Datasheet

AD8426
Rev. 0 | Page 24 of 28
INPUT BIAS CURRENT RETURN PATH
The other AD8426 terminals should be kept within the supplies.
All terminals of the AD8426 are protected against ESD.
The input bias current of the AD8426 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a current return path, one should be created, as shown
in Figure 66.
For applications where the AD8426 encounters voltages beyond
the allowed limits, external current limiting resistors and low
leakage diode clamps such as the BAV199L, the FJH1100, or the
SP720 should be used.
THERMOCOUPLE
+V
S
REF
–V
S
AD8426
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8426
TRANSFORMER
+V
S
REF
–V
S
AD8426
INCORREC
T
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8426
1
f
HIGH-PASS
=
2RC
THERMOCOUPLE
+V
S
REF
–V
S
10M
AD8426
TRANSFORMER
+V
S
REF
–V
S
AD8426
CORRECT
09490-007
RADIO FREQUENCY INTERFERENCE (RFI)
RF interference is often a problem when amplifiers are used in
applications where there are strong RF signals. The precision
circuits in the AD8426 can rectify the RF signals so that they
appear as a dc offset voltage error. To avoid this rectification,
place a low-pass RC filter at the input of the instrumentation
amplifier (see Figure 67). The filter limits both the differential
and common-mode bandwidth, as shown in the following
equations:
)2(π2
1
C
D
DIFF
CCR
uencyFilterFreq
+
=
C
CM
RC
uencyFilterFreq
π2
1
=
where C
D
≥ 10 C
C
.
R
R
AD8426
+
V
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
OUT
–V
S
R
G
C
D
10nF
C
C
1nF
C
C
1nF
4.02k
4.02k
09490-008
Figure 67. RFI Suppression
Figure 66. Creating an Input Bias Current Return Path
C
D
affects the differential signal, and C
C
affects the common-
mode signal. Values of R and C
C
should be chosen to minimize
RFI. Any mismatch between the R × C
C
at the positive input
and the R × C
C
at the negative input degrades the CMRR of the
AD8426. By using a value of C
D
one order of magnitude larger
than C
C
, the effect of the mismatch is reduced, and performance
is improved.
INPUT PROTECTION
The AD8426 has very robust inputs and typically does not
need additional input protection. Input voltages can be up to
40 V from the opposite supply rail. For example, with a +5 V
positive supply and a −8 V negative supply, the part can safely
withstand voltages from −35 V to +32 V. Unlike some other
instrumentation amplifiers, the part can handle large differen-
tial input voltages even when the part is in high gain. Figure 16,
Figure 17, Figure 19, and Figure 20 show the behavior of the
part under overvoltage conditions.