> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of yamanoor sairam
> Sent: 21 March 2010 02:18
> To: Microcontroller discussion list - Public.
> Subject: [PIC] ORP problem
> 
> Hello All,
>        I am sorry that I was in a hurry to shoot a mail. I had posted
the
> circuit while writing to the list on a previous occasion. The circuit
is
> available at:
> 
> http://mechatronicscraze.wordpress.com/pic-orp/
> 
> One of the jpeg image contains the Express PCB file image that I had
> posted and the other image contains the eagle schematic that I had
used
> for developing this ORP Probe interface. ( I would be happy if someone
> could offer an opinion on my PCB design skills).
> 
> The ORP probe is a glass probe which measures the oxidation reduction
> potential of a solution based on the potential of the solution. It is
> commonly used in aquariums by hobbyists and in water treatment plants
to
> measure the dissolved ozone in water.
> 
> Now the problem I am facing is that when I provide an input from a DC
> power supply, my display is providing me the correct voltage with an
error
> of 3 to 4mV. When I dip my probe into a calibration solution of 200mV,
it
> is displaying somewhere close to 203mV.  However, when I remove my
probe
> from the solution, the displayed voltage has to get reduced gradually
to
> zero. This is not happening. It is displaying all values between 0 and
> 900mV before getting reduced to zero. Some times it is displaying
> characters like "." "/" etc

Have you checked the output of your high impedance buffer op-amp (IC3A)
with an oscilloscope?  It's quite possible it could be oscillating
around a mean DC value.  I notice you don't have any components to limit
the bandwidth of this stage, and with the very high input impedance it
would take only a tiny amount of coupling (a few pF) from the output to
the non-inverting input to become unstable.

Regards

Mike

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