Datasheet

Data Sheet AD7951
Rev. A | Page 21 of 32
DRIVER AMPLIFIER CHOICE
Although the AD7951 is easy to drive, the driver amplifier must
meet the following requirements:
 For multichannel, multiplexed applications, the driver
amplifier and the AD7951 analog input circuit must be
able to settle for a full-scale step of the capacitor array at a
14-bit level (0.006%). For the amplifier, settling at 0.1% to
0.01% is more commonly specified. This differs significantly
from the settling time at a 14-bit level and should be
verified prior to driver selection. The AD8021 op amp com-
bines ultralow noise and high gain bandwidth and meets
this settling time requirement even when used with gains
of up to 13.
 The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7951. The noise coming from
the driver is filtered by the external one-pole low-pass filter
as shown in Figure 27. The SNR degradation due to the
amplifier is



















2
3
2
2
log20
N
dB
NADC
NADC
LOSS
NefV
V
SNR
where:
V
NADC
is the noise of the ADC, which is:
20
10
22
SNR
INp-p
NADC
V
V 
f
–3dB
is the cutoff frequency of the input filter (3.9 MHz).
N is the noise factor of the amplifier (+1 in buffer
configuration).
e
N
is the equivalent input voltage noise density of the op
amp, in nV/√Hz.

The driver needs to have a THD performance suitable to
that of the AD7951. Figure 15 shows the THD vs. frequency
that the driver should exceed.
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The AD8022 can also be used when a dual version is needed
and a gain of 1 is present. The AD829 is an alternative in applica-
tions where high frequency (above 100 kHz) performance is not
required. In applications with a gain of 1, an 82 pF compensation
capacitor is required. The AD8610 is an option when low bias
current is needed in low frequency applications.
Since the AD7951 uses a large geometry, high voltage input
switch, the best linearity performance is obtained when using
the amplifier at its maximum full power bandwidth. Gaining
the amplifier to make use of the more dynamic range of the
ADC results in increased linearity errors. For applications
requiring more resolution, the use of an additional amplifier
with gain should precede a unity follower driving the AD7951.
See Table 8 for a list of recommended op amps.
Table 8. Recommended Driver Amplifiers
Amplifier Typical Application
ADA4841-x
12 V supply, very low noise, low distortion,
low power, low frequency
AD829 ±15 V supplies, very low noise, low frequency
AD8021 ±12 V supplies, very low noise, high frequency
AD8022
±12 V supplies, very low noise, high
frequency, dual
AD8610/
AD8620
±13 V supplies, low bias current, low
frequency, single/dual
VOLTAGE REFERENCE INPUT/OUTPUT
The AD7951 allows the choice of either a very low temperature
drift internal voltage reference, an external reference, or an
external buffered reference.
The internal reference of the AD7951 provides excellent perform-
ance and can be used in almost all applications. However, the
linearity performance is guaranteed only with an external reference.