From: "Walter Harley"
Subject: linear power supply noise questions
Date: Tue, 12 Nov 2002 11:29:41 -0800
Organization: Posted via Supernews, http://www.supernews.com
Reply-To: "Walter Harley"
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This weekend I was fooling around with a simple noninverting audio opamp
circuit. The feedback divider was AC-coupled to ground, rather than
DC-coupled, to avoid amplifying the DC offset of the opamp along with the
audio. However, in order to avoid using a nonpolar electrolytic, I decided
to couple it to the negative supply, rather than to ground per se, reasoning
that since the AC source impedance of the negative supply should be very low
it wouldn't make any difference.
Bad plan! There was somewhere around 0.5mV rms of broadband noise (not hum)
sitting on the supply rail, and that of course coupled right through to the
output. In fact, the way I discovered it was by pulling the opamp from the
circuit and observing that the noise persisted, unless I turned off the
power switch. I changed the circuit to couple to ground instead of the
negative supply, and got a 30dB improvement in noise at the output.
I have three questions:
First, I'm using an LM337T as the negative regulator. It's bypassed at
input with a 0.1uF ceramic, and at output with a small tantalum (don't
remember value offhand). Am I right that the next thing I should do to
reduce the supply noise is bypass the ADJ pin, presumably with a ceramic or
polypropylene cap? Or would I be better off putting a poly or ceramic cap
across the output (in parallel with the tantalum), or should I do both?
Second, is this problem typical, and is that why these circuits usually show
a cap to ground instead of a supply rail? It seems like it would be better
engineering to use a cheaper, more widely available part in place of the
nonpolar cap, but not if it means increased noise.
Third, my Hitachi V212 oscilloscope's max calibrated sensitivity is 5mV per
division; with a x10 probe, that's 50mV per division, which means I can
barely see the 0.5mV supply noise (instead, I discovered it with an Audio
Precision analyzer). Do people generally use scopes with higher
sensitivity, or outboard preamplifiers, or is a scope just not the right
tool, or should I get my eyes checked?