PSN-L Email List Message

Subject: Re: nice online Opamp Input Noise Voltage Tutorial
From: Brett Nordgren brett3nt@.............
Date: Mon, 30 Sep 2013 10:38:12 -0400


I agree with you, Dave S has found a very nice tutorial.  It's 
probably worth watching more than once.  I note that Dave N regularly 
uses a similar H-P DSA instrument to measure the overall noise 
performance of his seismometers.

At 01:18 AM 9/30/2013, you wrote:
>VunderBar, I have never seen a better lecture on NOISE.
>What is the noise at DC ?? it seems a voltage regulator might do better.

Not sure how a voltage regulator applies here.  Just an aside: When I 
was designing power supplies we talked about noise as something you 
could see on an oscilloscope and drift as something you could see on 
a meter.  Now with seismometers we're working with 'noise' that's 
sometimes so low in frequency that it's hard to see even on a meter 
and has to be digitally recorded and analyzed later.

Actually, DC by definition, does not vary, and so has zero 
noise.  Attempting to plot the noise spectrum of a DC signal using an 
FFT would give zero at all frequencies, except that in practice FFT's 
usually think there's a big step at the beginning, from 0 to the DC 
value, so they will be showing noise that's not really there in the 
DC.  That effect can be minimized by starting out slowly turning up 
the voltage to its final DC value in a carefully planned way.

>I have heard below 1Hz you want to use bipolar devices only ??

The choice of bipolar vs FET input devices is more a function of the 
impedance of the source device being measured.  For an electrometer 
or pH meter (very high impedance) FETs might be better.  For 
thermocouple measurements (very low impedance) bipolars are 
definitely best.  In our circuits bipolars do quite well, even in 
some rather high resistance circuits, and so far Dave N hasn't found 
any FET devices that exhibit as low noise in his circuits.

There's also the possibility of using an auto-zero (chopper) 
device.  They work well with both high and low impedance sensors, but 
they are at their best at low frequencies, below a few Hz.  Above 
that their noise specs can be significantly worse than ordinary 
Op-Amps.  Again, in real circuits, Dave has found that his ordinary 
op-amps do somewhat better than any of the auto-zero parts he has tried.

>What about cosmic rays and ...

Sure, but they are relatively rare and the probability of an ion 
hitting a sensitive spot on an IC would seem to be rather small.  On 
average they don't add significantly to the total noise level.  In 
space however.......

>I hear that diodes can be used for a geiger counter and
>op amps are full of diodes.

I think diode detectors work best at higher flux levels as their 
sensitive areas tend to be small.  They also use diodes which are 
specially designed for the purpose (expensive).  Ion chambers and 
scintillation counters can have large volumes and are much more 
sensitive to low radiation flux 

>There has to be one best opamp for 100Hz to 100seconds,
>What Op Amp is it ??

Unfortunately, the choice will depend entirely on the circuit it is 
being used in.  In the FBV designs, after much testing, the AD706 has 
proven to be a truly excellent all-around device.  It is possible 
that some of the newer surface-mount parts might be slightly better, 
but so far we have tried to stay with DIP devices to make for easier assembly.

Hope that helps,



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