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pt 100 gain circuit , facing problem with nonlinear gain......

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fork03

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I am attaching here a my pt100 gain circuit..
In which i am using lm358 opamp for gain.
First i am calibrating this circuit for 0 - 300'c range.

1) Pt 100 resistance on
0’c -> 100 Ω
300 ‘c -> 212 Ω
First i am putting 100 Ω at input side and changing vr1 till vout will 0v.
Then i am putting 212 Ω at input and changing vr2 till vout will 5v.

2) So now pt100 (0 – 300 ) range calibration has done.
Now as resistance changes my V out will vary from 0 – 5v.

But gain variation is so much that I am getting the range from 2.6 – 1.5 v range.

Please somebody help that why this much non linear gain is coming...
Is this due to lm358 (error prone op-amp) ?
 

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  • circuit.jpg
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gain variation is so much that I am getting the range from 2.6 – 1.5 v range.
You would want to describe the problem more clearly.

There's no particular reason for non-linearity in the circuit. Assuming OP single supply (negative supply is rarely "vout" as indicated by the schematic), you can't adjust the output to exactly 0 V, which may be one problem. I also guess that it's rather difficult to adjust offset or range with the present potentiometer setup. Finally a certain nonlinearity will be involved by the Pt100 voltage divider circuit, output of a divider is not proportional to the resistance value.
 

Thanks for reply ..

You would want to describe the problem more clearly.

Means i wanted to say that ratio of o/p voltage to input voltage is not constant. it is varying in range from 1.5 - 2.7.
Then can you suggest me any changes in circuit to get more constant gain .....?
 

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