Hello Mr. Marcel, Thanks for your mail. 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. In order to avoid noises, I have used an RC filter combination at the PIC input & Opamp output. I used a 0.1UF & 10uF capacitor in parallel. I am also averaging 256 samples before displaying it. My display has stabilised now and I have also removed all junk characters on my display. When I remove my probe from the BNC connector, I am able to see that the displayed value is dropping to zero immediately. I shall check the oscilloscope output and get back to you. I am currently searching for a possible solution for controlling the erratic behavior when the probe is removed out of water but still connected to the PIC. Sairam ________________________________ From: Marcel Duchamp To: Microcontroller discussion list - Public. Sent: Tue, March 23, 2010 1:16:30 AM Subject: Re: [PIC] ORP problem 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. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist