the dynamic reference voltage y n ue89e a ue89e y ue8a0 n 1 2 u00b7 x ue8a0 n 1 4 u00b7 x ue8a0 n 1 1 8 u00b7 x ue8a0 n 2 u2026 this formula uses the present sampled value x n and the former sampled values x n u2212 1 x n u2212 2 x n u2212 3 to obtain the dynamic reference voltage y n by rearranging the equation a the relationship between the reference voltage y n and the sampled values can be rearranged to a transfer function y ue8a0 n 1 2 1 1 2 ue89e z 1 based on the transfer function the voltage generating unit 40 can be deemed as a low pass filter b ue89e y ue8a0 n ue89e 1 2 u00b7 x ue8a0 n 1 2 u00b7 x ue8a0 n 1 the above formula only utilizes a present sampled value x n and a previous sampled value x n u2212 1 to calculate the dynamic reference voltage y n the transfer function of equation b can be rearranged as y ue8a0 n 1 2 1 2 ue89e z 1 furthermore the sample and hold circuit 30 can be designed to perform two different sampling ways i e the full cycle sample and the half cycle sample the full cycle sample represent the sample action only occurs once at each complete cycle of the reference signal a with reference to fig2 for example if the sample and hold circuit 30 employs the full cycle sample way the sample action occurs at the moment.