PSN-L Email List Message
Subject: Re: OpAmp noise
From: Brett Nordgren brett3nt@.............
Date: Thu, 10 Oct 2013 11:02:25 -0400
Chris,
>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.
Regards,
Brett
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