The invention concerns a sigma-delta switched capacitor analog to digital converter (ADC) having: an input line for receiving a signal to be converted; first, second and third inputs for respectively receiving first, second and third test voltages; and switching circuitry adapted to apply, during a test mode of the sigma-delta ADC, a ternary test signal to the input line by periodically selecting, based on a digital test control signal, one of the first, second or third test voltages to be applied to the input line.