Switching Phenomenon
Precision analog switches introduce unwanted parasitic voltages into sensitive signal paths during transition events. This charge injection occurs when the control gate voltage of a metal-oxide-semiconductor transistor changes rapidly, forcing some of the channel charge into the source and drain terminals. The resulting offset alters the voltage stored on downstream sampling capacitors.
Measurement accuracy in multiplexed data acquisition systems is heavily affected by this behavior.
Error Mechanism
Parasitic capacitance between the gate electrode and the channel acts as the primary pathway for the unwanted charge transfer. When the gate control signal transitions from high to low, the capacitive coupling forces a specific quantity of coulombs into the signal path. This injected charge is proportional to the gate voltage swing and the size of the transistor switch.
The resulting voltage error varies inversely with the value of the holding capacitance.
Mitigation Strategy
Differential topologies and charge cancellation circuits reduce the magnitude of the disturbance. Switch designs often include dummy transistors driven by inverted control signals to absorb the injected charge.
Calibration Protocol
Experimental verification requires measuring the change in output voltage across a known capacitor when the switch transitions. Precision bench setups use low-noise electrometers to measure the resulting offset voltage and compute the total injected charge in picocoulombs. Designers perform these measurements at multiple common-mode voltages because the switch impedance is non-linear across the input range.
The collected offset values are then loaded into the system calibration tables to be subtracted during active measurements.
