A sensing circuit includes a delay chain and a decoder. The delay chain includes at least one delay unit, at least one cascading switch, and at least one feedback switch. The delay unit generates a delay signal according to an input signal and a reset signal. The cascading switch selectively passes the delay signal according to a control signal. The feedback switch selectively forms a feedback path of the delay unit according to the control signal. The decoder generates an output signal according to the delay signal. The delay unit is supplied by a work voltage. If the work voltage has noise, the noise will be detectable by analyzing the output signal of the decoder.