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'[PIC]: Floating analog input causing trouble?'
2001\05\20@215850 by Drew Vassallo

picon face
Yes, I did something careless yesterday... I have an external analog source
0-4.1 V that I have to use a clip lead to connect the output of one circuit
into the PIC's A/D input.  I have a 4.096 Vref into the PIC's Vref.

However, in my haste to get a test going, I accidentally forgot to clip the
analog source into the PIC's A/D input pin, so it was just left floating.
Later, I noticed that my Vref is no longer accurate at 4.096, but instead is
around 4.80 V.

My question is: could I have partially burned out the voltage reference by
leaving that pin floating?  The code always attempts to read the pin, so
yes, the PIC was actively trying to read the input.  Not being an EE myself,
I don't really understand what happens in these circumstances, I just try
not to do things like this :)

--Andrew
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2001\05\21@032059 by Vasile Surducan

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face
On Sun, 20 May 2001, Drew Vassallo wrote:

> My question is: could I have partially burned out the voltage reference by
> leaving that pin floating?  The code always attempts to read the pin, so
> yes, the PIC was actively trying to read the input.  Not being an EE myself,
> I don't really understand what happens in these circumstances, I just try
> not to do things like this :)
>

 I think not ( if I understand well your question  ) Your reference
voltage chip may be defective or you haven't use correct connections and
decoupling capacitors.
 I've done a lot of tests with 877 analogic inputs including negative and
out of spec experiments and never happent such damage, nor with
external reference chips or with analogic inputs even if where floating.
 Check if you have used decoupling capacitors on the reference output
( and pic reference input ) this may cause troubles or if you accidentaly
program pic vref as digital output.
 Take care,
Vasile

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2001\05\21@090516 by Olin Lathrop

face picon face
> However, in my haste to get a test going, I accidentally forgot to clip
the
> analog source into the PIC's A/D input pin, so it was just left floating.
> Later, I noticed that my Vref is no longer accurate at 4.096, but instead
is
> around 4.80 V.

I can't say whether the PIC got damaged, but it is certainly possible.  A
PIC A/D input can tolerate up to 10K ohms of source impedance and still
maintain accuracy.  I would put a resistor in series with the PIC input to
limit the current in case of connection to an out of range voltage.  A total
source impedance at the PIC pin of a bit under 10K (5k - 8K) is a good
target.  If the source has too high an impedance so that the extra resistor
would come out to less than 3K or so, you should buffer the signal thru an
op amp.  That will also allow you to add more ESD protection without cutting
into normal signals.

If you do this, remember that the A/D precharge time goes up with the source
impedance at the A/D pin.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2001\05\21@093234 by michael brown

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face
> > However, in my haste to get a test going, I accidentally forgot to clip
> the
> > analog source into the PIC's A/D input pin, so it was just left
floating.
> > Later, I noticed that my Vref is no longer accurate at 4.096, but
instead
> is
> > around 4.80 V.
>
> I can't say whether the PIC got damaged, but it is certainly possible.  A

I must be stupid, but I don't understand how leaving an input pin floating
could "damage" the PIC.  Perhaps you could enlighten me on this.

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2001\05\22@090705 by Olin Lathrop

face picon face
> > I can't say whether the PIC got damaged, but it is certainly possible.
A
>
> I must be stupid, but I don't understand how leaving an input pin floating
> could "damage" the PIC.  Perhaps you could enlighten me on this.

The floating part isn't the problem.  The damage can be done when the
floating wire is eventually connected to something.  This is like jumping
off a tall building.  The falling part is rather harmless.  It's the sudden
stop at the end that kills you.

He did say there was an allegator clip at the end.  Even a small static
charge is several thousand volts.  If that has enough capacitance back to
the PIC power supply, then you can get enough current for long enough to fry
something.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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