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'[PICLIST] [PIC] Q about internal A/D converters of'
2001\01\31@091916 by Adolfo Cobo

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Hello to all,
I have a problem with the internal A/D converters of a PIC16F877,
problably due to a misconfiguration of the port, but I can't find the
solution. The converter seems to work, but only for low voltages (close to
0V) and for high voltages (close to 5V). For voltages in between, the
values are very strange, with jumps, continuous changes, ...
¿any idea?
Thanks to all in advance,
  Adolfo.



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|         Adolfo Cobo Garcia  -  UNIVERSIDAD DE CANTABRIA                 |
|    E.T.S.I.I. y Telecomunicacion, Grupo de Ingenieria Fotonica          |
|        Avda. Los Castros s/n  E-39005  Santander  SPAIN                 |
|  Tfno. +34-942-201539  Fax +34-942-201873  Email: spam_OUTacoboTakeThisOuTspamteisa.unican.es |
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2001\01\31@093839 by Martin Hill

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What are you driving the ADCs with?  How long are you leaving it to
settle before taking the sample?

Martin


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2001\01\31@095125 by Michael Rigby-Jones

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> -----Original Message-----
> From: Adolfo Cobo [SMTP:acobospamspam_OUTTEISA.UNICAN.ES]
> Sent: Wednesday, January 31, 2001 2:18 PM
> To:   @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
> Subject:      [PIC] Q about internal A/D converters of 16F877
>
> Hello to all,
>  I have a problem with the internal A/D converters of a PIC16F877,
> problably due to a misconfiguration of the port, but I can't find the
> solution. The converter seems to work, but only for low voltages (close to
> 0V) and for high voltages (close to 5V). For voltages in between, the
> values are very strange, with jumps, continuous changes, ...
>  ¿any idea?
>  Thanks to all in advance,
>    Adolfo.
> Check the following:

1) You have a valid Tad (conversion bit time).  This must be >= 1.6us.
Check your A/D clock source and/or prescaler (bits ADCS1 and ADCS0)

2) Ensure you are waiting for the required acquisition time after setting
the input channel.  This depends on the source impedance of the parameter
being measured, and (to a lesser extent) temperature.  This will most likely
be in the 20-40us range.

3) Are you using an external reference, or using Vdd and Vss?  In either
case ensure that the reference is well decoupled, this is critical if you
are using Vdd/Vss as references, the PIC must be well decoupled.

4) Ensure your signal is noise free.  Consider adding a low pass filter if
it is noisy.

Mike

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2001\01\31@095944 by Bob Ammerman

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If you are sourcing your A/D with a potentiometer it's impedence will be
much lower near the limits of its travel than in the middle.

You may have too high a source impedence when at midrange values.


Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: Michael Rigby-Jones <RemoveMEmrjonesTakeThisOuTspamNORTELNETWORKS.COM>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Wednesday, January 31, 2001 9:51 AM
Subject: Re: [PIC] Q about internal A/D converters of 16F877


> {Original Message removed}

2001\01\31@173326 by Stuart O'Reilly

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Have you set the tris registers properly for the ports, they need to be
set to input.
Regards
Stuart

Adolfo Cobo wrote:
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'[PICLIST] [PIC] Q about internal A/D converters of'
2001\02\01@064126 by Vasile Surducan
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You may use ad in different way ( interrupt, polling etc. )
It's important if you have used +/- ref to increase AD gain, if you
do that for too much gain, ad resolution will decrease and you'll see
the result in steps as power of 2.
AD conversion clock must be set according with Xtal. The AD result format must be set as you expect to use it ( left-right
justified )
Be sure you read corret lsb and msb result by switching bank page
accordingly.
And finaly, use a good gnd connection between power and input source, if
no external reference is used, check with a scope if you have a nice and
clean VCC
There is an ad functional example at http://www.piclist.com, somewhere at 877
examples.

Vasile


On Wed, 31 Jan 2001, Adolfo Cobo wrote:

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2001\02\01@113420 by Mike Mansheim

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>I have a problem with the internal A/D converters of a PIC16F877,
>problably due to a misconfiguration of the port, but I can't find the
>solution. The converter seems to work, but only for low voltages (close to
>0V) and for high voltages (close to 5V). For voltages in between, the
>values are very strange, with jumps, continuous changes, ...

This may seem very basic, but the F87x's are 10 bit a/d's vs. 8 bit in
similar 'C' parts. Make sure you understand exactly how the data is
being justified, and that you are properly using the two registers that
the results are reported in.
For example:
if ADFM = 1 (right justified), but you are only looking at ADRESL, the
result will appear to go from 0FFh to 0 at 1.25V, as the 10 bit result
transitions from 0FFh to 100h.
Also basic, but if you have a scope available, always make sure you have
what you think you have on the a/d pin (voltmeters can mislead).

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