PSN-L Email List Message

Subject: OpAmp noise
From: Brett Nordgren brett3nt@.............
Date: Sat, 05 Oct 2013 14:51:37 -0400


You wrote:
>Subject: Re: nice online Opamp Input Noise Voltage Tutorial
> >What is the noise at DC ?
>Actually, DC by definition, does not vary, and so has zero noise.
>****Umm ? DC = Direct Current. The Voltage is NOT defined !

I agree.  An ideal DC source doesn't have zero noise, it's just that 
noise density simply has no meaning at DC.

>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.
>****These are VERY NOISY compared to TRUE chopper amplifiers.

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.

> >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.
>****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.

Here the noise is expressed as a constant noise density at higher 
frequencies and a corner frequency, below which the noise becomes 1/F 
in nature.

Voltage noise density, nV/root Hz     Corner frequency, Hz
AD706         14.6          8.45Hz
OPA227       2.47         19.9Hz
LT1007CN   3.14         36.9Hz

Current noise densiity - fA/root Hz,    Corner frequency - Hz
AD706           44.6        4Hz
OPA227         379        26.7Hz
LT1007CN     434        36.9Hz

As mentioned, it all depends on the circuit, and in particular the 
resistance, R, of the source circuit being sensed by the IC.  The 
AD706 has significantly lower current noise and lower 1/F corners and 
so should perform best with higher R values and at lower frequencies.
For R > 15k  The AD706 is the best from DC up to 1Hz
For R > 40k   The AD706 is the best at all frequencies

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.



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