I'm fairly new to using op amps, so this may be a simple issue. I am building an inverting amplifier for monitoring a negative voltage on an analog rail. Since the ADC I'm using requires a positive voltage an inverting amplifier seems to be the way to go.

First, here is the LTSpice model using the OP07 (I have tried a variety of op amps with little to no improvement) Non-ideal Inverting Amplifier

In this configuration the output voltage is higher than expected. With a -2V input and unity gain I would expect the output to be 2V, however it is slightly higher at 2.097V. The voltage at the inverting pin is 48.9mV, which seems to be the cause of the higher than expected output voltage.

Things I've tried

  1. Changing V- to -5V. This solves the issue, but I'd prefer to not require another voltage rail
  2. Using an ideal op amp the circuit works as expected, further indicating there is some parameter of non-ideal op amps that I don't understand

Any suggestions are appreciated

  • 2
    \$\begingroup\$ Did any of your "variety of op amps" include a device labelled as having "rail-to-rail" or "single-supply" capabilities? \$\endgroup\$
    – brhans
    Jan 20, 2022 at 19:50
  • \$\begingroup\$ @brhans One of them was (ADA4505-1) but I had to import the spice model. When I switched the an OP292 (a component in the default libraries) the issue was solved, so maybe I had an issue with the ADA's model as well, thanks for the help! \$\endgroup\$
    – soup
    Jan 20, 2022 at 20:07

1 Answer 1


OP07 input range is only guarranteed to within 2 V of the negative rail, typically 1 V. You need to use something like LM324 whose range includes the negative rail for it to work properly.

There are plenty of other opamps that will also include the negative rail. They tend to be called 'single supply' amplifiers, as they are intended to be used with ground as the negative rail. Most R2R (Rail to Rail) amplifiers will also handle inputs around 0 V properly.


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