6 Capacitance

For transient and AC systems, switch capacitance can impact signal quality. See Figure 6-1. There are two main capacitive components of a MOSFET. The capacitance across the switch and the capacitance on the switch to ground. In many PhotoRelay data sheets, the capacitance across the switch is specified as Coff. This capacitance defines the AC isolation of the switch as a high pass filter. The larger this capacitance, the more AC voltage will be passed over the switch when off. It is also important to note that this capacitance is not flat across frequency, and typically increases at higher frequencies. For Multiplexer devices, this parameter is often not defined in the data sheets and instead, off-isolation is defined directly as the performance when the switch is off. But these values can be easily converted using Figure 6-1.

The other key parasitic is the capacitance to ground on a multiplexer. This is described as Coff when the switch is off and Con when the switch is on in the data sheet. This capacitance defines the AC performance as a low-pass filter. The larger this capacitance, the more signals will be attenuated at high frequencies. In Photorelays, there is no direct ground reference. So, the equivalent to this capacitance is defined as total capacitance. Often times the key care about in a system is actually the RC of the switch. This is defined by the on-resistance and the capacitance multiplied together. Typically the lower the RC, the better the switch will perform at high speed.