[sdiy] dirty/clean ground again

media.nai at rcn.com media.nai at rcn.com
Sat Aug 17 18:00:37 CEST 2002

>I had an idea when I woke up this morning. Most of module circuit are made
>around opamp, right?

Yes, I would say that is true of most new designs.

>so basically current in ground pin of entire module should be 0.

I doubt that.  Most amplifier circuits are referenced to ground, and most
synth circuits have voltage dividers or signal capacitors going to ground.

>So why do we all decouple opamps with dual caps to ground.

Well, why do we choose monolithic caps for decoupling in the first place??
Isn't it because they are less inductive than other types of caps, and are
therefore better at high frequency decoupling??  So if the leads to the
capacitor are longer than a couple of millimeters, or if long board traces
are used, the inductance increases, and the advantages of using a low
inductance cap are minimized if not negated.  So one advantage of using
dual caps is their physical placement.  Being able to place each cap close
to ground minimizes inductance, as opposed to bridging one cap between both
power pins -- typically at opposite corners of the op-amp.

>Isn't it more justified to use single cap just across power rails?

I've seen that used in synth circuits, and it is the basis of IC sockets
with built-in bypass caps.

I'm in the process of ordering parts to convert a Blacet Time Machine to
MOTM format.  While compared to all other analog delay circuits I have ever
seen, the TM seems an extremely powerful and innovative design.  I have
also noted a shocking lack of bypass caps -- there are not any caps on the
supply pins of any of the chips.  So I am considering replacing the regular
sockets that come with the kit with sockets with integral decoupling
capacitors (eg. Mil Max 110-93-314-41-801).

Otoh, considering the length of the leads on these caps, these sockets
might not make a difference.  My thoughts are while they are not as good as
optimally mounted dual caps, they will provide some decoupling.  Any

While I'm on the subject, does anyone sell 8-pin DIP sockets with built-in
caps??  I can't seem to find them, and that leads me to believe that such
sockets are rarely used for op-amps.

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