From: Kevin McMurtrie
Subject: Re: linear power supply noise questions
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Tue, 12 Nov 2002 20:51:00 GMT
NNTP-Posting-Date: Tue, 12 Nov 2002 12:51:00 PST
In article ,
"Walter Harley" wrote:
>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?
> -walter harley
All signals must be referenced to a _single_ ground. Even amplifiers
using a negative ground must have a single point for signal references.
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