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ADS8556: Capacitance value of REFIO pin

Masanori Tachibana
Genius 5350 points
Part Number: ADS8556

Hello support team,

Our customer is using ADS8556 now.

According to it's datasheet, the optimum capacitance value of the decoupling capacitor connected to pin 51 (REFIO) is 470 nF.
But the customer would like to change to 1000 nF because it is difficult to obtain parts currently.

Looking at External load capacitance on page 9 of datasheet, the maximum value of it is not specified.
Is it OK to use 1000 nF?
And, are there any concerns regarding using 1000 nF?
(For example, the time to stable operation at power on becomes longer, and so on)

Best regards,
M. Tachibana

  • 0
    TI__Mastermind 24940 points
    Hello,
    I will confirm with the design team how increasing the cap size will affect the device, if it is okay with using such a large capacitor.
    With more than a double in size of the capacitor, the DAC will need to be able to support it as well as drive the three internal buffers following for each channel, which makes me hesitant to say it can be done; but I will confirm with design team and let you know.
    Regards
    Cynthia
  • 0 in reply to Cynthia
    Genius 5350 points

    Hello Cynthia-san,

    Thank you for your reply.
    We are waiting for feedback from your design team.

    Best regards,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Mastermind 24940 points

    Design team has responded and confirmed that the device can function with minimal degradation with a 1000nF capcitor on the REFIO pin.

    With that said, we cannot say exactly how this will effect the REF turn-on settling time. The datasheet specs a typical turn on time of 10ms and we do not predict it will be slowed down too much, but we do suggest to check the settling time of the reference with the new capacitor size to be aware of the present changes.

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Genius 5350 points

    Hello Cynthia-san,

    Thank you very much for confirming with the design team.
    Even if 1000 nF of capacitor was connected to the REFIO pin, I understood that it was not so problem.
    I advised our customer to evaluate enough.

    By the way, In Electrical Characteristics: General on the datasheet page 9, "At REFIO pins" of External load capacitance is stated as TYP : 470 μF and MIN : 100 μF. I think that this is a typo and these unit are not μF but nF.
    Can you revise it when the next updating of the datasheet?

    Sincerely,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Mastermind 24940 points
    Thank you Tachibana-san, I will be sure to make a note of this.