Datasheet

AD8231
Rev. B | Page 20 of 24
If more common-mode range is required, the simplest solution is
to apply less gain in the instrumentation amplifier. The extra op
amp can be used to provide another gain stage after the in-amp.
Because the AD8231 has good offset and noise performance at low
gains, applying less gain in the instrumentation amplifier generally
has a limited impact on the overall system performance.
RF INTERFERENCE
RF rectification is often a problem when amplifiers are used in
applications where there are 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 52.
The filter limits the input signal bandwidth according to the
following relationship
REDUCING NOISE
Because the AD8231 has no 1/f noise, reducing the bandwidth
corresponds directly to less noise. Table 8 shows the AD8231
performance at a gain of 1 at different bandwidths, assuming a
2-pole Butterworth filter roll off.
Table 8. AD8231 noise at various bandwidths
)(22
1
CD
Diff
CCR
FilterFreq
+π
=
C
CM
RC
FilterFreq
π
=
2
1
SNR
Single-Ended
1
SNR Differential
Output
2
where
C
D
≥ 10C
C
.
Bandwidth
(Hz)
Noise
(µV rms)
R
R
AD8231
+
V
S
+INA
–INA
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–V
S
C
D
10nF
C
C
1nF
C
C
1nF
4.02kΩ
4.02kΩ
06586-034
dB Bits dB Bits
1 0.07 148.3 24.3 154.3 25.3
3.2 0.12 143.2 23.5 149.2 24.5
10 0.21 138.3 22.7 144.3 23.7
32 0.37 133.2 21.8 139.2 22.8
100 0.66 128.3 21.0 137.63 22.0
320 1.17 123.2 20.2 129.2 21.2
1 k 2.07 118.3 19.3 124.3 20.3
3.2 k 3.71 113.2 18.5 119.2 19.5
10 k 6.55 108.3 17.7 117.3 18.7
32 k 11.73 103.2 16.9 109.2 17.9
1
SNR for single-ended output configuration calculated with output signal of
4.8 V p-p, which corresponds to 1.697 V rms.
Figure 52. RFI Suppression
2
SNR for differential output configuration calculated with output signal of
9.6 V p-p, which corresponds to 3.397 V rms.
Figure 52 shows an example where the differential filter frequency
is approximately 2 kHz, and the common-mode filter frequency
is approximately 40 kHz.
The AD8231 has two clocks: an auto-zero clock at 3.4 kHz and
a commutating clock at 54 kHz. While the auto-zero clock has
negligible energy and can generally be ignored, the commutating
clock has enough energy to significantly affect the noise of the
part. Therefore, in applications where low noise is critical, limiting
the bandwidth of the system below 54 kHz is recommended.
Values of R and C
C
should be chosen to minimize RFI. Mismatch
between the R × C
C
at the positive input and the R × C
C
at the
negative input degrades the CMRR of the AD8231. By using a
value of C
D
that is ten times larger than the value of C
C
, the
effect of the mismatch is reduced and performance is improved.
COMMON-MODE INPUT VOLTAGE RANGE
The 3-op amp architecture of the AD8231 applies gain and then
removes the common-mode voltage. Therefore, internal nodes
in the AD8231 experience a combination of both the gained
signal and the common-mode signal. This combined signal can
be limited by the voltage supplies even when the individual input
and output signals are not. To determine whether the signal could
be limited, refer to Figure 12 through Figure 14 or use the
following formula
V04.0
2
V04.0 −+<
×
±<+−
S
DIFF
CMS
V
GainV
VV