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adc unstable

 
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feitanx



Joined: 21 Mar 2010
Posts: 37

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adc unstable
PostPosted: Fri May 21, 2010 6:21 am     Reply with quote
what are the usual cause of unstable adc? how to correct it?
Gabriel



Joined: 03 Aug 2009
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PostPosted: Fri May 21, 2010 6:42 am     Reply with quote
your supply is unstable....

you have noisy signals close to your input...

you can try using a digital low pass filter like so:

(i do not take credit for this algorithm, another user posted it on an older thread)
Code:
filtered_value = (new_value + old_value+ old_value+ old_value)/4


or "olympic filter"---- search for "olympic" and youll get answers.

(i do not take credit for that algorithm ethier)

search for "filter" on the forum youll see several threads dealing with this.

you can put a small cap on your input... but that will limit your bandwith depending on the frequency of your input...and the size of the cap...


stabilize your supply....
keep your signal away from noise...
try using a digital filter...
try using a good voltage reference....
place 0.1uf caps on your PICs supply as close to the chip as possible.
try getting the signal from as close as you can to the source.... as to avoid long cables and noise....
try buffering your signal with op amp.


hope that helps...

gabriel
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CCS PCM 5.078 & CCS PCH 5.093
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Fri May 21, 2010 8:06 am     Reply with quote
Are you reading from multiple input channels? When yes, you have to take into account some time for the input voltage to settle before starting the data conversion. How long this wait should be depends on the connected source impedance. See the PIC datasheet for more details.

Another issue is the processor clock speed and the ADC conversion clock. More on this in: http://www.ccsinfo.com/forum/viewtopic.php?t=42413&start=7
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