Re: (2) dirty/clean ground again [sdiy]

jhaible at debitel.net jhaible at debitel.net
Tue Aug 20 16:54:30 CEST 2002


>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.

Hi Ian and list

this is true - but only for DC. And at DC, the PSRR of most opamps is excellent.
For higher frequencies, the "power supply impedance" which the opamp sees
is determined by the capacitors, i.e. rather low, regardless of the series resistor.

It's also possible to design the RC decoupling such that it has no effect for hum
(50 / 60 Hz), i.e making R rather low (10 Ohm ...) and keeping C in a reasonable
size. 
Example: 10 Ohm and 100uF make a corner frequency of 160Hz, so it won't
filter any 50Hz hum. Let the opamp do the hum rejection! But higher frequencies,
where the opamp is not good in its PSRR, are nicely filtered. And the 10 Ohm
series resistance will not cause much voltage drop, even when the opamp
drives some considerable load. (The PS-3x00 synths use this to decouple
complete boards: Separate GNDs for audio and for dirty, and two pairs of
10 Ohm / 100uF combinations for the audio and "dirty" +/- supplies.


> 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.


Yes. It's always a compromise, because PS outputs are often quite noisy,
even though they are well regulated and - thanks to caps parallel the outputs
- also quite low-impedance at higher frequencies. Fortunately, the stages
that really need the power are rarely sensitive to a little noise, so you
can always connect the power stuff directly, and the sensitive stuff with
RC filters.


JH.



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