Datasheet
AD8221
Rev. C | Page 20 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 49. The
filter limits the input signal bandwidth according to the following
relationship:
)2(π2
1
CD
Diff
CCR
FilterFreq
+
=
C
CM
RC
FilterFreq
π2
1
=
where C
D
≥ 10C
C
.
R
R
AD8221
+15V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–15V
R1
499Ω
C
D
C
C
C
C
10nF
1nF
1nF
03149-048
4.02kΩ
4.02k
Ω
Figure 49. RFI Suppression
C
D
affects the difference signal, and C
C
affects the common-
mode signal. 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 the negative input degrades the CMRR of the AD8221.
By using a value of C
D
one magnitude larger than C
C
, the effect
of the mismatch is reduced, and therefore, performance is
improved.
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 50, the bridge can be directly connected to
the inputs of the amplifier.
+5V
+2.5V
03149-049
10µF 0.1µF
AD8221
+IN
–IN
R
350Ω
350Ω350Ω
350Ω
+
–
Figure 50. Precision Strain Gage
CONDITIONING ±10 V SIGNALS FOR A +5 V
DIFFERENTIAL INPUT ADC
There is a need in many applications to condition ±10 V 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 V, single-ended instrumentation
amplifier to a +5 V, 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 51.
+12V
+IN
–IN
0.1µF
10µF
0.1µF
10µF
–12V
R3
1kΩ
+2.5V
R4
1kΩ
REF
R5
499Ω
R2
10kΩ
R1
10kΩ
C1
470pF
+12V
0.1µF
0.1µF
–12V
+12V
+5V +5V
0.1µF
10nF
0.1µF
–12V
+12V
0.1µF
0.1µF
–12V
R6
27.4Ω
R7
27.4Ω
C2
220µF
10µF 0.1µF 22µF
+5V
2.5V
220nF 10nF
AD8221
AD8022
OP27
AD8022
AD7723
VIN(+)
AV
DD
AGND DGND REF1 REF2
DV
DD
VIN(–)
AD780
GND
+V
IN
V
OUT
03149-050
(½)
(½)
Figure 51. Interfacing to a Differential Input ADC