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 INN or INP 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.