[sdiy] keeping signals seperate..avoiding noise

Tim Ressel madhun2001 at yahoo.com
Fri Nov 14 16:39:56 CET 2008


Dan,

There are 3 possibilities: noise in the grounds, noise coupling through adjacent wires, and noise coupling through shared power wires. The first two have been covered, so lets look at the third. Your noise problem is classic: digital circuits with their sharp little spikes of current during switching share power lines with analog circuits. Now wires are resistors, and when you pass spikey current through them you get spikey voltage drops. The opamps should in theory not care because they have what is called power supply rejection. But nothing is perfect and noise can still slip in. The first thing I would do is slap some tantalum caps across the power supplies. 10uF from +12 to gnd and -12 to gnd should do. These should be placed where the power comes into the board.

Keeping analog and digital separate is an art. I've been known to run separate power and ground wires all the way back to the power supply to keep the digital spikes away from the analog power.

--TimR


--- On Thu, 11/13/08, Dan Snazelle <subjectivity at hotmail.com> wrote:

> From: Dan Snazelle <subjectivity at hotmail.com>
> Subject: [sdiy] keeping signals seperate..avoiding noise
> To: synth-diy at dropmix.xs4all.nl
> Date: Thursday, November 13, 2008, 8:29 PM
> a few questions
> 
> if i have a clean source going through a buffer and then
> that splits into one half going into a PLL and a divider and
> the other half getting boosted by a x10
> opamp stage...WHY would i be hearing the divider sounds in
> the background of the clean stage?
> 
> this is very different than my lfo bleedthrough of the
> other night (into the vco) but it is similarly noisy.
> 
> i have all pcbs on metal standoffs going into a metal
> box....could this cause the high frequencys of the PLL
> signal (and octave down) to bleed into the clean opamp?
> (though none of the standoffs touch traces)
> 
> maybe i should put a regulator JUST on the divider board?
> 
> i have bypass caps.
> pretty good ground planes.
> and seperate lines for digital and clean grnd.
> 
> what about a bunch of wires laying all over each other or
> being bundled...can this cause signals to mix?
> 
> i think i really need to understand noise better.
> 
> any good books on this?
> 
> thanks
> 
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc  
> 
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle 
> 
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy




More information about the Synth-diy mailing list