In an analog-to-digital converter (ADC) having storage capacitors, active, top-plate, n-type, switch circuitry has an n-type transistor and gate-voltage control circuitry that generates the gate voltage to turn on and off the transistor. The control circuitry turns off the transistor by generating the gate voltage at a level that limits the gate-to-source voltage difference, thereby limiting GISL leakage current through the transistor that can otherwise jeopardize the accuracy of the ADC digital output value. In one implementation, when the transistor is to be off (for example, during the ADC conversion phase), the control circuitry generates the gate voltage to be at ground if the source voltage is below a reference voltage, and above ground if the source voltage is above the reference voltage. The switch circuitry can also be implemented using a p-type device or a transmission gate instead of the n-type device.