AD8222 Analog Devices, AD8222 Datasheet - Page 18

• Manufacturer Part Number: AD8222
• Description: Precision, Dual-Channel Instrumentation Amplifier Data Sheet (Rev. 0, 7/2006)
• Manufacturer: Analog Devices

AD8222
Differential Input Voltages at High Gains
When operating at high gain, large differential input voltages
can cause more than 6 mA of current to flow into the inputs.
This condition occurs when the differential voltage exceeds
the following critical voltage
This is true for differential voltages of either polarity.
The maximum allowed differential voltage can be increased by
adding an input protection resistor in series with each input.
The value of each protection resistor should be
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 48. The
filter limits the input signal bandwidth according to the
following relationship.
where C
V
R
FilterFreq
FilterFreq
PROTECT
CRITICAL
D
≥ 10C
= (V
= (400 + R
Diff
CM
C
.
DIFF_MAX
=
=
2
2
π
π
RC
1
R
G
) × (6 mA)
(2
− V
C
C
1
D
CRITICAL
+
C
C
)
)/6 mA
Rev. 0 | Page 18 of 24
Figure 48 shows an example where the differential filter
frequency is approximately 2 kHz, and the common-mode filter
frequency is approximately 40 kHz.
Values of R and C
Mismatch between the R × C
R × C
using a value of C
the mismatch is reduced and performance is improved.
COMMON-MODE INPUT VOLTAGE RANGE
The three op amp architecture of the AD8222 applies gain and
then removes the common-mode voltage. Therefore, internal
nodes in the AD8222 experience a combination of both the
gained signal and the common-mode signal. This combined
signal can be limited by the voltage supplies even when the
individual input and output signals are not. Figure 7 and Figure 8
show the allowable common-mode input voltage ranges for
various output voltages, supply voltages, and gains.
C
at negative input degrades the CMRR of the AD8222. By
4.02kΩ
4.02kΩ
R
R
D
C
should be chosen to minimize RFI.
10× larger than the value of C
C
1nF
C
10nF
C
1nF
Figure 48. RFI Suppression
C
D
C
499Ω
R1
0.1µF
0.1µF
C
+IN
–IN
at the positive input and the
+15V
AD8222
–15V
REF
10µF
10µF
V
OUT
C
, the effect of