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ADC problem with 16F877A

 
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mindstorm88



Joined: 06 Dec 2006
Posts: 102
Location: Montreal , Canada

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ADC problem with 16F877A
PostPosted: Wed Jan 06, 2010 2:48 pm     Reply with quote

Hi guys, I don't think that I have a code problem, so here is the question :why adjacent channels affect each others ?? Right now if I have a change on AN1 it affects my reading on AN0. Why ? They are 2 voltages measurement.
PCM programmer



Joined: 06 Sep 2003
Posts: 20137

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PostPosted: Wed Jan 06, 2010 3:36 pm     Reply with quote

Do you have a 20us delay after changing the ADC channel ?
You need one. Example:
Code:

set_adc_channel(0);
delay_us(20);
.
.
.
set_adc_channel(1);
delay_us(20);


Is the output impedance of the device that is supplying the analog voltage
to pins AN0 and AN1 equal to 10K ohms or less? That's the maximum
allowed by the 16F877 data sheet.
mindstorm88



Joined: 06 Dec 2006
Posts: 102
Location: Montreal , Canada

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PostPosted: Fri Jan 08, 2010 10:44 pm     Reply with quote

Thanks PCM , you were right , forgot the 20uS delay and one of the inputs was at 500 kOhms !!!! now down to 8 k , reading are stable now !!!
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