Outputs
The AD9240 output data is presented in positive true straight
binary for all input ranges. Table IV indicates the output data
formats for various input ranges regardless of the selected input
range. A twos complement output data format can be created by
inverting the MSB.
VREF
R2
SENSE
+5V
1kV
Figure 44. External Reference Using the AD1580 and Low
Impedance Buffer
AD9240
A1
820V
VINA
0.1mF
R1
1.25V
Figure 42. External Reference, VCM = 2.5 V (2.5 V on VINB,
Resistor Divider to Make VREF)
Table IV. Output Data Format
Single-Ended Input with 0 to 2 ⴛ VREF Range
Input (V)
Condition (V)
Digital Output
OTR
Figure 43 shows an example of an external reference driving
both VINB and VREF. In this case, both the common mode
voltage and input span are directly dependent on the value of
VREF. More specifically, the common-mode voltage is equal to
VREF while the input span is equal to 2 × VREF. Thus, the
valid input range extends from 0 to 2 × VREF. If, for example,
the REF191, a 2.048 external reference, were selected, the valid
input range extends from 0 V to 4.096 V. In this case, 1 LSB of
the AD9240 corresponds to 0.250 mV. It is essential that a
minimum of a 10 µF capacitor in parallel with a 0.1 µF low inductance ceramic capacitor decouple the reference output to ground.
VINA – VINB
VINA – VINB
VINA – VINB
VINA – VINB
VINA – VINB
< –VREF
= –VREF
=0
= +VREF – 1 LSB
≥ +VREF
00 0000 0000 0000
00 0000 0000 0000
10 0000 0000 0000
11 1111 1111 1111
11 1111 1111 1111
1
0
0
0
1
2xREF
VINA
0V
+5V
VINB
VREF
0.1mF
10mF
0.1mF
AD9240
VREF
0.1mF
+5V
SENSE
Figure 43. Input Range = 0 V to 2 × VREF
Low Cost/Power Reference
Out Of Range (OTR)
An out-of-range condition exists when the analog input voltage
is beyond the input range of the converter. OTR is a digital
output that is updated along with the data output corresponding
to the particular sampled analog input voltage. Hence, OTR
has the same pipeline delay (latency) as the digital data. It is
LOW when the analog input voltage is within the analog input
range. It is HIGH when the analog input voltage exceeds the
input range as shown in Figure 45. OTR will remain HIGH
until the analog input returns within the input range and another conversion is completed. By logical ANDing OTR with
the MSB and its complement, overrange high or underrange low
conditions can be detected. Table V is a truth table for the over/
underrange circuit in Figure 46 which uses NAND gates. Systems requiring programmable gain conditioning of the AD9240
input signal can immediately detect an out-of-range condition,
thus eliminating gain selection iterations. Also, OTR can be
used for digital offset and gain calibration.
The external reference circuit shown in Figure 44 uses a low cost
1.225 V external reference (e.g., AD580 or AD1580) along with an
op amp and transistor. The 2N2222 transistor acts in conjunction
with 1/2 of an OP282 to provide a very low impedance drive for
VINB. The selected op amp need not be a high speed op amp and
may be selected based on cost, power and accuracy.
OTR
1
0
0
+FS –1 1/2 LSB
DATA OUTPUTS
111111 1111 1111 OTR
111111 1111 1111
111111 1111 1110
–FS+1/2 LSB
0
0
1
000000 0000 0001
000000 0000 0000
000000 0000 0000
–FS
–FS –1/2 LSB
+FS
+FS –1/2 LSB
Figure 45. Output Data Format
–18–
REV. A
AD9240
Table V. Out-of-Range Truth Table
OTR
MSB
Analog Input Is
0
0
1
1
0
1
0
1
In Range
In Range
Underrange
Overrange
Most of the power dissipated by the AD9240 is from the analog
power supply; however, lower clock speeds will reduce digital
current slightly. Figure 47 shows the relationship between power
and clock rate.
400
380
360
340
MSB
POWER – mW
OVER = “1”
OTR
UNDER = “1”
MSB
Figure 46. Overrange or Underrange Logic
320
300
280
260
Digital Output Driver Considerations (DRVDD)
240
The AD9240 output drivers can be configured to interface with
+5 V or 3.3 V logic families by setting DRVDD to +5 V or 3.3 V
respectively. The AD9240 output drivers are sized to provide
sufficient output current to drive a wide variety of logic families;
large drive currents tend to cause glitches on the supplies and may
affect SINAD performance. Applications requiring the AD9240 to
drive large capacitive loads or large fanout may require additional
decoupling capacitors on DRVDD. In extreme cases, external
buffers or latches may be required.
