[sdiy] Digital / Analog Noises

[Scott Gravenhorst]

"Bill Berzinskas" <wberzinskas at nc.rr.com> wrote:
>Hello everyone..  as you might know by now i've been heavily modifing 
>the adv-bass circuit..   so currently, ive got it running off of 5v, and 
>got the digital pots all working..   Although, i'm having a few problems 
>with noise..   
>
>First..   At the final output stage, I am getting noise, although not 
>before it..  This sounds like it could be digital, or it could just be a 
>really low SN ratio..   So it's getting into the summing matrix 
>somehow..   I've changed this to just a regular non-inverting summing 
>amp..   which seems to me to be ok..    Any clues?
>
>Also,  I'm using the oscillator design from the LM158 Datasheet (Same 
>setup as the DS-7 Clone)..   And i'm getting a small bit of the square 
>wave into the signa..  whats up with that?!   i'm tinking it has 
>something to do with the virtual ground setup..  but i'm not sure..   
>Anyone?   :-)

I would doubt it unless you're trying to use it in a too high current
application.  IMHO, it sounds like you didn't adequately or correctly use
bypass capacitors.  I would have bypass caps from 5volts to Vground and
from 0volts to Vground.  This mimics how you would bypass a real dual supply.

If you think the Vground is a problem current wise, you can increase it's
current handling capacity by paralleling two or more opamps.  You can also
test for and measure this condition by using an oscilloscope to monitor the
voltage between any rail and Vground.  Ideally, the voltage should not change.

>Also, I can hear clicks when i change a digital pot.. i'm thinking this 
>is simply because it's stepped, but they seem rather loud.. what can i 
>do about that?

Still sounds like bypass to me.


>And while we're on this subject..  what precautions should I take in 
>layout, construction to help curb these problems? I'm thinking of 
>construction the Analog and Digital section on seperate boards and 
>interconnecting..  Would def be easier on me as far as working on it.. 
>anyway..  any input would be awesome!


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