Datasheet

AD8253 Data Sheet
Rev. B | Page 20 of 24
R
R
AD8253
+15
V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–15V
C
D
C
C
C
C
0
6983-057
Figure 58. RFI Suppression
Values of R and C
C
should be chosen to minimize RFI.
Mismatch between the R × C
C
at the positive input and the
R × C
C
at negative input degrades the CMRR of the AD8253.
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 ANALOG-TO-DIGITAL CONVERTER
An instrumentation amplifier is often used in front of an analog-
to-digital converter 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 AD8253
make it an excellent ADC driver.
In this example, a 1 nF capacitor and a 49.9 Ω resistor create an
antialiasing filter for the AD7612. The 1 nF capacitor also serves
to store and deliver 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 AD8253 and the voltage at the
input of the AD7612 but may destabilize the AD8253. A trade-
off must be made between selecting a resistor small enough to
maintain accuracy and large enough to maintain stability.
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
AD8253
DGNDDGND
06983-058
Figure 59. Driving an ADC