ssi2164 filter

[Gobotak]

I'm trying to use the ssi2164 for some low pass and high pass filters using this article as a reference: 
https://www.soundsemiconductor.com/downloads/AN701.pdf
I'm not sure how to go about coming up with a four pole high pass filter.  To quote the article, "Simply put, a high-pass filter is the output of a low-pass filter subtracted from its input".  I'm not sure if I should do four individual high pass filters in series

or if I can do four low pass filters and subtract the input from the first filter?

[toneman]

Hello,
This sounds similar to Thomas Henry's TH-201 "Mankato" Voltage-Controlled Filter.
:-)

[PRR]

do four low pass filters and subtract the input from the first filter?

That's gotta be unstable, oscillate, with four phase-shifts inside a feedback loop.

I think the first plan may be right.

[ElectricDruid]

Aside from the problems PRR mentions, the other trouble with the "subtract lowpass from the input to get highpass" method is that it depends on good matching of the levels to get the required cancellation. That works ok for a single stage, but it doesn't work so well after four stages. As the number of stages goes up, the accuracy requirement becomes severe and small errors (<1%) cause significant degradation of the stopband.

You'd get a better highpass with the cascaded highpass stages. Another option would be to do two 12dB/oct state variable filters one after the other. That might be slightly less parts.

[Gobotak]

Thanks for the help and suggestions!  I'm looking into state variable filters now and think I'll give that a go.  Seems like a good way to try out plenty of different configurations and hopefully get a better understanding of the chip and how the filters work.