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I have a circuit with an AC source. It is a low pass and high pass filter that have the same component values so the critical frequencies are the same. The filters are driven from the same source. I'd like to subtract the output voltage at the low pass filter from the output of the high pass filter. It seems like LTspice is plotting the absolute value of the difference. I thought that is might have something to do with the x-axis being a log scale but this does not change if I change it to linear.

The red trace is the difference between the output of the low pass filter (blue) and the high pass filter (green) enter image description here

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  • \$\begingroup\$ Keep in mind that .ac analysis results are in polar form with magnitude and phase. If you want to subtract just the magnitudes you need to do something like mag(V(vlp))-mag(V(vhp)) \$\endgroup\$
    – Ste Kulov
    Commented Feb 7 at 14:35
  • \$\begingroup\$ 1000 points for you Ste Kulov, thanks for the mag(V(vlp))-mag(V(vhp)) works like a charm \$\endgroup\$ Commented Feb 7 at 15:28

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LTspice is correctly plotting what you asked for. If you look at the subtracted result, the amplitude will always be constant but, the phase will change from 0° to -180° as expected: -

enter image description here

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    \$\begingroup\$ Dahhh. I felt as though it might have something to do with the phase. Thanks \$\endgroup\$ Commented Feb 7 at 15:25

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