Depending on how the analysis is done, noticeable errors can occur. You should also bear in mind that floating point accuracy is not infinite. To increase resolution at low frequencies you should either lower the stop frequency, or select octave or decade sweep (which is more appropriate for displaying a log frequency plot). The tiny voltage error at 15.26 Hz is caused by LTspice's method of extrapolating between these points. The first point after 0 Hz is at ~15.259 Hz, and the next is at ~30.518 Hz. But you have selected a log frequency display, which stretches out the plot at low frequencies and makes the extrapolation between points visible. The total number of points is limited to 65536, which normally wouldn't be a problem because 15.26 Hz is less than 1 pixel away from 0 Hz when not zoomed in. The error you are getting in LTspice is caused by selecting a linear sweep. This agrees with the 692.252 mV you quoted earlier, and in fact gives you quite a few more digits to chase :) If for some reason you really needed to know the response at precisely 15.26 Hz, you could make 15.26 Hz be the start point of your frequency domain simulation, and you would find: Frequency,MAG(V(out)) Simulate this circuit – Schematic created using CircuitLabīut if you really care about the Nth digit of precision, you can click "Export Plot CSV" and get a table printed at full resolution. You can click it and run the plot for yourself: I built your circuit in the CircuitLab circuit simulator and set it to run with 10000 points per decade. You're seeing a few types of numerical issues: quantization of the frequency sample points, interpolation between those points, as well as rounding in the plot display.
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