(2) dirty/clean ground again [sdiy]

[Ian Fritz]

At 02:43 PM 8/19/2002, Scott Gravenhorst wrote:
>harry <harrybissell at prodigy.net> wrote:
> >I say decouple positive and negative busses to ground with caps. Better 
> if you
> >can stand series resistance in the supply lines as well...
> >H^) harry
>
>This is an interesting technique.  For something like opamps, is there an
>optimum series resistor(s) value?  Does that vary from opamp type to opamp 
>type?
>  My guess is yes and it has to do with how much current the opamp can 
> supply and
>how much the designed load will to draw at maximum.  Are there applications
>where one ought not do this?


I know that decoupling with a series resistor is a popular technique, and I 
have used it in some circumstances, but it has one major drawback that must 
be considered. In fact, some time ago I read an article that vehemently 
insisted that this technique should never be used! The reason is that the 
series resistance effective increases the output impedance of the power 
supply regulator to the value of the resistor. So instead of your carefully 
designed and constructed low-impedance power supply, you now have a 100Ohm 
(or whatever) power source. In other words, you are throwing away some of 
the load regulation characteristics of the supply. Your supply pins will be 
quiet with respect to noise, but the dc voltage at the chip can vary with 
load current.

I've had good luck decoupling with ferrite beads. Odd that no one has 
mentioned this possibility in this thread. In my last VCO project I put 
beads on the supply lines where they enter the board and also from that 
point to several different sections of the circuit. This is the first VCO I 
have made where the switching transients don't couple back into the expo 
converter section.

If ferrite beads don't do the job, then probably it would be a good idea to 
go to local regulation (raw dc to the board plus an on-board regulator) or 
to re-regulate to a lower voltage using, say, a precision zener.

   Ian