Datasheet

AD8236
Rev. 0 | Page 17 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 40).
The filter limits the input signal bandwidth according to the
following relationship:
)2(π2
1
GC
D
DIFF
CCCR
FilterFreq
++
=
)(π2
1
GC
CM
CCR
FilterFreq
+
=
Mismatched C
C
capacitors result in mismatched low-pass filters.
The imbalance causes the AD8236 to treat what would have been
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 times C
C
. This sets the differential filter frequency
lower than the common-mode frequency.
R
R
AD8236
+
V
S
–IN
+IN
10µF0.1µF
V
OUT
REF
C
C
C
C
C
D
1nF
1nF
10nF
4.02k
4.02k
08000-141
Figure 40. 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 34 shows the internal nodes of the
AD8236. Figure 20 to Figure 23 show the common-mode
voltage ranges for typical supply voltages and gains.
If the supply voltages and reference voltage is 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 2 and Table 3
.
2.
Calculate the output of the internal op amp, A. The following
equation calculates this output:
4R
k52.5
24
5
G
REF
DIFF
DIFF
CM
V
V
V
VA −−
⎟
⎠
⎞
⎜
⎝
⎛
−=
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