mcf52235 AD have a big nonlinear!

[gxd]

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.

[gxd]

when input is 0mv. AD result is 3mV. but i do not subtract it .

if subtract 5mV. it seem normal.

the multimeter is FLUKE 15B or 17B ,same result use them.

Message Edited by gxd on 2008-06-20 04:38 AM

Message Edited by gxd on 2008-06-20 04:56 AM

[admin]

I don't own a Fluke multimeter, so the model number doesn't tell me anything. And still we don't know anything about the source of input signal.
See, I think you are jumping to conclusions in this case. Precision measurements do require attention to detail.

[admin]

Hmm, in the first column of your table you only preserve 3 digits, yet you show the third column with 5-digit accuracy. That's slightly misleading.
How accurate is the multimeter?
What's the source of the input voltage? Are you sure that the measuring devices (ADC and multimeter) do not disturb it?
Did you take the offset error into account?