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OPA191: Reduce reference noise

Dukel
Intellectual 610 points

Part Number: OPA191
Other Parts Discussed in Thread: REF6025

Hi,

I would like to reduce the reference noise of the underneath schematic. I tried to use the REF5025 (image under schematic) but according to TINA this only increases my output noise. Any advice?

  • 0
    Guru 140200 points
    Hi Luc,

    you could use a RC-filter at output of REF5025 to decrease the noise.

    Can you give some details about your application? What signals do you expect (levels, source impedance, etc.)? What is the frequency range of interest? ...?

    Kai
  • 0
    TI__Mastermind 23050 points
    Hi Luc,

    TINA voltage sources are ideal and as such do not contribute any noise, so adding a real reference is definitely going to add noise to your system. Are you primarily concerned about 1/f noise from your reference, or just the total output noise of this circuit? It is very difficult to filter 1/f noise because it is so low frequency, but here are some general steps you could take to help reduce your overall system noise:

    1) Increase the the value of your output capacitor on the reference. You don't really want to add resistance between the output of the reference and input to your op-amp because you want to drive the op amp with a low impedance source. The output capacitor could be increased to 47uF, just make sure to choose a cap with an ESR less than 1.5Ohms for stability. This capacitor should be placed directly at the output of the reference, and then an additional 10uF could be placed at the op-amp circuit input.

    2) Adding a 1uF capacitor to the trim pin will help reduce noise generate by the bandgap reference.

    3) If you want to further reduce your system noise, you could limit the bandwidth. The more bandwidth you can throw away, the less noise you will see on the output. I would recommend adding capacitors across R2 and R4 to accomplish this. This won't really help you with the 1/f noise of the reference, but it will reduce the noise contribution from your input and the op-amp itself.

    4) I was not able to find a lower noise reference, but you may consider using the REF6025. It includes and integrated buffer op-amp that you could configure as a filter to try to cut down on some of the 1/f noise. Just keep in mind designing a filter in the range of 1-100Hz will require high value passive components and this may be impractical for your design.