I'm designing an active low-pass filter for a differential input. The input is on a ±100V scale and needs to be scaled down to the standard ±10V scale.
I found this application note which describes how to design a 2nd order multiple feedback filter and convert it to differential inputs:
However, they state in their first step that the magnitude of the amplification factor H0 needs to be at least one, without giving a reason (in our case
H_0 = -0.1).
I don't see a reason for such a restriction, so I ignored it and designed such a filter (Bessel, with about 9kHz corner frequency) and it seems to work as expected, both when simulated with LT-Spice and in hardware.
Does anybody know where this condition originates? Is it a real restriction, or is it just arbitrary, maybe copied from somewhere without understanding the math behind it? Is it maybe because you would tend to use a different topology for gain less than one, though there is nothing really wrong in using multiple feedback?
Curiously, TI's Filter Pro also didn't let me enter anything less than 1 for the amplification.