220
Clock Input and Considerations
The AD9240 internal timing uses the two edges of the clock
input to generate a variety of internal timing signals. The clock
input must meet or exceed the minimum specified pulsewidth
high and low (tCH and tCL) specifications for the given A/D, as
defined in the Switching Specifications at the beginning of the
data sheet, to meet the rated performance specifications. For
example, the clock input to the AD9240 operating at 10 MSPS
may have a duty cycle between 45% to 55% to meet this timing
requirement since the minimum specified tCH and tCL is 45 ns.
For clock rates below 10 MSPS, the duty cycle may deviate
from this range to the extent that both tCH and tCL are satisfied.
All high speed high resolution A/Ds are sensitive to the quality
of the clock input. The degradation in SNR at a given full-scale
input frequency (fIN), due only to aperture jitter (tA), can be
calculated with the following equation:
SNR = 20 log10 [1/(2 π fIN tA)]
In the equation, the rms aperture jitter, tA, represents the rootsum square of all the jitter sources, which include the clock
input, analog input signal and A/D aperture jitter specification.
For example, if a 5.0 MHz full-scale sine wave is sampled by an
A/D with a total rms jitter of 15 ps, the SNR performance of the
A/D will be limited to 66.5 dB. Undersampling applications are
particularly sensitive to jitter.
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the
AD9240. As such, supplies for clock drivers should be separated
from the A/D output driver supplies to avoid modulating the
clock signal with digital noise. Low jitter crystal controlled oscillators make the best clock sources. If the clock is generated from
another type of source (by gating, dividing or other method), it
should be retimed by the original clock at the last step.
REV. A
200
2
4
6
8
10
12
14
16
CLOCK FREQUENCY – MHz
18
20
Figure 47. Power Consumption vs. Clock Frequency
(RBIAS = 2 kΩ)
GROUNDING AND DECOUPLING
Analog and Digital Grounding
Proper grounding is essential in any high speed, high resolution
system. Multilayer printed circuit boards (PCBs) are recommended to provide optimal grounding and power schemes. The
use of ground and power planes offers distinct advantages:
1. The minimization of the loop area encompassed by a signal
and its return path.
2. The minimization of the impedance associated with ground
and power paths.
3. The inherent distributed capacitor formed by the power
plane, PCB insulation and ground plane.
These characteristics result in both a reduction of electromagnetic interference (EMI) and an overall improvement in
performance.
It is important to design a layout that prevents noise from coupling
onto the input signal. Digital signals should not be run in parallel with input signal traces and should be routed away from the
input circuitry. While the AD9240 features separate analog and
digital ground pins, it should be treated as an analog component.
The AVSS, DVSS and DRVSS pins must be joined together
directly under the AD9240. A solid ground plane under the A/D
is acceptable if the power and ground return currents are carefully managed. Alternatively, the ground plane under the A/D
may contain serrations to steer currents in predictable directions
where cross-coupling between analog and digital would otherwise be unavoidable. The AD9240/EB ground layout, shown in
Figure 57, depicts the serrated type of arrangement. The analog
and digital grounds are connected by a jumper below the A/D.
–19–
AD9240
output bits: large capacitive loads are to be avoided. Note that the
internal correction logic of the AD9240 is referenced DVDD
while the output drivers are referenced to DRVDD.
Analog and Digital Supply Decoupling
The AD9240 features separate analog and digital supply and
ground pins, helping to minimize digital corruption of sensitive
analog signals.
The decoupling shown in Figure 51, a 0.1 µF ceramic chip
capacitor, is appropriate for a reasonable capacitive load on the
digital outputs (typically 20 pF on each pin). Applications
involving greater digital loads should consider increasing the
digital decoupling proportionally and/or using external buffers/
latches.
120
DVDD
PSRR – dBFS
100
AVDD
80
DVDD
0.1mF
DRVDD
AD9240
DVSS
0.1mF
DRVSS
60
Figure 51. Digital Supply Decoupling
40
1
10
100
FREQUENCY – kHz
A complete decoupling scheme will also include large tantalum
or electrolytic capacitors on the PCB to reduce low-frequency
ripple to negligible levels. For more information regarding the
placement of decoupling capacitors, refer to the AD9240/EB
schematic and layouts in Figures 54–58.
1000
Figure 48. PSRR vs. Frequency
Figure 48 shows the power supply rejection ratio vs. frequency
for a 200 mV p-p ripple applied to both AVDD and DVDD.
