Datasheet

Data Sheet AD8250
Rev. C | Page 19 of 24
RF INTERFERENCE
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 54. The filter
limits the input signal bandwidth according to the following
relationship:
)(22
1
C
D
DIFF
CCR
FilterFreq



C
CM
RC
FilterFreq
 2
1

where
C
D
≥ 10 C
C
.
R
R
AD8250
+15
V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
–15V
C
D
C
C
C
C
0
6288-061
OUT
Figure 54. RFI Suppression
Values of R and 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 the negative input degrades the CMRR of the AD8250.
By using a value of C
D
that is 10 times larger than the value of
C
C
, the effect of the mismatch is reduced and performance is
improved.
DRIVING AN ADC
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 AD8250 make it an excellent
ADC driver.
In this example, a 1 nF capacitor and a 100 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 100  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 AD8250 and the voltage at the
input of the AD7612 but may destabilize the AD8250. A trade-
off must be made between selecting a resistor small enough to
maintain accuracy and large enough to maintain stability.
0.1μF
0.1μF
1nF
100Ω
AD7612
ADR435
+12V –12V
+5V
+15
V
–15V
A0
A1
WR
+IN
–
IN
10μF0.1µF
10μF0.1µF
REF
AD8250
06288-062
DGNDDGND
Figure 55. Driving an ADC