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To be able to measure the value with an Arduino Nano I want to convert the negative rail voltage to positive. I tried to build a simple inverting amplifier with unity gain using LM324. The supply is +/- 10V.

The voltage is converted but scaled down. When I measure the voltages on the pins I see the values noted in below pic with red. It seems that the inverting pin can't be pulled to pin non-inverting pin value.

For the test purposes I didn't attach any output load to the circuit.

enter image description here

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    \$\begingroup\$ You're expecting the LM324 to be able to output +10 V when it is supplied by +10 V as well. That's not going to work, the LM324 can only drive the output to about 1 to 2 V below its positive supply, so at 10 V you can expect 8 V to 9 V, you get 8.6 V, that makes perfect sense. \$\endgroup\$ – Bimpelrekkie Jun 30 '19 at 14:54
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The LM324 datasheet says:

Large Output Voltage Swing 0 V to V+ − 1.5 V.

With a 10 V supply you can expect the output to go as high as 8.5 V which matches what you are seeing.

If you're feeding this into an Arduino 5 V ACD then you need a gain of only -0.5 to allow you to measure -10 V to 0 V. To achieve that you simply reduce R6 by a factor of two. The simple way to do this with reasonable accuracy is to put two 47k resistors in parallel.

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  • \$\begingroup\$ Thanks a lot for the advice. \$\endgroup\$ – AnilB Jun 30 '19 at 15:06

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