Not sure what you mean by input sensing of voltage, since "sensing of voltage" most often refers to a feedback situation, where you are sampling the output voltage, since it is the output voltage you wish to control.In a SMPS, both points A and B should be very low impedance at the maximum frequency of interest, which is related to the switching frequency and harmonics of the regulator, as well as to meet EMC requirements. You are least likely to have control of the source, but for the rest the PCB layout is important. The actual voltage appearing at the SMPS will be to a best approximation across the input filter.Cheers,Richard
