AD8231ACPZ-R7 datasheet(21/25 Pages) AD

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AD8231

Data Sheet

Rev. C | Page 20 of 24

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

)

Diff

FilterFreq







FilterFreq





where CD ≥ 10CC.

AD8231

+VS

+INA

–INA

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

4.02kΩ

Figure 52. RFI Suppression

Figure 52 shows an example where the differential filter frequency

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

is approximately 40 kHz.

Values of R and CC should be chosen to minimize RFI. Mismatch

between the R × CC at the positive input and the R × CC at the

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

value of CD that is ten times larger than the value of CC, 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

















DIFF

Gain

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.

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

Bandwidth

(Hz)

Noise

(μV rms)

SNR

Single-Ended1

SNR Differential

Output2

Bits

0.07

148.3

24.3

154.3

25.3

3.2

0.12

143.2

23.5

149.2

24.5

0.21

138.3

22.7

144.3

23.7

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.

2 SNR for differential output configuration calculated with output signal of

9.6 V p-p, which corresponds to 3.397 V rms.

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.

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