ColdFire/68K Microcontrollers and ProcessorsのトピックRe: mcf52235 AD have a big nonlinear!

mcf52235 AD have a big nonlinear!

gxd — Thu, 19 Jun 2008 14:39:34 GMT

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.

Re: mcf52235 AD have a big nonlinear!

admin — Thu, 19 Jun 2008 20:06:29 GMT

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?

Re: mcf52235 AD have a big nonlinear!

gxd — Fri, 20 Jun 2008 10:36:09 GMT

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

Re: mcf52235 AD have a big nonlinear!

admin — Fri, 20 Jun 2008 10:57:13 GMT

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.