> The Wilkinson ADC is based on the comparison of an input voltage with that produced by a charging capacitor. The capacitor is allowed to charge until a comparator determines it matches the input voltage. Then, the capacitor is discharged linearly. The time required to discharge the capacitor is proportional to the amplitude of the input voltage. While the capacitor is discharging, pulses from a high-frequency oscillator clock are counted by a register. The number of clock pulses recorded in the register is also proportional to the input voltage.

The capacitor is discharged using a resistor of known value.

Also take it down to the comparator level which basically all ADC use the op-amps use transistors and resistors and are themselves dissipating the input power in order to measure voltage.

The word transistor is a contraction of transfer resistance, they are electronically variable resistors and as everyone knows dissipate power as heat during function.