Off capacitance between the source and drain (CDS(OFF)) is a measure of an off switch’s ability to block a signal on the source from coupling to the drain. It’s a common specification seen in solid-state relays (also known as PhotoMOS, OptoMOS, photorelays, or MOSFET relays), and is often referred to as output capacitance (COUT) in solid-state relay datasheets.
CMOS switches don’t usually include this specification. However, the off-isolation spec is a different method to characterize the same phenomenon—i.e., the amount of a signal presented to the source of an off switch that couples to the drain.
This article will discuss how to derive CDS(OFF) from off isolation and how this can be used to compare the performance of solid-state relays and CMOS switches more effectively. This is important as CMOS switches are a fit for many applications where solid-state relays are used, such as switching dc and high-speed ac signals.