Sairam,

Russell said:
>>But if want to be certain that the opamp is working inside the manufacturers guaranteed output
>>voltage range you need to either reduce output voltage swing to say
>>0-2V (loss of 1 bit of measurement resolution) or increase the +ve
>>supply voltage or further change the circuit

___________

> Increase the + ve supply voltage alone? Did you mean that I can increase the voltage swing by increasing the +5V to +7V or greater?

Swing won't change, but it will 'fix' the problem.

It's not the SWING you want to change - you just need to ensure that
the opamp specification allow you to use the 'swing' that you already
have.
At present the op amp output goes within 1V of Vdd and the
specification sheet, while not as clear as it could be, very very
strongly suggests that this is an illegal condition for typical
versions of the IC. SOME may work there but others may not.

IF you can tolerate an eg 0-2 V output swing you can easily achieve it
very simply with the present system. Input is unity gain buffer =
Amplifier_1 = A1.
A2 connected also as unity gain buffer (inverting input to output).

Below R1 = R2 = some convenient & appropriate value.

A1 output via R1 to A2 + input.
A2+ input via R2 to +2 Volt point.
Vout = 1+ Vin/2.
So swing is halved and output is translated up by 1V.
For -2/+2 input you get 0-2V output.
You lose 1 bit resolution compared to 0-4V output.
You have to make +2V from some source. Or a higher voltage with a different R2.

> All ORP probes and monitors operate over a range of +2V to -2V. I chose this amplifiers over other options such as LM356. What are the factors that I should consider while choosing an opamp?

There are 2

- All datasheet parameters should never be violated under any
conditions of valid input.

- Other technical needs meet your need.

ie the present opamp does almost all this EXCEPT Vout is too high to
be safe based on the datasheets we have quoted so far. Adding 2V to
Vdd or possibly as little as 1V will cure this. Vdd = 7V looks good.

> As Mr.Olin had suggested, when I set the gain of the opamp using a pot for a pre determined offset voltage, the error should be corrected by storing the offset values in the EEPROM?
&
> In future, if I am interested to check the calibration of the probe, should I write a code that find the offset and stores it in EEPROM?

That's "up to you" - ie substituting programming for hardware
adjustment. An entirely valid option if it suits you.


   Russell

PS: Just fyi -
I know how hard it is to get language you aren't usd to correct. You
do very well. Re names -
'Mr Olin' & 'Mr Russell' is polite, but 'Olin' and 'Russell' is fine
in this context.
If you need to use "Mr" in a formal situation then use the last name
(surname, family name). eg Mr Lathrop or Mr McMahon - BUT just use
Olin and Russell here.
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