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Original question:

ADS1261-Q1: ADS1261 for thermistor measurements without IDAC

ADS1261-Q1: REFOUT Loading and ADC Accuracy

Drew Nakamura
Prodigy 130 points
Part Number: ADS1261-Q1
Other Parts Discussed in Thread: ADS1261

Hi, 

I recently started this discussion, but thought I should make another thread as the original question there was answered.

If the REFOUT pin on the ADS1261 is supplying close to the current limit (say 8mA out of 10mA), will that compromise the ADC accuracy at all? I'm aware that the 2.5V voltage will droop by ~400uV, but will the accuracy fall outside the typical room temperature max of 0.2% as the 10mA is reached?

Thanks,
Drew

  • +1
    TI__Guru 52296 points

    Hi Drew Nakamura,

    The ADS1261 VREF has a load regulation spec that I believe you have identified (50 uV/mA at 8mA would be 400uV). This is in addition to the room temp accuracy spec, or any drift specifications. Typically you would add these specs using the root sum of squares (RSS) method to determine an overall expected accuracy.

    However, you can see that the load regulation will be a much smaller percentage of the error compared to the initial inaccuracy. For example, a 0.1% error of 2.5V is 2.5mV, whereas you calculated 0.4mV (400uV) assuming a load current of 8mA. This error is >6x smaller compared to the initial accuracy spec, so it will be relatively insignificant in the overall error

    One other thing to consider is that the 10mA VREF current rating is a maximum, and is also the maximum allowable into any pin (see the Absolute maximum ratings table). It is therefore not a good idea to run the VREF at 10mA output current continuously. If you need a significant amount of continuous current e.g. >5mA, you might consider adding a buffer to the REFOUT, especially if you need to bias lots of resistive sensors.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 130 points

    Hi Bryan,

    Thanks again for the thorough response! I will be aiming for ~3mA of continuous current draw, but if that changes then adding a buffer sounds like a great idea.

    -Drew

  • 0 in reply to Drew Nakamura
    TI__Guru 52296 points

    Hi Drew Nakamura,

    I am glad we could help you resolve this issue

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 130 points

    Hi Bryan,

    Sorry, I have one more question on this topic. Regardless of the magnitude of the load regulation error, wouldn't the ratiometric measurement remove it anyway?

    -Drew

  • 0 in reply to Drew Nakamura
    TI__Guru 52296 points

    Hi Drew Nakamura,

    Yes, the ratiometric measurement should theoretically reduce the error due to shifts in the VREF voltage

    -Bryan