Datasheet

AD8235
Rev. 0 | Page 18 of 20
To eliminate high frequency common-mode signals while using
smaller source resistors, a low-pass RC network can be placed
at the input of the instrumentation amplifier (see Figure 42).
The filter limits the input signal bandwidth according to the
following relationship:
)
GC
D
C+2(2
1
DIFF
CCRπ
FilterFreq
+
=
)(2
1
GC
CM
CCRπ
FilterFreq
+
=
Mismatched C
C
capacitors result in mismatched low-pass filters.
The imbalance causes the AD8235 to treat what is a common-
mode signal as a differential signal. To reduce the effect of
mismatched external C
C
capacitors, select a value of C
D
greater
than 10× C
C
. This sets the differential filter frequency lower than
the common-mode frequency.
R
R
AD8235
+
V
S
–IN
+IN
10µF
0.1µF
V
OUT
REF
C
C
C
D
1nF
10nF
C
C
1nF
4.02k
4.02k
08211-040
SDN
Figure 42. RFI Suppression
COMMON-MODE INPUT VOLTAGE RANGE
The common-mode input voltage range is a function of the
input voltages, reference voltage, supplies, and the output of
Internal Op Amp A. Figure 35 shows the internal nodes of the
AD8235. Figure 20 to Figure 23 show the common-mode
voltage ranges for typical supply voltages and gains.
If the supply voltages and reference voltage are not represented
in Figure 20 to Figure 23, the following methodology can be
used to calculate the acceptable common-mode voltage range:
1.
Adhere to the input, output, and reference voltage ranges
shown in Table 1 and Table 2.
2.
Calculate the output of Internal Op Amp A. The following
equation calculates this output:
4
V
V
R
kΩ52.5
2
V
V
4
5
A
REF
DIFF
G
DIFF
CM
−−
⎟
⎠
⎞
⎜
⎝
⎛
−=
where:
V
DIFF
is defined as the difference in input voltages,
V
DIFF
= VINP − VINM.
V
CM
is defined as the common-mode voltage,
V
CM
= (VINP + VINM)/2.
If no gain setting resistor, R
G
, is installed, set R
G
to infinity.
3.
Keep A within 10 mV of either supply rail. This is valid over
the −40°C to +125°C temperature range.
−V
S
+ 10 mV < A < +V
S
– 10 mV