> Before getting reduced to zero, my voltage values are shooting upto those=
 values which is not possible

If it is not possible, then is there any way for it to NOT be a code
problem? :-)

(Actually, yes, but unlikely - eg the PIC could be behaving badly  eg
as may happen if you bias its pins badly as per prior discussions).

Looking at the voltages being measured with an oscilloscope (as
already suggested) is a very easy way to see if what is being reported
is anything like what is happening.

But, if your display shows strange characters or if the circuit does
not reach the voltages shown by the uP then the Holmes'  law of
necessary reality* [tm]  says that your code or your processor must be
malfunctioning.


               R

* After you have eliminated the impossible, what remains, no matter
how improbable, must be reality.

(The trick is in determining the bounds of "impossible").








i.e While getting reduced to 0 from 225mV, I am reading values as high
as 300 or 400mV. This cannot happen when my opamp input behavior is
normal. Can I attribute coding errors for the same?
>
> Sairam
>
>
> ________________________________
> From: Russell McMahon <apptechnz@gmail.com>
> To: Microcontroller discussion list - Public. <piclist@MIT.EDU>
> Sent: Sun, March 21, 2010 1:18:38 PM
> Subject: Re: [PIC] ORP problem
>
> Sairam,
>
> pH probe impedances are in the 10's to 100's of megohms, maybe 1000
> megohms, so signals induced into the probe leads cab be expected to
> need considering. =A0would expect the result might easily be much as you
> describe.
>
> It depends in part on how your probe behaves when it only has target
> solution on random parts of its exterior. If probe impedance rises
> when it is not fully immersed then induced signals in the probe leads
> may be significant. Placing a meter at the input is quite likely to
> make a difference. A 10 Mohn input resistance meter could swamp your
> probe and even a 100 Mohm input R meter may cause major problems.
>
> Measuring at Vsense (IC3 pin1 ) is safer. An oscilloscope on Vsense
> should show you if the signal is stable.
>
> Op amp input bias current is 20 pA so may not be an issue - although
> with a 1000 MOhn probe it would be noticeable.
> eg 20 E-12 x E9 ohm =3D 20 mV.
> So if the probe resistance varies greatly from immersed to 'in air'
> then it could cause problems.
>
> Having the display show "/" indicates software problems unless you are
> applying "/" volts at the time :-)
>
>
>
>
> =A0 =A0 =A0 =A0 =A0 Russell McMahon
>
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