Anyway, I have a PM129B LED display, circuit ground same as input ground. Input impedance guaranteed to be >1000Meg. Max input = 199.9 millivolts DC. Labeled as 3 1/2 digits which I believe means a one on the left and three digits of any integer. I am using a cell phone USB charger to give it 5VDC, isolated, and I want to measure the output of my Variac. First thought: Diode, capacitor, voltage divider. It takes 15 uf to filter the AC down to half a significant digit with a 1 meg voltage divider. That causes a time constant of 15 seconds. I cant wait nearly a minute to get the next reading to stabilize. I have been playing with the math to design a multi-stage R-C filter to get a faster time constant, but the math is tedious and the circuit gets bulky. I think I must be doing something wrong. Maybe my whole approach is wrong.

How to get 125.0V RMS down to 125.0 millivolts DC?