From: Brett Nordgren 
To: psnlist 
Sent: Sat, 5 Oct 2013 19:52
Subject: OpAmp noise

Chris
You wrote:
****TRUE chopper amplifiers.
SEE:- http://cds.linear.com/docs/en/application-note/an45f.pdf

We needed to keep our circuits small and simple and so took the
approach that if something couldn't demonstrate that it improved
performance we didn't use it.  In your reference, for one of the
chopper circuits I noticed the comment "In general, to maintain low
noise performance, source resistance should be kept below 500
Ohms."  so I'm not exactly sure what that implies for our circuits.

****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 !
    It is generally the first amplifier stage which needs to have very
low noise, but capacitative coupling stages may also need to be
optimised for low noise at long periods - eg in period extending
circuits.

 In the FBV designs, after much testing, the AD706 has
proven to be a truly excellent all-around device.
****Also look at the OPA227 and the LT1007CN?

Per your suggestion I looked at those devices vs the AD706.  I
measured the "typical" noise curves of all three to get the data below.

****Thanks for doing the calculations. Most amateur equipment uses
magnet + coil sensors. Geoff, to whom I was replying, uses an HS-10
with a resistance of 400 Ohms, which was why I suggested the two
alternative opamps. Some of the very old amateur equipment used
huge coils with much larger resistances, but when NdFeB rectangular
magnets became cheap and readily available, it was possible to build
quad magnet sensors with greatly increased coil outputs  >10x and
resistances of a few hundred Ohms - hence low noise. The soft Iron
backing plates also give good immunity to external fields.

Our position sensor has an output resistance of a little over 50k and
the integrator op-amp is seeing several megohms.
Probably with coil/magnet velocity sensors which generally have a
relatively low resistance, the other devices would be quieter than the
AD706.

****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 ?

    Regards,

    Chris Chapman




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