[sdiy] dare I say - decoupling?

[Magnus Danielson]

From: Antti Huovilainen <ajhuovil at cc.hut.fi>
Subject: Re: [sdiy] dare I say - decoupling?
Date: Sat, 3 Sep 2005 23:02:08 +0300 (EEST)
Message-ID: <Pine.OSF.4.61.0509032258410.224742 at kosh.hut.fi>

> On Sat, 3 Sep 2005, Magnus Danielson wrote:
> 
> > Just tossing it into the feedback loop of an op-amp will not help to acheive
> > the constant DC-pulling.
> 
> However, you could add a lowpass filter in the op-amp input. Depending on 
> the circuit this may help (no abrupt transient to cause EMI) or not (panel 
> controls wired to power supply).

This is a good point. Also, it adds to the coolness to have a smooth fade-in
and fade-out, which comes fairly cheap once you have taken on the cost.

> Please keep up the discussion. Bypassing and grounding seems a lot like 
> black magic - I haven't been able to find any definitive rules like "Do 
> this" and "don't do this".

As you go higher into frequency, the more black magic it may seem. However, in
the end you just have to learn the basics of inductance, capacitance,
resistance in action for various frequencies and edges. Then as the physical
layout comes up to wavelength size the time-delay for propagation plays in.
It is all about practical electromagnetic fields. The more you learn, the
less there is to recall. 

> Do you (or others) have ideas what to do when the circuit uses several 
> DACs so you just can't "tie analog and digital grounds together at one 
> point near the DAC"?

One trick-of-trade that I use is to convert the digital signal levels to the
local power-lines by use of a buffer. The buffer is tied in to the local power
and only has AC-coupling to the dirty world so that the edges goes through
undisturbed. That way the digital and analog parts is tied in over at the PSU
and you have have several analog sections each sitting on its own branch of
the star-ground. Infact, the point of the star-ground is to separate the
current into different paths so that the inductance and resistance sees less
current that develop a potential difference.

Star-grounding is one of your friends so be sure to make good friend with it.
It doesn't solve all your problems, but it can certainly be a good starting-
point.

Maybe the most important thing about power/ground-rails/planes is that they are
just as much electrical conductors which have source impedance like any other
source. When you think of it as a "stable" source that you can do whatever you
like with, it will bite you. When you think it as a potentially weak source
and that it may be lowfrequency stable over there (think of me pointing at the
PSU) it is not true over here (pointing at a VCO). Also, the PSU can more or
less weak, which depends on the loading (oh, how if I had a penny each time I
seen an almost overloaded PSU causing trouble, I would be rich now) and the
voltage to the PSU.

Cheers,
Magnus