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INA849: Voltage noise

Takumi Suzuki1
Mastermind 8510 points
Part Number: INA849

Hi,

I would like to ask you about voltage noise.

According to the datasheet specification, the voltage noise changes with input stage gain.

(Fig 7-24, page 11)

I do not understand why the noise changes...

Will you tell me the reason or give me the documents which explain the reason?

As a result, can I understand that input stage noise is " 1nV/√Hz", correct?

Thanks and Best Regards,

Takumi

  • +1
    Guru 140200 points

    Hi Takumi,

    don't overlook the short word "RTI". "RTI" means "referred to the input". To get the noise at the output of OPAmp multiply the noise in figure 7-24 by the gain.

    Keep in mind that the OPAmp has two different noise sources, the noise of the input stage (1nV/SQRT(Hz)) and the noise of the output stage (45nV/SQRT(Hz)), see page 6 of datasheet. With a very low gain the noise is mainly determined by the higher noise of the output stage, while with a very high gain the noise performance profits from the very low noise of the input stage. This absolutely makes sense since instrumentation amplifiers are normally used in very high gain applications.

    Kai

  • +1 in reply to kai klaas69
    TI__Guru 64530 points

    To follow up on Kai's explanation see below graph:

  • 0 in reply to Marek Lis
    Mastermind 8510 points

    Hi Kai-san and Marek-san

    Thank you very much for your explanation !!

    RTI and RTO... I understand well.

    Regards,

    Takumi