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OPA1632: .1to10Hz pp noise vs 1/f specs

David Burton
Expert 1330 points
Part Number: OPA1632
Other Parts Discussed in Thread: PGA281, OPA188, PGA280, OPA333

I'm looking for a fully differential amplifier with very low noise and band of interest .5-40Hz.  The OPA1632 seems quit nice so far, but there is an issue of concern with lower frequency noise performance,  The customer is particularly sensitive about not having an actual .1-10 Hz noise plot, since they say they have in the past been burned by amps that have good noise specs but are very bad below the range on the published graph.

Of course it's 1/f and you can just draw the line further to extrapolate lower frequencies, but apparently there are other sources of noise down there that cannot be estimated by the 6 sigma rule.

How should I interpret data sheet 1/f specs vs the 0.1-10Hz sec plots given with certain other amps?  Is 1/f performance completely useless as a prediction of .1-10Hz noise?

Can I assume an auto zero amp (i.e. PGA281) will completely remove 1/f effect?  I see from the spectral plot in the data sheet it seems this is so, but again no 10 sec time plot of .1-10Hz is given so it makes it a harder sell.

Does the auto zero function eliminate shot/popcorn/whatever noises in the .1-10 Hz band as well as 1/f?

Thanks.

  • 0
    TI__Genius 12631 points
    Hello David,
    The reason we do not publish a noise spectrum below 10 Hz is because of the time it takes to do this and also many standard mid to high frequency spectrum analyzers do not go so low. Its certainly possible that there may be non-idealities below this 10 Hz region which cause the noise curve to deviate from the theoretical 1/f shape. The best way to measure the noise performance at these low frequencies is to simply use a bandpass filter and measure the total noise using a digitizer such as an ADC or scope.

    With regards to choppers and auto-zero amps (such as the OPA188, OPA333, PGA280, etc) it will be best to post the question on the precision amplifier forum since we do not have any such amplifiers on the high speed team. As far as I know the issue with popcorn however will not be eliminated using the chopper architecture.

    -Samir