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ADS131E08 VCAP1 with external reference

Bruce Lott
Prodigy 220 points
Other Parts Discussed in Thread: ADS131E08S, ADS131E04

When using the ADS131E04/6/8 with an external reference, does the VCAP1 cap value matter?  Is it even necessary?  I was thinking of using a small value such as 470pF like suggested for the ADS131E08S.

Your thoughts?

  • 0
    TI__Mastermind 38745 points
    Hey Bruce,

    The VCAP1 voltage is used for other things besides just the input to the internal reference buffer. I would recommend using the ADS131E04/6/8 datasheet specified capacitor value.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 220 points

    Brian,

    22uF is not a cheap cap in tantalum. Since I'm not using the internal reference, do I still need to avoid vibration-induced ferroelectric cap noise on this node? This unit will be exposed to some 60 Hz vibration.

    Alternatively, if I were to use a smaller value (maybe 1uF?) in my application, would that only affect the power-up sequence (RESET-related) timing (the only other use mentioned in the datasheet)?

    Bruce

  • 0 in reply to Bruce Lott
    TI__Mastermind 38745 points
    Hey Bruce,

    I apologize, the information I previously gave to you was incomplete. It is true that the VCAP1 net extends to other places on the die besides the internal reference buffer input, but after speaking with the analog designer, he says that the degradation to noise performance would most likely be marginal if you were to decrease the size of the capacitor (by the way in case you want to benchmark, you would only measure the difference in performance at |Vin| >= 0 V).

    That being said you could probably use the ADS131E08S's recommended capacitor value of 470 pF for VCAP1. In addition, tantalum is not absolutely necessary for that capacitor.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 220 points
    Brian,

    Sorry to nitpick, but what does he mean by "most likely be marginal"? Is it neglectable? If not, any ballpark delta dB or ENOB for a 22uF to 470pF VCAP1 change?

    What is sensitive to noise on (or downstream of) the VCAP1 net when the internal reference buffer is disabled?

    This prompted a related question: Considering that going from 22uF to 470pF is a huge increase in bandgap filter cut-off frequency, it seems strange that the noise tables in the two datasheets (08 and 08S) are identical. Why is this?

    Thanks,
    Bruce
  • 0 in reply to Bruce Lott
    TI__Mastermind 38745 points
    Hey Bruce,

    I just followed up with him again. Again, I misinterpreted his words. He said the effect should be marginal before I told him you were using an external reference. He actually doesn't think it will affect performance at all to put a smaller cap but recommends putting a cap nonetheless since we have never tested it otherwise.

    The reason that the noise doesn't seem to properly scale with the bandgap capacitor is that reference noise is not a factor in an input short noise test. The output of a data converter is proportional to Vin/Vref. Imagine if there is some error "e" that models noise on the reference voltage. The inputs of the data converter are shorted together which means the output will be proportional to 0/(Vref+e) = 0. Even with offset from the PGA and modulator, the numerator in that fraction is still very small in comparison to the denominator. Reference noise scales with the magnitude of the input voltage. Thus expanding the noise bandwidth of the bandgap does not affect the datasheet specification. However, since you are using an external reference, this should not matter.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 220 points
    Thanks for your thorough followup on this.
    Bruce