Driver Amplifiers for Analog-To-Digital Converters

Driver Amplifiers For
Analog-To-Digital Converters
Don Tuite Analog/Power Editor
FREQUENTLY ASKED QUESTIONS
ED Online 20791
What amplifiers are used to drive analog-to-digital converters (ADCs)?
Possibilities include single-ended
and differential inputs and outputs,
plus voltage feedback (VFB) or current feedback (CFB) in the control
loop. Specialized amplifiers may provide level shifting, interstage isolation,
single-ended to differential conversion,
differential to single-ended conversion,
plus attenuation or gain.
What are the considerations with
VFB and CFB amplifiers?
RF
+VS
+DIN
–DIN
RG
RG
+IN
VOCM
VOUT, dm
–IN
–VS
RF
1. Some differential voltage feedback amplifiers
include an additional VOCM input that allows the
0326FAQs-FIGURE
1
common-mode voltage
of the output
signal to
be shifted.
without requiring tightly matched
external components. Thus, differential outputs are very close to the ideal
of being identical in amplitude and are
exactly 180° out of phase. Also, if it is
necessary, preserving the dc content
of a signal can be accomplished via the
VOCM function.
When would I need a single-ended,
attenuating, level-translating ADC
driver? How does it work?
Industrial applications often involve
sensors driven by ±10-V signals. That’s a
With CFBs, closed-loop gain is largely
problem with single-ended input ADCs
independent of frequency. Also, CFB
fabricated to today’s smaller design
amps provide faster slew rate and lower ing and non-inverting inputs, some dif- rules because those ADCs are condistortion and perform well at higher ferential VFB ADC drivers have another strained to a smaller input signal swing.
gains than VFB amps. VFBs can offer input, VOCM, that shifts the common- A level-translating ADC driver takes a
lower noise and better dc performance mode voltage of the differential output large signal, reduces the amplitude,
than CFB amps. Other tradeoffs lie (Fig. 1). Like a VFB op amp, closed-loop and level-shifts the output commonin design constraints. With a VFB op gain is set by means of input and mode voltage so it is compatible with
amp, the circuit designer has consider- feedback resistances, but there must low-voltage, single-supply ADCs (Fig. 2).
able freedom in choosing the value of be separate, matched resistors for the For example, a 20-V p-p (±10-V) input
the feedback resistor, although higher inverting and non-inverting inputs.
signal riding on 0 V might be converted
resistance values may limit stability.
The internal common-mode feedback to a 4-V p-p signal riding on 2.5 V.
CFB amplifier datasheets specify the loop produces outputs that are highly
There are a number of other ways to
feedback resistor values. CFBs therefore balanced over a wide frequency range perform level-translation. It has been
lend themselves to applications
accomplished using multiple
that require higher gain levels.
amplifiers, single differential driv+5 V
ers like those described above,
What are the advantages of
or ADC drivers designed for
+4.048 V
differential ADC drivers?
level translation. The approach
+2.048 V
+VS
+10 V
+0.048 V
These drivers facilitate singleusing a single differential driver
SENSE
–IN
ended-to-differential
and
is simpler than the multi-ampliVDD
–
IN+
OUT
differential-to-differential confier approach, and the specialversions, common-mode level
function level translation driver
IN–
+
GND
REF
shifting, and amplification of
approach is simpler yet.
VIN
REF2
differential signals. They also –10 V
Such amps use internally laser+IN
VREF
REF1
exhibit lower distortion and
trimmed
resistors, ensuring high
–VS
faster settling time than singlegain accuracy, along with high
ended drivers.
common-mode rejection and low
offset. A final advantage is that,
How can a differential VFB 2. A level-translating driver, optimized for specific ADCs, provides
since the amp and ADC use the
ADC driver differ from a a convenient way to accurately match the wide voltage output of
same supply voltage as the ADC,
single-ended amplifier?
sensors excited by ±10 V (common in industrial applications) to a
there is no need for multiple
In addition to the usual invert- single-ended ADC input with
a
more
restricted
voltage
range.
power supplies.
0326FAQs-FIGURE 2
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If a driver has a 1-GHz, –3-dB bandwidth, can I use it at that frequency
to drive converter inputs?
If you’re driving a high-resolution
ADC, look beyond the –3-dB spec and
consider gain flatness and, in particular, harmonic distortion as a function
of frequency. Recall that in a VFB amp,
the –3-dB bandwidth figure simply
reflects the half-power point after the
amp’s open-loop gain starts its –6-dB/
octave roll-off. That provides a rough
figure for comparing amplifiers.
