Datasheet
AD8420 Data Sheet
Rev. 0 | Page 22 of 28
CAPACITIVELY COUPLED
+V
S
C
R
R
C
–V
S
AD8420
1
f
HIGH-PASS
=
2RC
THERMOCOUPLE
+V
S
–V
S
10M
AD8420
TRANSFORMER
+V
S
–V
S
AD8420
CORRECT
V
OUT
V
OUT
THERMOCOUPLE
+V
S
–V
S
AD8420
CAPACITIVELY COUPLED
+V
S
C
C
–V
S
AD8420
TRANSFORMER
+V
S
–V
S
AD8420
INCORRECT
V
OUT
V
OUT
V
OUT
V
OUT
09945-061
Figure 65. Creating an I
BIAS
Path
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8420 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a return current path, create one, as shown in Figure 65.
RADIO FREQUENCY INTERFERENCE (RFI)
All instrumentation amplifiers can rectify high frequency out-of-
band signals. Once rectified, these signals appear as dc offset errors
at the output. High frequency signals can be filtered with a low-pass
RC network placed at the input of the instrumentation amplifier, as
shown in Figure 66. 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
V
OUT
R1 R2
+IN
+
V
S
–V
S
C
C
330pF
5%
C
C
330pF
5%
10µF
10µF
C
D
3300pF
0.1µF
0.1µF
R
20k
1%
R
20k
1%
AD8420
–IN
09945-064
Figure 66. Suggested RFI Suppression Filter
C
D
affects the differential signal and C
C
affects the common-mode
signal. Values of R and C
C
are chosen to minimize out of band
RFI at the expense of reduced signal bandwidth. Mismatch
between the R × C
C
at the positive input and the R × C
C
at the
negative input degrades the CMRR of the AD8420. By using a
value of C
D
that is at least one magnitude larger than C
C
, the
effect of the mismatch is reduced and performance is improved.
C
.