PSN-L Email List Message
Subject: Re: OpAmp noise
From: Brett Nordgren brett3nt@.............
Date: Mon, 07 Oct 2013 15:23:04 -0400
Dave,
Many thanks for the AN link, that's an interesting device. Clearly,
as you suggest, it is aimed at low source-resistance
applications. Consistent with that, its differential input
resistance seems to be spec'd at only 225K.
Geoff's original question about what is the best OpAmp got me to work
plotting up the noise curves for a few devices. I find that when I'd
said that "It depends" it was even more the case than I had
thought. Of prime importance is the resistance (impedance) of the
circuitry feeding the OpAmp input terminal. For high-impedance
circuits such as Dave normally uses, the total noise plot for a
device will be quite different from what you see for low resistances
like velocity-sensor coils. And if the circuitry has some capacitors
involved, that changes things, too.
I made an Excel spread sheet to help me visualize what's going on,
plotting the various devices' input-referred Voltage and Current
noise densities. Those I got from measuring manufacturers' plots of
typical, noise values. In addition I plotted the Voltage noise
density with an assumed source resistor, which you can change by
entering the desired value in the 'Plot Data' worksheet. This adds a
voltage noise term that is the current noise times the resistance
value. And something I almost forgot is that the source resistance
is a noise source itself, which must also be added to get the
effective device noise.
Something else that we don't often think about is the output voltage
swing available from the device. Most older devices can swing at
least +/- 15V, whereas many modern parts, like the ADA4528 have much
lower supply voltage limits such as +/- 2.5V. In seismometer
circuits, the key parameter is dynamic range, the maximum signal
available divided by the instrument noise level. In a good design,
that will be a function mainly of the lowest level OpAmps (first
stage) and their associated circuits. When compared with a 15V
device, a 2.5V part will have only 1/6 the clipping level, and in
order for a seismometer to have the same dynamic range, it must
exhibit a noise level that is smaller by a factor of 6. For a fair
comparison with higher-voltage devices, such parts should have their
noise levels increased by that factor of 6, or else noise should be
plotted as dB relative to the device clipping levels. I may try that
in the future.
If you want to play, the spreadsheet is OpAmpNoise.xls,
in http://bnordgren.org/seismo/OpAmpNoise.zip
Regards,
Brett
At 11:18 AM 10/6/2013, you wrote:
>See this app note on the ADA4528 for noise calcs vs source resistance
>
>http://www.analog.com/static/imported-files/application_notes/AN-1114.pdf
>
>My testing has not found any better opamp for telesesimic frequencies and
>source resistances from 100 ohms to 10k ohms (geophones to coil/magnets).
>
>Ciao
>
>Dave
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