PSN-L Email List Message

Subject: Re: OpAmp noise
From: Brett Nordgren brett3nt@.............
Date: Thu, 10 Oct 2013 11:02:25 -0400


>You wrote:
>****The two chip chopper amp design gives such low noise that
>in any circuit with a source resistance greater than about 500
>Ohms, the intrinsic input resistor noise will dominate the overall
>circuit noise. And Chopper amplifiers DON'T show ANY 1/f noise !
>THIS circuit uses +/-15 V SUPPLIES !

I completely agree.  But if we are monitoring a sensor having 
significantly higher resistance than 500 Ohms which, itself, may have 
some modest amount of 1/F noise, it's not clear what that added 
complexity buys us.  When Dave tried replacing the AD706 with an 
LTC1151, which also should have zero 1/f noise, the instrument 
performance did not visibly improve.  Conclusion: we are limited by 
sensor noise at long periods, though it may be interesting to try 
that substitution again now that we have better measuring techniques.

>****I am puzzled as to what you are quoting when you say your
>position sensor has an OUTPUT resistance of 50 K ? I thought that
>you / we had the switched capacitors connected to an opamp ? Is this
>impedance resistive or purely reactive ?

There was a time when I, too, believed that switched capacitor 
circuits were inherently noiseless, the only noise coming from such 
things as imperfect switches.  However I have since found that's not 
true.  A simple switched capacitor circuit exhibits an apparent 
resistance = 1/(f C), which resistance will exhibit the noise 
associated with a discrete resistor of that value.

The 52k value I used is actually the sensor DC output resistance, 
both as measured and simulated in Spice.  Whether that has a 
corresponding sqt(4kTR) voltage noise density, I don't know yet, but 
to be conservative I was assuming that it did.  In any case, it 
definitely does affect the degree to which the OpAmp's current noise 
adds to its total low-frequency noise.  I'll try some more 
simulations to see if I can discover how that 52k varies with frequency.

I sure wish I understood switched-capacitor circuits better.



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