AD8221
CD affects the difference signal, and CC affects the common-
RF INTERFERENCE
mode signal. ꢀalues of R and CC should be chosen to minimize
RFI. Mismatch between the R × CC at the positive input and the
R × CC at the negative input degrades the CMRR of the AD8221.
By using a value of CD one magnitude larger than CC, the effect
of the mismatch is reduced, and therefore, performance is
improved.
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 48. The
filter limits the input signal bandwidth according to the following
relationship:
PRECISION STRAIN GAGE
The low offset and high CMRR over frequency of the AD8221
make it an excellent candidate for bridge measurements. As
shown in Figure 49, the bridge can be directly connected to
the inputs of the amplifier.
1
FilterFreqDiff =
2πR(2CD +CC)
1
FilterFreqCM =
2πRCC
+5V
where CD ≥ 10CC.
10µF
0.1µF
+15V
350Ω
350Ω
350Ω
350Ω
+IN
–IN
+
0.1µF
10µF
R
AD8221
C
1nF
C
D
C
–
R
+IN
R1
+2.5V
4.02kΩ
V
OUT
C
10nF
1nF
AD8221
Figure 49. Precision Strain Gage
499Ω
R
REF
CONDITIONING 10 V SIGNALS FOR A +5 V
DIFFERENTIAL INPUT ADC
–IN
4.02kΩ
C
There is a need in many applications to condition 10 ꢀ signals.
However, many of today’s ADCs and digital ICs operate on
much lower, single-supply voltages. Furthermore, new ADCs
have differential inputs because they provide better common-
mode rejection, noise immunity, and performance at low supply
voltages. Interfacing a 10 ꢀ, single-ended instrumentation
amplifier to a +5 ꢀ, differential ADC can be a challenge.
Interfacing the instrumentation amplifier to the ADC requires
attenuation and a level shift. A solution is shown in Figure 50.
+12V
0.1µF
10µF
–15V
Figure 48. RFI Suppression
+2.5V
+12V
R3
1kΩ
0.1µF
+5V
10nF
+5V
R6
27.4Ω
+12V
10µF
0.1µF
AD8022
C1
470pF
(½)
0.1µF
+IN
–IN
AV
DV
DD
DD
VIN(+)
VIN(–)
0.1µF
–12V
AD8221
OP27
R1
C2
220µF
AD7723
10kΩ
REF
R5
+12V
499Ω
R2
10kΩ
0.1µF
AGND DGND REF1 REF2
0.1µF
10µF
0.1µF
R7
27.4Ω
–12V
AD8022
–12V
R4
(½)
1kΩ
220nF
10nF
0.1µF
–12V
2.5V
22µF
+5V
+V
V
OUT
IN
10µF
0.1µF
AD780
GND
Figure 50. Interfacing to a Differential Input ADC
Rev. B | Page 19 of 24
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