Gong xd

mcf52235 AD have a big nonlinear!

Discussion created by Gong xd on Jun 19, 2008
Latest reply on Jun 20, 2008 by Guest
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.