Hello, Thanks for the kind reply. I will have to get back to you after setting the potential divider circuit. However, I tried what you had told me on a particular occasion. I set the potentiometer to 0V and I applied a signal 3V or so, I got a negative voltage at pin 7. I believe the opamp is working properly. I shall double check this. >If your probe puts out +800mV to the input, the first stage will turn >this into +4V. If the offset pot is set to +2V, you will get 0V at the >output. Now if the ORP voltage drops towards +200mV, the output will >indeed rise to 3V. This is probably what you want. The total circuit >gain at DC is +5. Your input span is 800mV -200mV = 600mV; .6V * 5 = 3V. You were right on the input voltage. When there is no signal, the circuit displayed 4.2V and there was a gradual decrease in voltage later. So let me understand what you had meant. my circuit has a span of 3V. I am supposed to set the offset voltage such that 800mV is 0V and 200mV is 3V. I shall check this and get back to you. I had been so dumb applying a voltage of 3V using a DC RPS and I was worried that I was not able to get an output. Sairam ________________________________ From: Marcel Duchamp To: Microcontroller discussion list - Public. Sent: Sat, March 6, 2010 12:15:22 AM Subject: Re: [PIC} Orp Probe interface with PIC16F877A On 3/5/2010 10:01 AM, Marcel Duchamp wrote: > > As far as reading ORP signals from 200mV to 800mV, you MUST connect the > probe such that it presents a negative voltage to the BNC input jack. > This is because of the inverting second stage. Looking back on this, I may be incorrect about the polarity. It all depends on where you set your offset pot. If your probe puts out +800mV to the input, the first stage will turn this into +4V. If the offset pot is set to +2V, you will get 0V at the output. Now if the ORP voltage drops towards +200mV, the output will indeed rise to 3V. This is probably what you want. The total circuit gain at DC is +5. Your input span is 800mV -200mV = 600mV; .6V * 5 = 3V. -- 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