Placing an RC filter in front of the isolated amplifier improves signal-to-noise performance of the signal path. In practice, however, the impedance of the resistor divider is high and only a small-value filter capacitor can be used to not limit the signal bandwidth to an unacceptable low value. Design the input filter such that:

• The cutoff frequency of the filter is at least one order of magnitude lower than the sampling frequency (20 MHz) of the internal ΔΣ modulator.
• The input bias current does not generate significant voltage drop across the DC impedance of the input filter. Because the device has an input bias current in the low nanoampere range, IR drops across the filter impedance are usually not a concern.

Most voltage-sensing applications use high-impedance resistive dividers in front of the isolated amplifier to scale down the input voltage. In this case, a single capacitor (as shown in Figure 8-2) is sufficient to filter the input signal.

Figure 8-2 Input Filter