On 3/20/2010 7:17 PM, yamanoor sairam wrote:
>
> 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
>
> I measured the input voltage to the op amp. My multimeter showed 215
> mV and the voltage on the input got reduced gradually to zero when I
> removed the probe from the beaker of water. I measured the opamp
> output. The opamp output spans between 4.139V and 0V for 0mV to
> 900mV. My opamp also seems to be sending out the correct voltage.
>
> Could this be a problem with my code?

As others have pointed out, this is normal operation for an ORP probe. 
It is extremely high impedance when out of the solution.  It then is 
susceptible to picking up electrical noise (line frequency, etc.) or 
simply floating to one rail or the other.  When you connect your DVM to 
the input, you are pulling the input to ground.  But with no DVM on the 
input, the circuit will produce indeterminate signals.

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