Here’s an interesting situation that’s giving me fits, and I’m wondering if what I see is typical of the default Sigma second-order high-pass filter, or do I just not understand something basic here.
I need a fairly aggressive high-pass function to get rid of subaudible noise in a project. I cascaded a pair of second-order, 15Hz HPFs, fixing the Q of one at the default 1.41, and the Q of the other at 0.53. According to the Stimulus/Probe utility, this gives me ruler-flat response down to 20Hz and the expected 24dB/octave below.
Once this was loaded onto the Eval Board, the first thing I noticed was that the filter didn’t clip symmetrically. Sure enough, there’s a negative DC component in the output. What’s worse, though, is that there is quite a bit of low-frequency bumping and rumbling coming out of this high-pass. That certainly doesn’t seem right... it's a HIGH-pass, right?
The first test I made was to connect a 0V DC source to the filter input. This killed the DC offset and LF noise. So the offset and noise are in the filter output only when its input is connected to the eval board’s A/D converter. Putting a Sigma DC Blocking filter in the input of the HPF does nothing, however.
I thought that maybe the filter had a mysterious low-end tip-up, which was just boosting the noise from the A/D. But actually sweeping the filter with a function generator did not show up any huge peaks in the response down at 1-5Hz.
The project is attached, and if you load it you can see the problem even without external test gear. The DC offset at the output can be verified from the Readback module, and the LF noise shows up on the AC-coupled meter. With the filter set to 1000Hz, there’s no problem. But take it down to 10Hz and the offset and noise are there... and killing me!
If this can’t be fixed easily, what other options are available in Sigma for a steep high-pass function with a low turnover frequency?