Ultra-strange filter problem..


While evaluating the second revision of my DSP board, I noticed some super strange behaviour that is caused by a 2nd order lowpass filter. 

See for yourself.

This is basically what you would expect. This uses a single-precision, no slew, 2nd order LP filter set to 2000Hz. The measurement below is not meant to show the filter function, rather the device output. You'll see what I mean when we get to the problem.


Now, this is the exact same thing, but with a double precision filter (same type of course): 

What the hell?

It gets even stranger..

This is the two filters above in one design, with a switch to easily compare them (at least that was my approach to troubleshooting the issue)

This results in this:

What is this ? It makes zero sense.

The switch doesn't change the behaviour either. 

Thankful for any help.


[edited by: nneu at 2:35 AM (GMT 0) on 14 Jan 2020]