On 5/10/2010 8:58 AM, yamanoor sairam wrote:
> Dear All, I would like to thank Mr.Russell McMahon, Mr. Marcel Du
> Champ and Mr.Sergio for their kind help. I was successfully able to
> develop the PIC based ORP circuit. I am sorry that I was not able to
> thank them earlier since I have a very restricted access to the
> internet.
>
> I am currently trying to use the same op amp circuit to modify such
> that I am able to read signals from probe between -2V to +2V. I
> wanted to level shift the signal such that I am able to read the
> signal against a scale of 0-4V. I removed the 40K resistor and I
> replaced the same with a 10K resistor such that my opamp gain is 2 in
> the first stage. Now when I set an offset voltage such that my output
> is 4V for a -2V input, I see that I am not able to find linearity in
> the output. When I provide a 2V input to my opamp, the output reads
> zero. When I provide a -1V input to my opamp, the output is close to
> 3.81V and 4.1V for     -2V. Have I adopted a wrong technique for
> level shifting my signal? Please find the circuit diagram in the link
> given below:
> http://mechatronicscraze.files.wordpress.com/2010/03/orp-circuit1.jpg
>
>  Please correct me if I am wrong.
>
> Yours Sincerely, Sairam
>
>
>

You are very close to the answer.  Try removing R9 - this will change 
the gain of the input amp from +2V to +1V.  With an offset adjust value 
around +1V, you should get what you are looking for.

Why the change?  You have an input voltage range of -2V to +2V ==> 4V 
span.  You want an output span of +4V to 0V ==> 4V span also.  Thus, you 
need a system gain of -1 which your second stage already has.
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