Datasheet

AD8251
Rev. B | Page 20 of 24
RF INTERFERENCE DRIVING AN ADC
RF rectification is often a problem when amplifiers are used in
applications where there are strong RF signals. The disturbance
can appear as a small dc offset voltage. High frequency signals
can be filtered with a low-pass RC network placed at the input
of the instrumentation amplifier, as shown in Figure 58. The
filter limits the input signal bandwidth according to the following
relationship:
An instrumentation amplifier is often used in front of an ADC
to provide CMRR. Usually, instrumentation amplifiers require a
buffer to drive an ADC. However, the low output noise, low
distortion, and low settle time of the AD8251 make it an
excellent ADC driver.
In Figure 59, a 1 nF capacitor and a 49.9 Ω resistor create an
antialiasing filter for the AD7612. The 1 nF capacitor stores and
delivers the necessary charge to the switched capacitor input of
the ADC. The 49.9  series resistor reduces the burden of the
1 nF load from the amplifier and isolates it from the kickback
current injected from the switched capacitor input of the AD7612.
Selecting too small a resistor improves the correlation between
the voltage at the output of the AD8251 and the voltage at the
input of the AD7612 but may destabilize the AD8251. A trade-
off must be made between selecting a resistor small enough to
maintain accuracy and large enough to maintain stability.
)CC(R
1
FilterFreq
C
D
DIFF
+
=
2π2
C
CM
RC
1
FilterFreq
π2
=
where C
D
≥ 10 C
C
.
R
R
AD8251
+15
V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–15V
C
D
C
C
C
C
0
6287-057
0.1F
0.1F
1nF
49.9
AD7612
ADR435
+12V –12V
+5V
+15
V
–15V
A0
A1
WR
+IN
–
IN
10F0.1µF
10F0.1µF
REF
AD8251
DGNDDGND
06287-058
Figure 58. RFI Suppression
Values of R a n d C
C
should be chosen to minimize RFI. A
mismatch between the R × C
C
at the positive input and the
R × C
C
at negative input degrades the CMRR of the AD8251.
By using a value of C
D
that is 10 times larger than the value of
C
Figure 59. Driving an ADC
C
, the effect of the mismatch is reduced and performance is
improved.