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From: firstname.lastname@example.org (staniam)
Subject: Re: linear power supply noise questions
Date: 13 Nov 2002 12:46:17 -0800
NNTP-Posting-Date: 13 Nov 2002 20:46:17 GMT
"Walter Harley" wrote in message news:...
> 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
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.
> 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,
Yes! Put a cap from the ADJ pin to gnd. It needs to be large however -
10UF or so. The output noise at the adjust pin is amplified by the
ratio of the output voltage over the reference voltage. Adding the cap
reduces the the noise gain to one. This info is in the LM337 app notes
BTW. My recollection is that the 337 is much noisier than its positive
counterpart (the LM317).
Even if you discard the idea of bypassing to the rails (as you
should!) lowering the supply noise is good practice. Your op-amps are
not perfect regarding power supply rejection and become less-so with
increasing freq. Depending on the gain of your circuit, the power
supply can contribute to output noise.
> 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.
Yes! It is bad practice to tie caps to the supply rails if they have a
path into the audio. Even if you did not have a noise problem, there
will inevitably be signal-induced ripple on the lines. It may be 60 dB
down but if you have 40 dB gain in your circuit, the ripple is now
only 20 dB down. You should be able to use a non-polar cap to
AC-couple the feedback network by proper choice of resistors. You can
also use two polar caps in series (C will drop in half). Tie like
polarities together and tie the junction to the appropriate rail with
a large (say 1Mohm) resistor. I would think you could get a 1UF
non-polar pretty cheap though.
> 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?
5 mV/div is a typical max sensitivity. In this case you could probably
dispense with the 10X probe.
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