Transient effects when turning a switch off or on are of concern in certain
applications. Short duration spikes are generated (Figure 2 (a)) as a result of
capacitive coupling between digital signals and the analog output. These have
the effect of creating an acquisition time interval during which the output
level is invalid even when little or no steady state level change is involved.
A large charge injection will change the output voltage level.
The total charge injection coupled to the analog circuit is of concern when
switching the voltage on a capacitor, since the injected charge will change the
capacitor voltage at the instant the switch is opened (Figure 2 (b)).
Charge injection is measured in picocoulombs;
a general equation for the voltage transferred to the capacitor is computed by:
V = Charge (pC)
Both of these effects are, in general, considerably less for CMOS switches than
for equivalent resistance JFET or PMOS devices, since the gate drive signals
for the two switching transistors are of opposite polarity.