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

Most voltage-sensing applications use high-impedance resistor dividers in front of the isolated amplifier to scale down the input voltage. In this case, a single capacitor (as shown in Figure 9-2) is sufficient to filter the input signal.