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I am trying to get -5V from 5V to use it on another opamp for my flexiforce pressure sensor.

Following circuit outputs something around 0V instead of -2.5V either on a real thing or in simulator, so I'm pretty sure I am doing something wrong here:

enter image description here

Thanks!

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It's impossible that this circuit would output -2.5V, since you don't have any negative voltage power supply. The output voltage of the opamp can never exceed the voltage of the power supply rails.

enter image description here

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  • \$\begingroup\$ I was counting on "Inverting OpAmp" circuit to deliver Vout = -Vin*(R2/R1). You're right.. when I inserted -5 instead of ground on opamp power supply, I got -2.5V on output. What can I do to reverse 5V pin to -5V? \$\endgroup\$ – Mike Jackson Feb 21 '20 at 10:31
  • \$\begingroup\$ Because unfortunately this is not how do opamp works, It would be nice, but you have to supply an additional negative rail. There are tinny charge pumps for that purpose or better bipolar PSUs. \$\endgroup\$ – Marko Buršič Feb 21 '20 at 10:34
  • \$\begingroup\$ Could you suggest some of available options? I see SRS0505. Is there any other cheaper alternatives? \$\endgroup\$ – Mike Jackson Feb 21 '20 at 11:21
  • \$\begingroup\$ What do you have now? A dc/dc converter is noisy. You will have to add LDOs at both rails, so you will likely end with smaller voltages. Let me see, if I can find. \$\endgroup\$ – Marko Buršič Feb 21 '20 at 11:55
  • \$\begingroup\$ @MikeJackson CJMCU-2662 LM2662 board is the only thing that I could find. \$\endgroup\$ – Marko Buršič Feb 21 '20 at 12:30
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You need an OP-Amp with beyond-the-rail output. (fun fact, there aren't many out there with this feature, so don't try to get one).

A common op-amp has no means to generate voltages lower than its negative supply or higher than its positive supply. That's because it doesn't contain real sources. This is due to basic laws of electronics.

To generate voltages beyond the rails, charge pumps or more intricate devices containing coils, transformers or capacitors are needed.

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