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Original question:

ADS1263: Noise reference spec

ADS1263: Current noise of IDAC outputs ?

Damien Buisson
Prodigy 70 points
Part Number: ADS1263

Tool/software:

Hello,

I have seen in another post that the the internal reference has a huge voltage noise (290nV/sqrt(Hz) above 10 Hz).

- But what is the current noise output at boths the IDACs pins as it seems they use the internal reference ?

- Are these noise currents correlated or not ?

Multipled by the reference resistor in a high-side RTD measurement, it will create a voltage noise at reference input.

Thanks !

  • 0
    TI__Mastermind 45455 points

    Hi Damien,

    As you can see in the post you mentioned, it is true that the ADS1263 datasheet states that the IDACs track the internal reference, but that is more with regards to the matching between the two IDACs. 

    We do not have a specification about the noise output on the IDACs, it is also true any noise on the IDAC current will generate errors while the current is flowing through external reference resistors in a RTD design, however RTD measurement is usually implemented with a ratiometric measurement which removes any error caused by IDACs including noise and drift over temperature, it generally resulting in more repeatable and accurate measurements.

    As I recommended in your another post, please keep your queries about the same ADC into the same thread. Thanks.


    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 70 points

    It is true for one measurement with a ratiometric value, but the IDAC chopping mode makes four measurements.

    Between each measurement the IDAC noises have changed so IDAC noises are NOT canceled when chopping current sources.

    I need to know the order of magnitude of them.

  • 0 in reply to Damien Buisson
    TI__Guru 52146 points

    Hi Damien Buisson,

    We don't have a current/voltage noise to share with you with respect to the IDACs. But I also would not expect the noise at the output of the IDAC to vary much between adjacent measurements, is this something you have experienced previously? Were you using a discrete (programmable maybe?) current source IC, or were you building current sources out of transistors?

    If you are trying measure a 3-wire RTD and compensate the lead resistance using two IDACs, I would suggest considering using a single IDAC and two measurements instead. This will generally be the highest accuracy solution, and the noise will cancel out since you are not rotating / swapping the IDACs.

    This method of course requires an extra ADC channel to measure the voltage between the 2nd and 3rd RTD wires, while the main differential measurement is taken between the 1st and 2nd RTD wires.

    -Bryan