Hello Mr. Mike, Thanks for your mail. 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. You are right about the DC voltage oscillating around the mean value. I am yet to gain access to an oscilloscope. I checked the output of my opamp using a multimeter. The opamp remained at 3.3V when I removed the probe out of water instead of increasing to 4.139V. It took a long time before increasing to the desired value. How am I supposed to prevent amplification of all noises at this stage? Sairam ________________________________ From: Michael Rigby-Jones To: Microcontroller discussion list - Public. Sent: Mon, March 22, 2010 11:43:50 PM Subject: RE: [PIC] ORP problem > -----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 ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= -- 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