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The below is the circuit in question using LTSpice XVII(x64) (17.0.21.0)

enter image description here

The two sources have the exact same sine wave, just with different DC offsets. One with a DC offset of 5V, the other with 0V.

So, when you subtract, or measure the voltage between Vcc and Vref, you should get a perfect DC value. And we do. But, when you measure the voltage between Vdiv and Vref, it has some AC signal.

I'm going to assume there is a setting or a way LTspice functions that I don't know about.

Here is the output measuring the voltage between Vcc and Vref. A perfect 5V DC enter image description here

Here is the output measuring the voltage between Vdiv and Vref. enter image description here

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Unfortunately, this is a known problem that's been haunting LTspice XVII since a year or so. It gets even worse, just try making R1 to be 1, then plot the same differential voltage. Adding .opt plotwinsize=0 makes no difference.

The solutions are:

  • impose a small timestep, but it can get very small and the effects seem to be linearly decreasing
  • add .opt method=gear for LTspice XVII from 2020 and beyond
  • use the version of LTspice XVII from May 6 2019 or before (as it appears in the Help > About)
  • use the discontinued LTspice IV.
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