[sdiy] keeping signals seperate..avoiding noise

Dan Snazelle subjectivity at hotmail.com
Fri Nov 14 19:05:17 CET 2008


since there are actually 4 pcb's in this project, and ALL the digital chips are on ONE board...i think i will run seperate lines all the way to the PSU for JUST that board.

i was even thinking of powering JUST that circuit off of a 9v battery but then i figured it would be a hassle to always have to make sure a battery was in the box. and since there will be a back panel...it would have to be unscrewed,etc to change the battery

and then i thought about a regulator for the digital board. BUT that STILL wouldnt solve the dirty ground issue.

SO....i think running all the way back to the PSU is the best bet.

someone had mentioned sensing (on my psu) and i think i have it hooked up properly....(to bypass it) but i am wondering if that could help at all ( turning it on...anyone here actually USE the sensing within the context of SYNTH DIY?_

it's funny how you can spend months building something, it all works fine (as separate elements) and then once you hook it
all TOGETHER at the end, all kinds of NEW problems arise.

i still am not quite sure why this didnt show up until NOW as i had tested these boards many many times. the only difference i can find 
is that this time i am not using a battery and this time i have the boards screwed into the case (on metal standoffs)

i am going to try plastic spacers just for the hell of it too.

i will test this all out this weekend and report back

ANYWAY...thanks so much for the clarification.

check out various dan music at:



(or for techno) http://www.myspace.com/snazelle 

> Date: Fri, 14 Nov 2008 07:39:56 -0800
> From: madhun2001 at yahoo.com
> Subject: Re: [sdiy] keeping signals seperate..avoiding noise
> To: synth-diy at dropmix.xs4all.nl; subjectivity at hotmail.com
> 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  wrote:
>> From: Dan Snazelle 
>> 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 
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