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ADS131M08: What is the external reference current into REFIN?

Kuba Sunderland-Ober
Prodigy 150 points

Part Number: ADS131M08
Other Parts Discussed in Thread: REF3312

I have a design that's going to use ADS131M08 with an external reference. I wonder:

1. What is the average maximum external reference input current (at fCLKIN=8.192MHz)?

2. What is the largest capacitance being dynamically switched to this input?

I want to make sure that the reference buffer supports the load presented by the ADC. I couldn't find any details about the reference input current in the datasheet.

  • 0
    TI__Guru 69446 points

    Hello Kuba,

    Thank you for your post.

    Sampling capacitance is typically on the order of 10 - 20 pF per channel, so an external bypass capacitor of 100 nF should be plenty to keep the reference voltage input at a stable level while supplying much of the instantaneous current draw during sampling. You might consider an external reference IC such as the REF3312.

    Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 150 points

    Thank you, Ryan. That answers my question.

    Since this is a reference input and thus fairly sensitive, we need a C0G capacitor to prevent vibration from being transduced piezoelectrically into voltage, and from thus coupling into the reference input. I've went with two 10nF C0G 0603 capacitors in parallel. That was cheaper and still somewhat tighter in layout than a 100nF C0G, the smallest of which I could only find in a fairly massive 1206 package. As an anecdote, DigiKey at the moment has no stock of 10nF C0G in 0402, and there's only one part# available as a non-stocked full-reel-only item. Surely this will change in a year or two.

  • 0 in reply to Kuba Sunderland-Ober
    TI__Guru 69446 points

    Hello Kuba,

    Thanks for the tip! Glad I was able to help.

    Regards,