With the filter wizard it would be good to have a tool that calculates a 3rd order Sallen Key filter - this is a standard 2nd order Sallen Key filter, but with an extra RC circuit on the front end to give an extra pole. The maths behind this is hard to do manually.
If I understand what you are asking, you are saying, “If I pick an odd order filter, the tool creates an RC stage with a buffer amplifier. A lot of times, you can get away with not having that buffer amplifier. It would be great if the tool allowed me to not have that buffer.” Is that right?Our tool is architected assuming each stage does not load the next, so it needs the buffer amplifier in the RC stage. Here’s a decent workaround: create your 3rd order circuit in the tool. Then export to spice, delete the buffer amp, and see if the performance is good enough.
I would recommend keeping your extra RC filter impedance at 10x less than the Sallen Key components. In the filter wizard tool, you can increase the Sallen Key component impedance if you go into the “Choose for me” mode and move the RC slider. This video covers multiple pole passive filters. While not directly addressing a passive filter before a Sallen Key, it might give some insights.