Datasheet

AD8422 Data Sheet
Rev. 0 | Page 22 of 24
+
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 63. 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 16, Figure 17,
Figure 18, and Figure 19. 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 64.
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 64. RFI Suppression
The filter limits the input signal bandwidth, according to the
following relationship:
)2(π2
1
C
D
DIFF
CCR
uencyFilterFreq


C
CM
RC
uencyFilterFreq
π2
1

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 × C
C
at the positive input and R × C
C
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.