Analog switch charge injection ($Q_{\mathrm{inj}}$)

Analog switch charge injection is charge transferred to a signal node when the switch control changes. Capacitive coupling from the internal control transition can cause an output step even with a constant analog input. The primary tutorial models coupling through gate-drain capacitance and shows the resulting step separately from retained-charge transients.^[ref]

For a node modeled by total capacitance $C_L$, define $Q_{\mathrm{inj}}$ as the net charge deposited there. Charge balance givesIn an original example, $Q_{\mathrm{inj}}=0.50\mathrm{pC}$ on $C_L=50\mathrm{pF}$ produces a $10\mathrm{mV}$ step. Doubling the capacitance would halve that step if the deposited charge were unchanged. In an actual circuit the charge distribution can also depend on the node impedances and signal level, so that last assumption needs testing.

A useful measurement holds the analog input constant, toggles the control and records the signal node at a specified delay after the transition. Varying load capacitance helps distinguish an approximately fixed charge from other transient effects. Probe capacitance belongs in the node model.

The sign is determined by the circuit and switching direction. A charge step is not the same as a finite switch-on delay or a residual from incomplete discharge. When the switch opens an integrating feedback path, its consequence can be a reset charge-injection pedestal; a longer preceding reset need not remove charge deposited afterward.