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OPA2182: Cause of 1/f to comes out?

Part Number: OPA2182
Other Parts Discussed in Thread: OPA182

Hi Experts,

Please assist us with the customer's inquiry below.

What might cause 1/f to come out of the OPAx182?  Can e.g. iffy grounding, lack of power regulation, and/or insufficient bypass caps cause this?   (which, for the record, displays all three of the problems I listed).  

The noise density at the output out of the amp shows a substantial 1/f starting at about 100Hz.  The spec sheet shows a flat response, as would be expected of a zero-drift/chopper amp.  I know it isn't the sensor, as I have a different amp/DAQ chain that doesn't display 1/f from that sensor.
This is an issue as our signal of interest is in the 0.5-3Hz range.  I don't need you to solve the problem for me. I'm just wondering if there are any known cases where 2182 would, to put it plainly, act as if it wasn't a chopper amp.

Thank you in advance. 

Best regards,

Jonathan

  • Hi Jonathan, 

    The supply noise on the supplies could be an issue.  Although, the power supply rejection ratio vs frequency is quite high at around the ~120-dB level at low 0.5-Hz -3-Hz frequency range.

    One thing worth mentioning that could degrade the performance of zero-drift (chopper) amplifiers is high source impedance, or high impedance mismatch at the inverting and non-inverting inputs of the op-amp.  Since the OPA182 is a chopper, the input bias current does exhibit periodic switching transients. These input bias current transient become transformed to a voltage offset when the bias currents flow through the high value circuit resistors.  Hence, in order to eliminate the noise generated by IB spikes across the input resistors and offset shift caused by large unmatched input impedances in chopper amplifiers like OPAx182, we recommend to attempt to match the equivalent impedances at the inverting and non-inverting input terminals, as well at keeping the equivalent impedances relatively low, around the ~10kΩ range.  In addition to reducing the impedances and feedback components, adding the feedback capacitors and matching the capacitors can help improve results.

    Below are a couple of posts related to this topic:

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1190764/effect-of-the-input-impedance-of-the-source-to-the-op-amp/4494822

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1165061/opa2188-noise-reduction-in-howland-current-source/4382915

     Please let us know if you have an schematic you would like us to review.

    Thank you and Kind Regards,

    Luis