[sdiy] tri-sin shaper and THD

[ryan williams]

that discussion of tri-sin converters and the mention of using feedback 
around the OTA caused me to write this note to show myself how it works. 
I posted it the other day, but I added some THD calculations to it and 
also another version with negative feedback.

http://www.sdiy.org/destrukto/notes/tri-sin_converters_using_otas.pdf

for anyone that knows about THD calculation. take a look at it and see 
if that is corrent (pg.4). Or if thats to much work, just tell me if 
this sounds right: I took the sum of the FFT of the converted sine and 
divided by the FFT of a pure sine then subtract 1. expressed in percent.

I ended up with 1.87% distortion with feedback and 3.52% without. but 
the one with feedback looks much better (the sharp corner is almost 
completely gone). Those were the best numbers I could get.

-ryan