Differential Drivers for Precision ADCs

MT-074
TUTORIAL
Differential Drivers for Precision ADCs
DIFFERENTIAL INPUT ADC CHARACTERISTICS
Many high performance ADCs are now being designed with differential inputs. A fully
differential ADC design offers the advantages of good common-mode rejection, reduction in
second-order distortion products, and simplified dc trim algorithms. Although they can be driven
single-ended, a fully differential driver usually optimizes overall performance.
One of the most common ways to drive a differential input ADC is with a transformer. However,
there are many applications where the ADCs cannot be driven with transformers because the
frequency response must extend to dc. In these cases, differential drivers are required. This
tutorial focuses on driving high resolution 16- to 18-bit ADCs with sampling rates up to 10
MSPS. The bandwidth of the input signals is generally limited to a few MHz. Tutorial MT-075
discusses differential amplifiers suitable for driving higher speed ADCs.
Most high performance CMOS switched capacitor pipelined ADCs have differential inputs.
similar to that shown in Figure 1.
S5
SWITCHES SHOWN IN TRACK MODE
S3
CP
S1
VINA
CH
+
5pF
ZIN
S7
S2
A
CH
-
VINB
5pF
CP
S4
S6
ZIN IS A FUNCTION OF:
‹ TRACK MODE VS. HOLD MODE
‹ INPUT FREQUENCY
Figure 1: Simplified Input Circuit for a Typical Unbuffered Switched
Capacitor CMOS Sample-and-Hold
The differential structure is typically carried through most of the ADC. This makes matching
requirements easier as well as reduces second-order products. In addition, the differential
structure helps in common-mode noise rejection.
Rev.0, 10/08, WK
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MT-074
Note that the SHA switches are connected directly to each of the inputs. Switching transients can
be significant, because there is no isolation buffer. The drive amplifier settling time to the
transients must be fast enough so that the amplifier settles to the required accuracy in less than
one-half the sampling period (this settling time must include the effects of any external series
resistance).
The differential input impedance of this structure is dynamic and changes when the SHA
switches between the sample mode and the hold mode. In addition, the impedance is a function
of the analog input frequency.
In the track mode (shown in the figure), the input signal charges and discharges the hold
capacitors, CH. When the circuit switches to the hold mode the switches reverse their positions,
and the voltage across the hold capacitors is transferred to the outputs.
It is highly recommended that this type of input be driven differentially for common-mode
rejection of the switching transients. While it is possible to drive them single-ended (with one
input connected to the appropriate common-mode voltage), degradation in SFDR will occur
because the even-order distortion products are no longer rejected.
Figure 2 (A) shows each of the differential inputs of a typical unbuffered CMOS ADC as well as
the sampling clock. The inputs were driven with a 50 Ω source resistance. Note that a transient
occurs on each edge of the sampling clock because of the switching action previously described.
Figure 2(B) shows the differential input signal to the ADC under the same conditions as (A).
Note that most of the transient current glitches are cancelled because they are common-mode
signals. Note that for cancellation to be optimum the two inputs must be driven from a balanced
source impedance (the real and reactive components of the impedance must be matched).
(A) SINGLE ENDED
(B) DIFFERENTIAL
SAMPLING CLOCK
SAMPLING CLOCK
‹ Differential charge transient is symmetrical around mid-scale and
dominated by linear component
‹ Common-mode transients cancel with equal source impedance
Note: Data Taken with 50Ω Source Resistances
Figure 2: Typical Single-Ended (A) and Differential
(B) Input Transients of CMOS Switched Capacitor ADC
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MT-074
DRIVING PRECISION 16- AND 18-BIT DIFFERENTIAL INPUT ADCs
Figure 3 shows the ADA4941-1 driving the 18-bit PulSAR family of ADCs which have switched
capacitor inputs. This is a common application where the signal is single-ended and bipolar and
the ADC input is differential. Because of the high resolution, the drive amplifier must have low
distortion, low noise, and high dc accuracy, as well as the capability of performing the singleended to differential conversion. The ADA4941-1 is a low power (2.2 mA @ 3.3 V), low noise
(10.2 nV/√Hz @ 1 kHz), low distortion (110 dBc @ 100 kHz) differential driver for ADCs up to
18 bits. Small signal bandwidth is 31 MHz. The amplifier has rail-to-rail output, high input
impedance, and a user-adjustable gain.
The ADA4941-1 consists of two op amps. The lower one in the figure is configured as a noncommited non-inverting buffer (with external feedback resistor) and drives an inverting
amplifier. The feedforward and feedback resistors for the inverting amplifier are included in the
IC. Although there is extra phase shift and delay through the inverting amplifier, this does not
introduce significant error at the frequencies of interest (up to 1 MHz or 2 MHz).
