# I am wondering why the unity gain does not work to convert -0.548 V to +0.548 V for a opamp and PT10

Discussion in 'The Projects Forum' started by joniengr081, Sep 17, 2016.

1. ### joniengr081 Thread Starter New Member

Sep 15, 2016
4
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I generate a current source of 0.5 mA using an Op Amp which I am feeding to PT1000. The voltage across the PT1000 which I measure is -0.548 V. The resistance of PT1000 can be calculates as (548 mV /0.5 mA = 1097 Ohm) which corresponds to 25 C and this match with reference temperature in the lab.

Now I need to invert the polarity of the measured voltage from -0.548 V to +0.548 V using an OpAmp AD844 in unity gain inverting configuration. The output of the inverting amplifier is Vout = -(R2/R1) Vin, where R2 is the feedback resistance. If I use R1 = R2 = 1K Ohm, the output of the OpAmp at pin number 6 is only 0.250 V which shows that gain approximately half instead of 1. In order to get approximately +0.548 V I need to use R2 = 2.2 K Ohm feedback resister. I am wondering why the unity gain does not work to convert -0.548 V to +0.548 V ?

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