Analog & Digital PSU rails in one application - Page 1
Forum section: electronics / projects

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 28, 2014, 01:24:42 pm
content: Howdy there, from the, let's say, lower tier intermediate electronics fiddler ;-)Say I build a circuit that uses analog audio processing circuitry with dual, i.e. +5V, -5V rails, and also some digital circuitry, and both are "connected" with a DAC in between.The digital stuff runs mostly on 3V3, save for some not fully r-to-r opamps which get +5V; 0V rails.I guess this also means I need an analog 3V3 supply as well, for the VDDA of the DAC.Now, I already got a more or less nice ;-) self-built +/-5V linear PSU and I try whenever I can to just give the things I build a PSU jack to plug in that PSU.I wonder, is it wise, in a reasonably high quality audio application, to hook up to the positive 5V rail that's powering the analog stuff, and use it to also power the digital stuff, maybe with some RF choke coil in between or so, or will that still be too noisy?And then if I have my analog +5V and digital +5V rails, just give both of them a 3V3 LDO and that's my analog and digital 3V3 rails?If there are distincly better solutions, what are they?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2014, 04:06:16 pm
content: Honestly, I doubt it's noisy enough to even matter. At least, if it's well bypassed. If you're really concerned about noise and crosstalk, you can add isolation between sections -- CLC filters, usually. Typical values might be 10uF (ceramic or tantalum*) and anything from 1uH to 1mH. (Mind that you're making a resonant LC circuit as well, which needs to be damped with some R. Typically you'll put an even bigger C in parallel with the 'output' side C, which has enough ESR to dampen it.)*Assuming your supply is current limited. Tantalum doesn't like surge currents.Tim

author: German_EE
author_rank: Super Contributor
timestamp: September 28, 2014, 04:52:18 pm
content: The key here is the phrase "high quality audio application". If you really must run the digital and analog sections from the same supply then have everything wired in a 'V' formation where the bottom of the V is your power supply connection and the top ends are the analog and digital Vcc. Add lots and lots of decoupling on both rails and you should be fine.

author: nctnico
author_rank: Super Contributor
timestamp: September 28, 2014, 07:18:58 pm
content: Don't forget to put ferrite beads in the positive supply rails. This keeps the noise from the digital section inside the digital section.

author: Warhawk
author_rank: Frequent Contributor
timestamp: September 28, 2014, 07:43:38 pm
content: Quote from: TinkeringSteve on September 28, 2014, 01:24:42 pmHi , I am a bit OT, but you may find some ideas here:http://www.analog.com/library/analogdialogue/archives/39-06/chapter%209%20hardware%20design%20techniques%20f.pdf

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 30, 2014, 06:15:07 pm
content: Ah yes, that document, I think I've seen it in some other incarnation if that's possible. Some interesting things in there, thanks.Quote from: German_EE on September 28, 2014, 04:52:18 pmWell, "must". That's relative ;-) I could build a PSU that provides separate 5V digital, if that's worth it.