Place a differential RC filter (R1, R2, C5) in front of the isolated modulator to improve signal-to-noise performance of the signal path. Input noise with a frequency close to the ΔΣ modulator sampling frequency (typically 20MHz) is folded back into the low-frequency range by the modulator. The purpose of the RC filter at the input is to attenuate high-frequency noise below the desired noise level of the measurment. Design the input filter such that:

• The filter capacitance (C5) is a minimum of 10nF
• The cutoff frequency of the filter is at least one order of magnitude lower than the sampling frequency (f_CLKIN) of the ΔΣ modulator
• The dynamic input bias current does not generate significant voltage drop across the DC impedances (R1, R2) of the input filter
• The impedances measured from the analog inputs are equal (R1 equals R2)

Capacitors C6 and C7 are optional and improve common-mode rejection at high frequencies (>1MHz). For best performance, make sure C6 matches the value of C7 and that both capacitors are 10 to 20 times lower in value than C5. NP0-type capacitors offer low temperature drift and low voltage coefficients, and are preferred for common-mode filtering. For most applications, the structure shown in Figure 7-2 achieves excellent performance.

Figure 7-2 Input Filter