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'[PIC:] PIC16F87X ADC channels'
2003\12\02@190112 by Keith L. Kovala

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While on the topic of jittery A/D conversion, will the use of an external
osc provide better stability in the A/D conversion as well?  I'm considering
using a 16F88 for a voltmeter project that is supposed to go inside a power
supply and trying to weigh the options of using the internal osc to minimize
components or use external osc.  I guess the answer of using an external
voltage reference is answered... if I want the accuracy to be better than
+/-20mV then I guess I'll use the external.  My other concern is I've read
that crystal osc's are susceptible to vibration... and well the transformer
does get some vibration going on inside the power supply.  Thoughts on using
an external osc or internal in this case?

Keith L. Kovala
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> {Original Message removed}

2003\12\02@190734 by Wouter van Ooijen

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> Thoughts on using an external osc or internal in this case?

Maybe a stupid question, but if you don't need the accuracy of an
external crystal why bother? Less components, and a few bonus pins!

Wouter van Ooijen

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2003\12\02@193907 by Dave Dilatush

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>Keith L. Kovala wrote...

>While on the topic of jittery A/D conversion, will the use of an external
>osc provide better stability in the A/D conversion as well?

Hmmm.  I doubt it.  The PIC's A/D is a successive-approximation
type, and I wouldn't think oscillator jitter would affect it
unless it got really extreme.  Clock phase noise can cause poor
performance in delta-sigma A/D converters, but PICs don't use
those.

>...My other concern is I've read
>that crystal osc's are susceptible to vibration... and well the transformer
>does get some vibration going on inside the power supply.  Thoughts on using
>an external osc or internal in this case?

Crystals can be damaged by high levels of shock, but I wouldn't
think the kind of vibration you're likely to get from the PS
transformer would do that.  Ceramic resonators are less fragile,
if you need the clock frequency accuracy; or you could just use
the RC oscillator if you don't.

Dave D.

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2003\12\02@194321 by

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He might have ment external crystal osc vs. internal *crystal* osc.
Not the internal *RC* osc.
If I read his post correctly...

And the question is, will the external crystal osc interfere
less with the ADC ? I think not, meybe better to sleep the
PIC during the conversion.

Jan-Erik.

> {Original Message removed}

2003\12\02@231953 by Michael J. Pawlowsky

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I remember reading an application note on Microchip's site about getting steady ADC results.
I think it called for putting it in sleep mode or something before the measurement.
I really don't recall right now, but do a search on their site

If you get the AN post the URL to the list.
If I find it I will do the same.

Regards,
Mike

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2003\12\03@030226 by

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Michael J. Pawlowsky wrote:

> I remember reading an application note on Microchip's site
> about getting steady ADC results.
> I think it called for putting it in sleep mode or something
> before the measurement.

Actualy, most (all?) data sheets for a PIC with builtin ADC
has a section headed "A/D operation during sleep" or
something like that. There is says by running the A/D
cinversion suring sleep, you "eliminate all digital switching
noise from the conversion" (this is from the 16F87x data sheet,
but the wording is similar on others).

Jan-Erik.

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2003\12\03@073159 by Olin Lathrop

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Keith L. Kovala wrote:
> While on the topic of jittery A/D conversion, will the use of an
> external osc provide better stability in the A/D conversion as well?

I doubt it.  The important thing is that the minimum Tad time requirement is
met.  As long as you set up the divider so that Tad is met with a
comfortable margin, but not way too long, I don't see how the clock speed
makes a difference.

By the way, I've found the PIC A/Ds to be very stable.  If youre readings
are jittery, then there is noise coming in from somewhere.  Is the input
signal properly filtered, of low enough impedence?  Have you given attention
to ground return currents of other parts of the circuit to make sure they
don't cause a measurement offset?

In any case, it's a good idea to read the A/D as often as you can manage,
then low pass filter in software for the final answer.


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2003\12\03@073408 by Olin Lathrop

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> He might have ment external crystal osc vs. internal *crystal* osc.

There is no internal crystal oscillator.

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2003\12\03@090227 by

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Olin Lathrop wrote:

> > He might have ment external crystal osc vs. internal *crystal* osc.
>
> There is no internal crystal oscillator.


I ment the canned external crustal oscillators where
you just feed a clock signal into the PIC.
And the internal oscillator (with uses an external crystal, yes...)

Jan-Erik.

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2003\12\03@090440 by

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Olin Lathrop wrote:
>
> Keith L. Kovala wrote:
> > While on the topic of jittery A/D conversion, will the use of an
> > external osc provide better stability in the A/D conversion as well?
>
> I doubt it....  I don't see how the clock speed makes a difference.

I don't think he ment different clock *speed*, but having
different clock *source* (int/ext osc).

Jan-Erik.

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2003\12\03@095748 by Mike

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On 3 Dec 2003 at 7:33, Olin Lathrop wrote:

> > He might have ment external crystal osc vs. internal *crystal* osc.
>
> There is no internal crystal oscillator.
>
Then all the specsheets I've read are wrong. You should call up all those companies
and complain :-)

"Internal Crystal Oscillator" has been touted in every spec sheet I've read from PICs
to RF chips for those chips that have them. The term Indicates that the active device
is on-chip.

BRs,
Mike

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2003\12\03@101056 by Herbert Graf

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> In any case, it's a good idea to read the A/D as often as you can manage,
> then low pass filter in software for the final answer.

       I agree, in fact, any time I have an A/D conversion I always try to do
this, and not just with the PIC's A/D. I've found this to be a good idea
even when using external A/Ds and chips that do A/D inside them (i.e.
digital temperature sensors). No matter how "clean" you make things there is
always the possibility of some external temporary influence from affecting
your readings slightly, this method brings some protection against this.
TTYL

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2003\12\03@125917 by Olin Lathrop

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Jan-Erik Soderholm XA (TN/PAC) wrote:
> I don't think he ment different clock *speed*, but having
> different clock *source* (int/ext osc).

He specifically mentioned jitter as an issue, which is a short term
variation of the clock speed.  I was trying to point out that small
variations in the bit resolution time (Tad) from one bit to the next
shouldn't effect the A/D conversion much.


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