The compression distortion calculation only takes the amplitudes at the fundamental frequency into account. For 1 mV input, you get 20*log10((10 mV - 100 nV)/10 mV) = 20*log10(0.99999) = -86.86 udB. This matches the result I get with SPECTRE16 ISR1 in IC617 ISR8: AC Compression Distortion Summary Results in ac_distortion Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq Instance Distortion(dB) freq=1 freq=2 freq=3 Total -86.86u 100n(180) 1.031a(180) 33.33n(180) G0 -86.86u 100n(180) 1.031a(180) 33.33n(180) The phase is shown as 180 degrees, so the values are negative. For 1 V input, I get the following result: AC Compression Distortion Summary Results in ac_distortion Nonlinear Mag(Phase)[V(Deg)] at 1st 2nd & 3rd harm of linear freq Instance Distortion(dB) freq=1 freq=2 freq=3 Total 19.08 100(180) 11.23f(-161.6) 33.33(180) G0 19.08 100(180) 11.23f(-161.6) 33.33(180) This is calculated as 20*log10(|(10 V-100 V)/10 V|) = 20*log10(9) = 19.08 dB. Your older version does not take the phase into account and calculates for 1 mV 20*log10(1.00001) = 86.86 udB and for 1 V 20*log10(11) = 20.83 dB.
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