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OPA387: Low frequency noise with cascaded choppers

Expert 3925 points
Part Number: OPA387

Dear e2e support,

My customer is looking for the lowest noise possible AOP solutions at low frequency.

Choppers technologies such as the OPAx387 seems to be the best approach.

Question is: when we cascade several choppers, can it generate low freq noise, considering the HF noise from this chopper which could potentially create aliasing frequencies?

If yes, in that case, would that be better to choose different PNs to mix different chopper HF frequencies?

Regards,

  • TISL,

    When you cascade several choppers, you presumably do this to gain up the signal and by doing so the first gain stage will dominate the total output noise - see below.  Thus, you do not need to worry about aliasing issues.  For unity gain, G=1, in the graph on the left below, the output voltage noise spectral density matches 8.5nV/rt-Hz given in the datasheet while on right the total output noise approaches 30uVrms.  Of course, the total output noise can be minimize by adding a low-pass filter at the output.

    Cascading two G=10 stages shows that the total output noise is 100x greater than the output noise of unity gain op amp; thus, second stage noise contribution is insignificant. 

    Having said that, so far we have only considered voltage noise without taking into account the conversion of the input current noise and IB chopping spikes across the gain resistors (these effects are NOT simulated). It is not clear from your description but considering that you chose a chopper amplifier for your application, you most likely also want to minimize the input offset voltage and temperature drift.  To this end, please review the table below recommending for OPA387 maximum source and feedback resistance (Rf|Rg) to be at or below 10kohm.

    However, if you want the OPA387 input voltage noise to dominate the total output noise, you would also need to keep the Rf||Rg resistance even smaller, down to about 500ohm - see table below.

    Since depending on choice of resistors, chopper current noise may dominate the total noise, please review the white paper discussing the trade-offs by clicking on link below:

    https://www.ti.com/lit/wp/sboa586/sboa586.pdf?ts=1711729121849&ref_url=https%253A%252F%252Fwww.google.com%252F

    OPA387 voltage noise.TSC