On 3/5/2010 5:49 PM, yamanoor sairam wrote:
> 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
>

I think you have a good understanding now.  One more point you may know
but if you don't, you need to understand.  With a high impedance input
circuit like this, if you simply remove the ORP probe then the output is
indeterminate.  With the input floating, random noise, leakage currents,
power line influence, etc. will dominate and the output may go to any
voltage between the power supply rails.

And yes, you understand correctly about setting the offset voltage for 
your particular input span.

Good luck and come back if you have more questions.
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