AD8235 datasheet(18/20 Pages) AD

18 / 20 page

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:

)

(

DIFF

FilterFreq

)

(

FilterFreq

Mismatched CC 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 CC capacitors, select a value of CD greater

than 10× CC. This sets the differential filter frequency lower than

the common-mode frequency.

AD8235

+VS

–IN

+IN

10µF

0.1µF

VOUT

REF

1nF

10nF

1nF

4.02kΩ

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:

Adhere to the input, output, and reference voltage ranges

shown in Table 1 and Table 2.

Calculate the output of Internal Op Amp A. The following

equation calculates this output:

kΩ

52.5

REF

DIFF

−

⎟

⎠

⎞

⎜

⎝

⎛

−

where:

VDIFF is defined as the difference in input voltages,

VDIFF = VINP − VINM.

VCM is defined as the common-mode voltage,

VCM = (VINP + VINM)/2.

If no gain setting resistor, RG, is installed, set RG to infinity.

Keep A within 10 mV of either supply rail. This is valid over

the −40°C to +125°C temperature range.

−VS + 10 mV < A < +VS – 10 mV

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