To avoid aliasing, attenuate noise above half of the sampling frequency of the isolated modulator, depending on the oversampling ratio of the sinc³ decimation filter. Although the AMC1306M05 integrated amplifier has a bandwidth of 800 kHz and a high common-mode rejection ratio, an external anti-aliasing low-pass filter and optional common mode input filter capacitors are added for systems with either lower than 20-MHz modulator clock frequency or systems with higher frequency system noise. The cutoff frequency of the differential input filter R26, R29, and C40 is set for 770 kHz, and can be adjusted to the transient noise spectrum of the 3-phase inverter PCB. This input filter provides additional input signal attenuation of around 22 dB at 10 MHz, when a modulator clock of 20 MHz is used.

As a general guideline, the common mode capacitor C36 and C66 are chosen to be 10- to 20-times smaller than the differential filter capacitor. In this design 470 pF was chosen, which gives a cut-off frequency of 15-MHz. A mismatch of the two common mode capacitors 336 and C66 changes the corresponding low-pass filter cut-off frequency and hence can convert common mode noise around and above the cut-off frequency into residual differential noise. Capacitors with tolerance of 5% or better are chosen to make sure any residual differential noise is attenuated respectively by the lower cutoff frequency of the 770-kHz differential low-pass filter.