Low propagation delay and high data rate: The digital isolator operates at a high SPI frequency (10 MHz-15 MHz) since it has to handle the aggregate data of all the channels in a serialized manner. To be able to support high SPI frequency, the isolator needs to have very low propagation delay and the ability to operate at high data rate.

Small footprint and high channel density: Since most PLC I/O modules don’t deal with voltages greater 60 V, there are no electrical safety considerations for the digital isolators. Hence digital isolators that support 2.5 kV_RMS withstand voltage in the smallest footprint and the highest channel densityare preferable. Some analog input modules support higher input common mode: up to 200 V. Even for these modules, since the input voltages are not directly connected to AC mains, basic isolation is sufficient.

Transient Immunity: While 2.5-kV_RMS isolation suffices, the isolation barrier must still withstand IEC ESD, EFT and surge transients which when applied to the analog inputs with respect to safety earth can stress the isolation barrier _[1].