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INA331: Noise issue

Markus Kuderer
Prodigy 20 points

Other Parts Discussed in Thread: INA331, TINA-TI

Hi,

I'm using an INA331 for amplifying the output signal of an wheatstone bridge. That output signal is digitized with a high precission ADC. That works fine, but when I take a look at the spectrum of the digital output signal the noise floor seems to be quite high.

I tried to figure out the origin of this noise and calculated back from the output noise to the input noise at the single components. According this calculation, the input referred noise of the INA331 should be around 10 nV/Sqrt(Hz) at a frequency of 1kHz.

BUT: According the datasheet of the INA331 @ 1 kHz is about 46nV/Sqrt(Hz)!
I'm used that the measured performance is usually about or slightly worse than expected, but to be a factor of nearly 5 better than given in the datasheet is very strange...

Might this be an issue of the external components of the INA331?
When I geht this right out of the data sheet the input referred noise (RTI) was determined using a gain of 25 - so with external components present. In my circuit I use the INA331 without external components and hence with a gain of 5 - which is a factor 5 less than the configuration in the datasheet and is nearly about the factor I get at the comparison of ideal and real noise. But from my point of view this doesn't make any sense....

Has anybody an idea?

I can give you more details in case you need some input...

Thank you very much,

Markus

  • 0
    Genius 5430 points

    Markus;

    Have you calculated the noise of your Wheatstone bridge?

  • 0
    TI__Genius 12375 points

    Hi Markus,

    It might be helpful if you walked me through the steps you took to calculate the noise spectral density of the INA331 from your ADCs output spectrum. It's good to remember that the ADC will have it's own noise as will the voltage reference that you are using and that these noise sources will add to the total noise as a root sum of squares.If you can provide a schematic or at least specs/part numbers of the ADC, voltage reference, and the impedance of the wheatstone bridge you are using we can begin to track down the dominant noise source in your system.

    To answer your question, yes external components will affect the noise output as every resistor contributes thermal noise to the output. However, I would expect that these contributions would be accounted for in the noise specifications in the datasheet.

  • 0 in reply to Neil Albaugh
    Prodigy 20 points

    Hello Neil,

    yes I have. It is about 5.62 nV / Sqrt(Hz) - so much less than the measured one :-(

    I'm really sure that the INA331 defines the noise performance, but I don't know why it is so much less than given in the datasheet...

    Regards,

    Markus

  • 0 in reply to Markus Kuderer
    Genius 5430 points

    Markus;

    Perhaps if you post a TINA-TI file of your circuit (only the analog portion) we can answer this puzzle. Also a bit more on how you did your calculation will be helpful.