AD8231

Rev. 0 | Page 16 of 20

R

R

AD8231

+INA

–INA

0.1µF

10µF

10µF

0.1µF

REF

10nF

1nF

1nF

4.02kΩ

4.02kΩ

Figure 33. RFI Suppression

Figure 33 shows an example where the differential filter

frequency is approximately 2 kHz, and the common-mode filter

frequency is approximately 40 kHz.

negative input degrades the CMRR of the AD8231. By using a

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 3 through Figure 5 or use the following

formula:

V

04

.

0

2

V

04

.

0

−

+

<

×

±

<

+

−

S

DIFF

CM

S

V

Gain

V

V

V

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.