In general, AVDD, the analog supply, should be decoupled to
AVSS, the analog common, as close to the chip as physically
possible. Figure 49 shows the recommended decoupling for the
analog supplies; 0.1 µF ceramic chip capacitors should provide
adequately low impedance over a wide frequency range. Note
that the AVDD and AVSS pins are co-located on the AD9240
to simplify the layout of the decoupling capacitors and provide
the shortest possible PCB trace lengths. The AD9240/EB power
plane layout, shown in Figure 58, depicts a typical arrangement
using a multilayer PCB.
AVDD
0.1mF
AVSS
AD9240
AVDD
0.1mF
AVSS
Figure 49. Analog Supply Decoupling
The CML is an internal analog bias point used internally by the
AD9240. This pin must be decoupled with at least a 0.1 µF
capacitor as shown in Figure 50. The dc level of CML is approximately AVDD/2. This voltage should be buffered if it is to
be used for any external biasing.
CML
0.1mF
AD9240
Figure 50. CML Decoupling
The digital activity on the AD9240 chip falls into two general
categories: correction logic and output drivers. The internal
correction logic draws relatively small surges of current, mainly
during the clock transitions. The output drivers draw large
current impulses while the output bits are changing. The size
and duration of these currents are a function of the load on the
APPLICATIONS
Direct IF Down Conversion Using the AD9240
Sampling IF signals above an ADC’s baseband region (i.e., dc
to FS/2) is becoming increasingly popular in communication
applications. This process is often referred to as Direct IF Down
Conversion or Undersampling. There are several potential benefits
in using the ADC to alias (or mix) down a narrowband or wideband IF signal. First and foremost is the elimination of a
complete mixer stage with its associated amplifiers and filters
reducing cost and power dissipation. Second is the ability to
apply various DSP techniques to perform such functions as
filtering, channel selection, quadrature demodulation, data
reduction, detection, etc. A detailed discussion on using this
technique in digital receivers can be found in Analog Devices
Application Notes AN-301 and AN-302.
In Direct IF Down Conversion applications, one exploits the
inherent sampling process of an ADC in which an IF signal
lying outside the baseband region can be aliased back into the
baseband region in a similar manner that a mixer will downconvert
an IF signal. Similar to the mixer topology, an image rejection
filter is required to limit other potential interfering signals from
also aliasing back into the ADC’s baseband region. A tradeoff
exists between the complexity of this image rejection filter and
the sample rate as well as dynamic range of the ADC.
Until recently, the actual implementation of Direct IF Down
Conversion has been limited by the lack of cost-effective ADCs
with sufficiently wide dynamic range and high sample rates for
IFs beyond 10.7 MHz. Since the performance of the AD9240
in the differential mode of operation extends well beyond its
baseband region, it may be well suited as a mix-down converter
in narrowband as well as some wideband applications. Also,
with the full-power bandwidth of the AD9240 extending beyond
60 MHz, various IF frequencies exist over this frequency range
in which the AD9240 maintains excellent dynamic performance.
Figure 52 shows the AD9240 configured in an IF sampling
application at 37.5 MHz. To reduce the complexity of the
digital demodulator in many quadrature demodulation applications, the IF frequency and/or sample rate are selected such that
–20–
REV. A
AD9240
the bandlimited IF signal aliases back into the center of the ADC’s
baseband region (i.e. FS/4). At a sample rate of 10 MSPS, an
image of the IF signal centered at 37.5 MHz will be aliased back
to 2.5 MHz which corresponds to one quarter of the sample rate
(i.e. FS/4). Note, the IF signal in this case will have undergone a
frequency inversion that may easily be corrected for in the digital
domain.
50V
50V
WORST CASE SPURIOUS AND SNR – dBc
G1 = 20dB
G2 = 12dB
AD8009
200V
AD9240
50V
VINA
AD8009
280V
22.1V
CML
0.1mF
200V
93.1V
MINI-CIRCUITS
T4-6T
60
SFDR w/5V SPAN
50
SFDR w/2V SPAN
SNR w/5V SPAN
40
30
SNR w/2V SPAN
20
10
–70
–50
–20
–60
–40
–30
INPUT POWER LEVEL – dBFS
–10
0
Figure 53. Single-Tone SNR/SFDR vs. Input Amplitude
@ 37.45 MHz
Figure 52. Simplified AD9240 IF Sampling Circuit
Figure 54 compares the two tone SFDR performance of the
AD9240 in the 2 V span with and without the use of the AD8009
gain stage. No degradation in distortion performance was noted
with the inclusion of the AD8009 gain stage provided that the
AD8009 2nd order distortion products are sufficiently attenuated by the bandpass filter.
