Hello Mr. Russell, Thanks for your mail. I shall check the op-amp output and get back to you. Having the display show "/" indicates software problems unless you are applying "/" volts at the time :-) Before getting reduced to zero, my voltage values are shooting upto those values which is not possiblei.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 To: Microcontroller discussion list - Public. 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. would 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 = 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 :-) Russell McMahon -- 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