FPGAARCADE minimig compatible board, comments?

[jkonstan]

Check out this NSC app note for Linear VTT regulator.

http://www.national.com/an/AN/AN-1254.pdf

[jkonstan]

Mikej,

DACs tend to have poor output drive capability; thus, adding buffers (after the series cap) to the DAC outputs is a good idea. The simpliest approach is using a single ended MC33204 (VCC and GND)with a "virtual ground (VCC/2)" as a buffer or as a non-inverting amplifier so that you will not need a -VEE rail.

http://www.onsemi.com/pub_link/Collateral/MC33201-D.PDF

[jkonstan]

To modify the example WM8729 Low pass filter to run from a single ended positive power supply:

1. Use single ended opp amp such as MC33204 where ground is now used for the -VEE supply for opp amp.
2 Make VCC/2 supply via VCC to 1K/1K divider to Ground & use @ 1uF cap from VCC/2 supply to ground.
3. Ground side of 47K resistor should now connect to VCC/2 supply.
4. Ground side of 4.7K resistor should now connect to VCC/2 supply. Note that this part of filter forms a noninverting amp with a DC gain K=2.
5. Ground side of 680pf connect to VCC/2 supply or keep to ground.

[jkonstan]

This is a Sallen & Key 2nd order VCVS low pass filter.
I have used this for LPF for audio in the past and here is a link for a description of it.

http://www.ecircuitcenter.com/Circuits/opsalkey1/opsalkey1.htm

The transfer function for this filter is interesting.
http://en.wikipedia.org/wiki/Sallen_Key_filter

You would need another series cap after the 51R.

I would simulate it in PSpice or TinaPRO.