To maximize its distortion performance, the AD9240 is configured in the differential mode using a transformer. Preceding the
AD9240 is a bandpass filter and a 32 dB gain stage. A large
gain stage may be required to compensate for the high insertion
losses of a SAW filter used for image rejection. The gain stage
will also provide adequate isolation for the SAW filter from the
transient currents associated with AD9240’s input stage.
100
The gain stage can be realized using one or two cascaded AD8009
op amps. The AD8009 is a low cost current-feedback op amp
having a third order intercept of 33 dB for a gain of +10 MHz
at 37.5 MHz. A passive bandpass filter is required after the
AD8009 to reduce the resulting second order distortion products and limit its out-of-band noise. The specifications of this
filter are application dependent and will affect both the total
distortion and noise performance of this circuit.
Figure 53 shows the single-tone SNR and SFDR performance of
the AD9240 configured in the 2 V and 5 V span without using
the AD8009 gain stage. Only a slight degradation in SNR performance (i.e., 1 dB) was noted with the inclusion of the AD8009
gain stage and a bandpass filter. Note, the tradeoff in SNR and
SFDR (dBFS) performance between the 5 V and 2 V spans at
different signal levels.
REV. A
70
0
–80
VINB
–21–
WORST CASE SPURIOUS – dBc AND dBFS
SAW FILTER
OUTPUT
80
90
80
w/o AD8009 – dBFS
w/AD8009 – dBFS
70
w/AD8009 – dBc
60
50
w/o AD8009 – dBc
40
85 dB REFERENCE LINE
30
20
10
0
–90
–80
–70
–60 –50
–40 –30 –20
INPUT POWER LEVEL (f1 = f2) – dBFS
–10
0
Figure 54. Two Tone SFDR vs. Input Amplitude
@ f1 = 36.40 MHz and f2 = 38.60 MHz
AGND J7
DGND J6
J11
+5_OR _+3
–VEE J5
+VCC J4
+5D J3
+5A J2
VINB1
VINB2
VINA1
VINA2
TP23
TP22
TP27
TP21
TP20
TP19
TP18
L5
L4
L3
L2
L1
2
JG1-WIRE
ETCH
CKT SIDE
C39
22mF
25V
C31
22mF
25V
A
C30
22mF
25V
A
C29
22mF
25V
A
A
C28
22mF
25V
+
+
+
+
+
3
JP5
JP4
JP3
C4
0.1mF
C3
+
0.1mF
A
C5
10mF
16V
JP6
3
1
C40
0.1mF
A
C35
0.1mF
A
C34
0.1mF
A
C33
0.1mF
A
C32
0.1mF
2
A
A
5 SETS OF
PADS TO
CONNECT
GROUNDS
VCC
+DRVDD
A
C2
0.1mF
RBIAS
C6
0.1mF
+DRVDD
VEE
VCC
+5VD
+5VA
1
A JP11 B
C7
0.1mF
A
A JP12 B
A
A
R2
10kV
R1
10kV
C1
10mF
16V
28
CAPT
C12
0.1mF
A
C43
0.1mF
5
6
2
29
OTR
VOUT
A
TP24
+5VA
25
A
1
28 J8
29 J8
30 J8
31 J8
10 J8
12 J8
14 J8
38 J8
24 J8
36 J8
NC 37 J8
20 J8
22 J8
NC 35 J8
18 J8
34 J8
32 J8
NC 39 J8
3
2
PRI
6 J8
8 J8
16 J8
R5
10kV
A
7
22
21
20
19
18
17
16
15
14
13
12
11
+5VD
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
T1
R3
15kV
4
5
A
A
R37
33V
C13
10mF
16V
A
JP1
JP22
JP21
C37
15pF
R38
33V
C36
15pF
A
R36
200V
C42
0.1mF
SEC
6
R4
50V
J9
