PSN-L Email List Message

Subject: Re: OpAmp noise
From: Brett Nordgren brett3nt@.............
Date: Mon, 07 Oct 2013 15:23:04 -0400


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, 


At 11:18 AM 10/6/2013, you wrote:

>See this app note on the ADA4528 for noise calcs vs source resistance
>My testing has not found any better opamp for telesesimic frequencies and
>source resistances from 100 ohms to 10k ohms (geophones to coil/magnets).


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