From: "Walter Harley"
Subject: Re: linear power supply noise questions
Date: Tue, 12 Nov 2002 14:47:46 -0800
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Reply-To: "Walter Harley"
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"Kevin McMurtrie" wrote in message
> What you saw was only half the problem. The power carrying lines are
> modulated by the amplifier's own load, regardless of how good your
> regulator is. It happens in the wires. It results in the pull-up and
> pull-down phases having different levels of amplification, or
> oscillating badly.
Thanks, Kevin. I don't quite entirely understand this, though; maybe you
can help me. I think what you're saying is that the supply line looks like
a possibly-ideal voltage source, plus some finite source impedance. The
power connection of the opamp, and any signals I refer to the power rail,
connect to the far side of that impedance. So, any current that flows out
the opamp's output causes a voltage drop on the supply rail, and therefore a
corresponding induced voltage at the opamp input, in my case where I am
referencing a feedback divider to the supply rail rather than to ground.
But, it seems to me that induced voltage should show up only as a tiny
(compared to the much higher impedance of my feedback divider) error term; I
don't see why it would cause oscillation, since it's no different in nature
(that is, it's not appreciably phase-shifted) from my feedback signal and
it's much smaller. I do see why it could introduce distortion, because the
error would exist on only one half the output cycle, but if my feedback
network is on the order of 10k and my supply source impedance is on the
order of 10 milliohms, that distortion should be on the approximate order of
1ppm, right? Just trying to see if I understand.