# 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 expertengr1, Sep 17, 2016.

1. ### expertengr1 Thread Starter New Member

Sep 15, 2016
1
0
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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This thread was split from -- RTD td-1000 circuit.

2. ### AlbertHall Well-Known Member

Jun 4, 2014
2,282
450
Your amplifier circuit will have an input resistance (to virtual ground) of 1K and this will appear in parallel with the 1K of the PT1000 thus halving the voltage.