odd filter?

[Styx]

I am auditing a cct at work atm and there is this cct arrangement

Sorry I don't have actual value's atm (at home now) but I just dont reconise this topology. Tried to break the cct down to work out its cut-off but was having a hard time.

Anyone reconise a filter like this? it is a 2nd-order that much I know

 

[audioguru]

It is similar to a single opamp bandpass filter. It has the same input resistor as the "standard" bandpass filter, except its other resistors and capacitors are swapped in their positions.

 

[Styx]

mmm yer I can see that now, but this most definitly is not a band-pass filter.

Where this filter is found is in a motor-drive of the position feedback signals

position feedback: SIN/COS are generated in the electrical machine, the freq of which is equal to the mechanical freq. This SIN/COS is digitised and via CORDIC function within an FPGA position & velocity are calculated.

speed-range is: +/-15,000rpm (inc zero with an accuracy of 30rpm).


Now the problem is in doing some drive-tuning we are getting position-lag that is screwing up the tuning (phas advance and all that).

So I look at the SIN/COS feedback and on SIN there is two of these filter (one after another, different component values) and then two R-C low-pass networks (with a buffer between) - same on COS

3ppl have gone over this arrangement and we couldn't reconsise or calc the cutoff (network problems so all the OrCAD/SABER/MATLAB licenses were unavailable)

Just seems odd, some excessive filtering which isn't helping the performance as well a 4, one after another??

oh well ill wait until mon and run it through a sim get the cut-off

thanks

--EDIT--
Got it!!!

nothing like google for "active filter" and then look at "images"

**broken link removed**

2nd order lRC active low-pass filter, now to find hte equation



--EDIT 2--

Got it!!!
**broken link removed**
good paper on filters