A
2
3
7
R17
1kV
6
1
13
U8
3
U8
11
C15
0.1mF
R9
50V
A
J1
A
A
C38
A
C18
0.1mF
C24
0.1mF
C25
0.1mF
JP24
B
A
3
1
5
JP13
4
2
6
R13
10kV
JP23
74HC04
U5
U5
U5
1
2
3
A
C22
0.1mF
+5VD
2
7
VCC
SJ6
TP26
A
VEE
4
40 J8
CW
3
C21
0.1mF
6
C19
0.1mF
R34 TP9
22.1V
R33 TP8
22.1V
R32 TP7
22.1V
R31 TP6
22.1V
A
A
R30 TP17
22.1V
R29 TP5
22.1V
R28 TP4
22.1V
R27 TP3
22.1V
R26 TP16
22.1V
R25 TP15
22.1V
R24 TP14
22.1V
R23 TP13
22.1V
R22 TP12
22.1V
R21 TP11
22.1V
R20 TP10
22.1V
BUFFER AD845
R14
10kV
U5
DECOUPLING
C20
0.1mF
R10
500V
U4
JP18
JP17
R11
500V
20 +DRVDD
11
12
13
14
15
16
17
18
TPD
74HC541N
G1
Y7
G2
Y6
A7
Y5
A6
Y4
A5
Y3
A4
Y2
A3
Y1
A2
Y0
A1
A0
GND +5VD
U7
74HC541N
20 +DRVDD
11
12
13
14
15
16
17
18
DIRECT COUPLE OPTION
SPARE GATES
A
C26
0.1mF
+5VA
U6
G1
Y7
G2
Y6
A7
Y5
A6
Y4
A5
Y3
A4
Y2
A3
Y1
A2
Y0
A1
A0
GND +5VD
AC COUPLE OPTION
Q1
2N2222
C17
10mF
16V
1
19
CLK 9
D0 8
D1 7
D2 6
D3 5
D4 4
D5 3
D6 2
10
1
19
D7 9
D8 8
D9 7
D10 6
D11 5
D12 4
D13 3
2
10
JP14
A
U8
DECOUPLING
TPC
12
4
VIN
R7
1kV
TP25
R8
316V
R41
R40
CLK
JP16
CLKB
JP15
C16
0.1mF
TPD
10
A
R18
5kV
CW
74HC14
U8
2
U8
8
U8
A
A
8
12
R16
5kV
R6
820V
C14
0.1mF
9
A
CML
TPD
TPC
VINB1
U5
+5VA
+5VA
VINA1
VEE
4
AD817
U3
VCC
A
R19
50V
9
13
U5
C23
0.1mF
5
10
U8
U5
TP2
6
11
ADC_CLK
CLKIN
ADC_CLK
D13
EXTERNAL REFERENCE DRIVE
JP10
4 J8
A
JP8
C11
0.1mF
26 J8
R35
50V
A
J10
A
GND
3
CLK
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
BIT9
BIT10
BIT11
BIT12
BIT13
BIT14
2 J8
AIN
VIN
REF43
U2
C10
0.1mF
1
36
CAPB
39
CML
41
VINA
42
VINB
35 BIAS
37
4
C8
0.1mF
U1
24 D13
AD9240MQFP BIT1 23 D12
VREF
31
SENSE
33
REFCOM
32
DRVSS
CML
C41
0.1mF
+
A
AVDD2
JP7
AVSS2
TPD
JG1
DVSS
+5VA
SJ3
AVDD1
JP2
SJ1
DRVDD
TPC
SJ2
DVDD
TP1
SJ5
–22–
SJ4
AVSS1
C9
0.1mF
D2
1N5711
A
D1
1N5711
+5VA
R15
33V
A
D4
1N5711
D3
1N5711
AC COUPLE OPTION
+5VA
R39
R12
33V
15 J8
17 J8
19 J8
21 J8
23 J8
25 J8
27 J8
33 J8
1 J8
3 J8
5 J8
7 J8
9 J8
11 J8
13 J8
VINB2
VINA2
AD9240
Figure 55. Evaluation Board Schematic
REV. A
AD9240
Figure 58. Evaluation Board Ground Plane Layout
(Not to Scale)
Figure 56. Evaluation Board Component Side Layout
(Not to Scale)
Figure 57. Evaluation Board Solder Side Layout
(Not to Scale)
REV. A
Figure 59. Evaluation Board Power Plane Layout
(Not to Scale)
–23–
AD9240
OUTLINE DIMENSIONS
Dimensions shown in mm and (inches).
44-Lead MQFP (S-44)
1.03 (0.041)
0.73 (0.029)
C2937a–0–11/98
13.45 (0.529)
12.95 (0.510)
2.45 (0.096)
MAX
10.1 (0.398)
9.90 (0.390)
08
MIN
33
23
34
22
SEATING
PLANE
8.45 (0.333)
8.3 (0.327)
TOP VIEW
(PINS DOWN)
44
0.25 (0.01)
MIN
0.23 (0.009)
0.13 (0.005)
12
1
0.8 (0.031)
BSC
0.45 (0.018)
0.3 (0.012)
PRINTED IN U.S.A.
2.1 (0.083)
1.95 (0.077)
11
–24–
REV. A