Help with Analog circuit

SteveW719

I was looking at the BOE schematic DAC section sheet 4 looking at how the PWM is filtered and feed through the op amp. The only part I don't really understand is the C6 1000pf cap. What is the purpose ?? and how were r7 and r9 calculated??

http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/32900_PropBOE_RevA_Schematic.pdf


Many Thanks
Steve

Phil Pilgrim (PhiPi)

You can use this online design tool to analyze the Prop BOE's "Sallen & Key" filter:

http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

-Phil