AD8422 Data Sheet
+V
S
AD8422
–V
S
V
IN+
+
–
V
IN–
+
–
+V
S
AD8422
R
PROTECT
R
PROTECT
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
+V
S
AD8422
R
PROTECT
R
PROTECT
–V
S
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
–V
S
+V
S
AD8422
R
PROTECT
R
PROTECT
–V
S
I
V
IN+
+
–
V
IN–
+
–
I
SIMPLE CONTINUOUS PROTECTIONTRANSIENT PROTECTION
LOW NOISE CONTINUOUS
OPTION 2
LOW NOISE CONTINUOUS
OPTION 1
11197-064
Figure 61. Input Protection Options for Input Voltages Beyond Absolute
Maximum Ratings
At the expense of some noise performance, another solution is
to use series resistors. In the overvoltage case, current into the
inputs of the AD8422 is internally limited to a safe value for the
amplifier. Although the AD8422 inputs must still be kept within the
Absolute Maximum Ratings, the I × R drop across the protection
resistor increases the maximum voltage that the system can
withstand to the following values:
For positive input signals,
V
MAX_NEW
= (40 V + Negative Supply) + I
IN
× R
PROTECT
For negative input signals,
V
MIN_NEW
= (Positive Supply – 40 V) − I
OUT
× R
PROTECT
Overvoltage performance is shown in Figure 14, Figure 15,
Figure 16, and Figure 17. With gains greater than 100 and
supply voltages less than ±2.5 V, overdrive voltages beyond
the rails may cause the output to invert as far as the REF pin
voltage.
RADIO FREQUENCY INTERFERENCE (RFI)
RF rectification is often a problem when amplifiers are used in
applications that have 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 62.
R
R
AD8422
+V
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–V
S
R
G
C
D
10nF
C
C
1nF
C
C
1nF
2kΩ
2kΩ
11197-065
Figure 62. RFI Suppression
The filter limits the input signal bandwidth, according to the
following relationship:
)2(π2
C
D
DIFF
CCR
uencyFilterFreq
+
=
where C
D
≥ 10 C
C
.
C
D
affects the difference signal, and C
C
affects the common-mode
signal. Choose values of R and C
C
that minimize RFI. A mismatch
between R × CC at the positive input and R × CC at the negative
input degrades the CMRR of the AD8422. By using a value of C
D
that is one order of magnitude larger than C
C
, the effect of the
mismatch is reduced, and performance is improved.
Resistors add noise; therefore, the choice of the resistor and
capacitor values depends on the desired tradeoff between noise,
input impedance at high frequencies, and RF immunity. The
resistors used for the RFI filter can be the same as those used for
input protection.
Rev. A | Page 22 of 24
