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 so high that adding a filter capacitor on the IN pin limits the signal bandwidth to an unacceptable low limit, such that the filter capacitor is omitted. When used, 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

Most voltage-sensing applications use high-impedance resistor dividers in front of the isolated amplifier to scale down the input voltage. In that case, no additional resistor is needed and a single capacitor (as shown in Figure 8-2) is sufficient to filter the input signal.