Power supply and power/ground layout questions

Hi Todd,
Two questions:

1) Is power to the analog and digital sections separate enough to allow relatively easy connection of 2 supplies? I have a super low-noise 5v PS that I would use for the analog section, and use a standard wall wart for the digital.

2) Do you use a star ground layout on the board (all ground returns kept separate)?

Thanks much,
Mike

Comments

  • Hey Mike --

    The answer is 1.) Yes and 2.) Sort of -- but not really.

    Both ground and Vcc are routed independently to three sections of the PCB. The digital section (the MCU, RAM, latches, and MIDI) gets one send, the dirty analog (the dual op-amp which contains the clock circuit) gets another set, and the quiet analog (the quad op-amp that handles audio stuff) gets a third. You could cut any or all of these and isolate them. All three sets of power and ground are returned to the large electrolytic filter cap on the output of the regulator.

    As to a star ground, each of those "sections" has different ideas of specific grounding. Both analog sections have their own ground planes to which all grounds in that section (the quiet, and the dirty) are tied. The digital section grounding is essentially a bunch of daisychains to anything else in the digital section. In my experience, on a PCB, ground planes are almost always your friend. When trying to reduce power system noise, what really matters is thinking about current flow in your grounds. When you have large currents flowing through relatively high resistance (both VERY relative terms) traces you will see voltage drops or spikes which manifest as noise. You basically don't want current from circuits from anything transient-heavy to return through the same copper as something sensitive or high impedance. And you can reduce voltage drops in current heavy circuits in the first place by keeping their gound returns short, and heavy.
    Because ground planes are a TON of copper, they usually are pretty good UNLESS you have a big current passing underneath something sensitive. And frequency has a lot to do with how current passes through ground planes, but at AF it's pretty simple. So separating the ground planes is usually a good way to do this.
    In general, a lot of dorky PCB designers think about ground noise in terms of current loops. I do, whenever I can -- and whenever I have to :-)

    Finally, I'm not sure how much improvement you'll see in the noise floor by using a separate quiet supply. I did a LOT of testing on that circuit and most noise that's left in it (of which there's not much) is due to cross-talk getting capacitvely picked up in the output summing amp because of the relatively high impedances and relatively big loop size (like I talked about before) from VR3 to the quad op amp. But go ahead and do it if you feel like and let me know if you think it helps! I'd be curious.

    If _I_ wanted to pimp my WTPA for minimum noise floor, I'd scale down the impedances in the output amp stage. If I was using WTPA at line level I would reduce the 1M resistor on the input to a 47k or maybe 100k resistor. I might replace the venerable 7805 with a newer regulator (Micrel makes good ones) with better line and load regulation -- a generally faster regulator. _Maybe_ I would put an LC filter on the supply for the quiet analog section. And I would CERTAINLY cut the unused traces going from the quiet analog section to the overdub summing amp (in the dirty TLV2472 section) since all that happens in the digital domain now, and those traces go past the high impedance output summing amp.

    All this for the cleanest 8-bit audio around :-)

    Hope this helps, and best,

    TB

    ps -- Don't even get me started about ENOB. There's a duck on my grave over that one...
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