who can help ?
used a standard reference TPS3033. Reference Voltage=3.3V
and the VA used a LC filter , L=10uH (1A current) and a 10uF//0.1uF
measure a analog DC voltage ,10240 timers then sum them.
4 layers PCB. only have a CPU ,not add other circult component. and a ground layers. a 3.3V power layers.
A=input voltage(mv) B=10240 timer AD value C= B/A
117 mV 1373690 11740
190 mV 2331084 12268
288 mV 3570993 12399
683 mV 8578059 12559
1667mV 21032711 12617
2651mV 33473643 12626
input voltage is measure by FLUKE 17B multimeter.
I find when the voltage is low ,the C value is low.
when voltage is high, the C value is high.
my another 2 layers PCB go near to same result.
NOW it is not linear? whether this AD can not used? only do sth for show.