> From:    Brett Nordgren 
> Date:    Sat, 05 Oct 2013 14:51:37 -0400
>
> Chris
>
> 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.
>>
>>SEE:- http://cds.linear.com/docs/en/application-note/an45f.pdf

for a very low noise chopper opamp with NO 1/F noise, see the ADA4528

http://www.analog.com/static/imported-files/data_sheets/ADA4528-1_4528-2.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.
>
>> >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

ADA4528       ~6          0Hz (no 1/F noise)

Since we are often interested in teleseismic signals <0.1Hz
the chopper ADA4528 with no 1/F is much superior at these low frequencies.

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

ADA4528        500        0Hz (no 1/F noise)

The ADA4528 looks highest noise here at first glance
but the steep increase of 1/F noise below their 1/F corner frequencies
for the others negates this again.

> 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

See this app note on the ADA4528 for noise calcs vs source resistance

http://www.analog.com/static/imported-files/application_notes/AN-1114.pdf

My testing has not found any better opamp for telesesimic frequencies and
source resistances from 100 ohms to 10k ohms (geophones to coil/magnets).

Ciao

Dave
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