Your concern as a mixed-signal circuit designer must be to minimize
the effect of amplifier distortion on
the ADC’s effective number of bits
(ENOB) performance. ENOB is a
function of signal-to-noise ratio (SNR)
+ distortion (SINAD) across the whole
analog signal chain: ENOB = (SINAD
– 1.76)/6.02. So, look to the data sheet
graphs of harmonic distortion to make
your decision.
Drivers That Maximize ADC Performance
Voltage Feedback Differential ADC Driver Offers Desirable
Distortion/Power Consumption Ratio At 70 MHz
The ADA4939-1 achieves 82-dB spurious-free dynamic
range (SFDR) at 70 MHz while consuming less than 120
mW of power on a single 3.3-V supply. The device can be
used in either single-ended-to-differential or differential-todifferential configuration.
Ultralow-DistoRtion CurrentFeedback Differential ADC Driver
The ADA4927 is a low-noise, ultralow-distortion, highspeed, current-feedback differential amplifier that is an ideal
choice for driving high-performance ADCs with resolutions up
to 16 bits from dc to 100 MHz.
Single-Ended, Attenuating, Level-Translating ADC Driver
The AD8275 is a G = 0.2 difference amplifier that can be used to translate ±10-V signals to
a +4-V level. The device has fast settle time and low distortion, making it suitable for driving
16-bit SAR ADCs.
Why would I want to use an active
driver instead of a passive transformer?
To view Analog Devices’ complete list of ADC drivers,
please visit www.analog.com/ADCdrivers.
The main reasons are to get better pass-band flatness and to isolate
the signal from the noisy ADC input.
Transformers have a rather “lumpy”
frequency response. An amplifier
should produce less variability, typically
±0.1 dB over the frequency range. If
the design calls for wideband gain, an
amplifier provides a better match than
a transformer to the ADC’s inputs.
Still looking at frequency response,
some amplifiers provide dc coupling.
Transformers can’t deal with slowly
varying signals.
Because transformers are passive
devices and provide no interstage isolation, noise generated on the secondary
coil of the transformer from the ADC
input will pass through it back to the
original signal source. In contrast,
amplifiers buffer the signal source with
a low output impedance, providing 70
to 80 dB of interstage isolation from
the ADC input back to the original
signal source.
On the other hand, a consideration
that favors transformers is that at higher
frequencies, they may maintain better
SNR and spurious-free dynamic range
(SFDR). Nevertheless, within the first
or second Nyquist zone, a transformer
or an amplifier can be used.
Differential ADC Drivers
Supply
Voltage
(V)
-3 dB
BW
(MHz)
Slew
Rate
(V/μs)
HD2/
HD3
(-dBc)
@ BW
(MHz)
Voltage
Noise
(nV/
√Hz)
ADA4938-1
5, ±5
1000
4700
82/82
50
ADA4939-1
3, 5
1400
6800
83/97
70
ADA4927-1
5, ±5
2300
5000
87/89
100
Part #
IS/
Amp
(mA
Typ)
IS/Amp
(mA
Typ)
Price
($U.S./
1K)
2.6
37
16-LFCSP
3.84
2.3
36.5
16-LFCSP
3.84
1.4
20
16-LFCSP
3.79
3.63
AD8139
3, 5, ±5
410
800
85/89
1
2.25
24.5
8-LFCSP/
SOIC
AD8138
3, 5, ±5
320
1150
94/114
5
5
20
8-SOIC/
MSOP
3.8
3, 5
1900
6000
84/91
70
2.2
39.5
16-LFCSP
3.84
3 to 5.5
2500
9000
82/84
140
2.7
37
16-LFCSP
3.49
12
8-SOIC/
MSOP
1.67
2.2
8-LFCSP/
SOIC
2.42
Package
Price
($U.S./
1K)
8-MSOP
1.6
ADA4937-1
AD8352
AD8132
ADA4941-1
3, 5, ±5
2.7 to 12
350
30
1200
22
83/98
101/98
5
0.1
8
10.2
Difference Amplifier for Driving ADCs
Part #
AD8275
Supply
Voltage
(V)
3.3 to 15
-3 dB
BW
(MHz)
15
Settling
Time to
0.001%
(ns)
450
Input
Voltage
(VPP)
20
Output
Voltage
(VPP)
Output
Type
4
Singleended
Voltage
Noise
(nV/
√Hz)
40
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