VREF = +4.096V
VREF = +4.096V
0.1µF
+5V
+5V
+5V
0.1µF
11.3kΩ
9.53kΩ
+2.1V
ADR444
ADA4941-1
+
8.45kΩ
41.2Ω
REF
10.0kΩ
–
0.1µF
+2.1V +/– 2V
R
R
3.9nF
+2.1V – /+ 2V
+1.75V
+
VIN = ± 10V
IN+ INPUT RANGE=
8.192V p-p DIFF.
AD7690, 400kSPS
AD7691, 250kSPS
18-BIT
PulSAR
ADCs
VCM = +2.1V
10.2nV/√Hz
0.1µF
VDD
IN–
41.2Ω
SNR = 100dB
For AD7690
–
3.9nF
4.02kΩ
LPF CUTOFF = 1MHz
CF
806Ω
‹ After filter, noise = 13µV rms due to amp
‹ Signal = 8V p-p differential
‹ SNR = 107dB @ ADC input
Figure 3: ADA4941-1 Driving AD7690 18-Bit
PulSAR® ADC in +5V Application
In this application, the two resistor dividers set the output common-mode voltage of the
ADA4941-1 to +2.1 V so that the output only has to go to within 100 mV of ground. This allows
sufficient headroom for the rail-to-rail output stages of the amplifier and allows the entire circuit
to operate on a single +5 V supply.
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MT-074
The input range of the AD7690 and AD7691 is 2⋅VREF p-p differential. The reference used is the
ADR444 which is a 4.096 V reference. The 41.2 Ω resistors and the 3.9 nF capacitors for a
lowpass filter with a cutoff frequency of 1 MHz, suitable for use with the AD7690 which has an
input bandwidth of 9 MHz. The ADA4941-1 has an output noise spectral density of 10.2 nV/√Hz
for the configuration selected. Integrated over the filter bandwidth, this is 13 µV rms. This
corresponds to an SNR due to the op amp of 107 dB, which is 7 dB better than the 100 dB SNR
of the ADC.
Figure 4 shows another example of driving a high performance iCMOS™ Pulsar™ ADC (e.g.,
AD7634). There are many industrial applications where signals as great as ±10 V are standard.
The iCMOS family of ADCs was developed to handle these applications. Most iCMOS Pulsar
ADCs have differential inputs. Here, the ADA4922-1 is driving a 16-bit or 18-bit iCMOS
PulSAR ADC. It performs a single-ended to differential conversion.
+12V
0.1µF
VIN = ± 10V
ADA4922-1
+
+5V
LPF CUTOFF = 1MHz
41.2Ω
VCC
–
+ / – 10V
R
R
3.9nF
0.1µF
IN+
18, 16-BIT
iCMOS
PulSAR
ADCs
(e.g., AD7634)
OUTPUT
VN = 12nV/√Hz
– / + 10V
–
VDD
IN–
41.2Ω
VEE
+
3.9nF
‹ After filter, noise = 15µV rms due to amp
‹ Signal = 40V p-p differential
‹ SNR = 119dB @ ADC input
0.1µF
AD7634
SNR = 100dB
–12V
Figure 4: ADA4922-1 Driving AD7634 18-Bit iCMOS PulSAR ADC
in ±12V Industrial Application
The ADA4922-1 is a differential driver for 16-bit to 18-bit ADCs that have differential input
ranges up to 40 V p-p. Small signal bandwidth is 38 MHz. The ADA4922-1 is manufactured on
ADI’s proprietary second-generation XFCB process that enables the amplifier to achieve
excellent noise and distortion performance on high supply voltages.
Noise calculations using the 1 MHz lowpass filter yield 15 µV rms for the op amp. The signal
range of the ADC is 40 V p-p, which is 14.14 V rms. This yields an SNR of 119 dB due to the op
amp alone.
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MT-074
Using the AD7634 SNR of 100 dB, the rms ADC input noise contribution is calculated to be 141
µV rms. The combined input ADC noise is therefore 142 µV rms, and the contribution due to the
op amp is almost negligible.
REFERENCES
1.
Hank Zumbahlen, Basic Linear Design, Analog Devices, 2006, ISBN: 0-915550-28-1. Also available as
Linear Circuit Design Handbook, Elsevier-Newnes, 2008, ISBN-10: 0750687037, ISBN-13: 9780750687034. Chapter 2.
2.
Walter G. Jung, Op Amp Applications, Analog Devices, 2002, ISBN 0-916550-26-5, Also available as Op
Amp Applications Handbook, Elsevier/Newnes, 2005, ISBN 0-7506-7844-5. Chapter 3.
3.
Walt Kester, Analog-Digital Conversion, Analog Devices, 2004, ISBN 0-916550-27-3, Chapter 6. Also
available as The Data Conversion Handbook, Elsevier/Newnes, 2005, ISBN 0-7506-7841-0, Chapter 6.
4.
Walt Kester, High Speed System Applications, Analog Devices, 2006, ISBN-10: 1-56619-909-3, ISBN-13:
978-1-56619-909-4, Chapter 2.
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