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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: line level and op amps
Keywords: op amp, line level
Message-ID: <14082@milton.u.washington.edu>
Date: 10 Jan 91 01:43:23 GMT
References: <1991Jan8.072135.15225@wam.umd.edu>
Distribution: usa
Organization: University of Washington, Seattle
Lines: 38

In article <1991Jan8.072135.15225@wam.umd.edu> elwood@wam.umd.edu (Jim Bogard) writes:
>2. What's the lowest noise op amp?
>   (Don't read too much into this; nothing esoteric, just whatis
>   the quietest general purpose op amp. eg, 741,386, etc.)

	Op amps have at least four kinds of noise, and it matters a LOT
which is most important to you.  There is broadband voltage noise,
broadband current noise, low-frequency voltage noise (a combination
of 'excess noise' and drift), and low-frequency current noise (bias
current and bias current drift).
	Most op amps are not tested extensively for noise, but
rather are spot-checked; as a result, many manufacturers don't guarantee
noise performance, but only give typical values.
	Most applications don't need high input impedances on
op amps, and as a result the current noise (especially the bias current)
is relatively unimportant.  For input impedances of 1k ohms or
less, where DC offsets are important, typical low noise op amps
would be uA725, OP-07, OP-27, LM308A, NE5532, RC4136.
	For input impedances of 1k ohms, where DC offsets
are NOT important, one can add some JFET input op amps: LM356 is
adequate, and the TL071 is the low-noise version of the TL081.
	For input impedances of higher than 100 k ohms, generally
one considers ONLY the JFET input op amps.
	Additionally, one can get pre-trimmed op amps (from PMI and
Burr-Brown and Analog Devices, mainly) at higher cost with some
pretty fantastic characteristics.  Unless the source is terribly
low in noise to begin with, though, there's no real point in scrutinizing
the poor op amp.  
	A 16-bit D/A converter, for instance, has quantization
error of a dozen microvolts relative to a 1V output; any
op amp with less than 100 nV/root Hertz noise will have negligible
noise by comparison (over the audio frequency range).  In fact,
good CD players have to add noise generators to mask the low-output
'blurble' of digitized sound, 'cuz the op amps don't add in
ENOUGH noise to do it.

	John Whitmore
	whit@milton.u.washington.edu