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ADS8556: ADS8556 Capacitor Values with DC BIAS on REFC_x

DAVID SABALESKY
Prodigy 225 points
Part Number: ADS8556

Hello, I am inquiring on the change in capacitance on REFC_x (A,B,C) with a 10uF X7R 10V capacitor in 0603 package.

With a 3.3V DC BIAS, the capacitance drops to  6.1uF. 

Is it acceptable to use this part with this change in capacitance?

The current design uses a 10uF X7R 50V capacitor 1206 package and has lower C drop at 3.3V DC BIAS, almost negligible.

Should ESL parameters be considered as the loop area is reduced using 0603 or 0805 versus using a 1206 package?

How should the 10uF capacitor be selected, i.e. parameters, for best performance of the ADS8556?

Sincerely,

David

  • 0
    TI__Expert 8625 points

    HI David,

    Thanks for your question. The capacitance drop you see is unideal as we recommend 10uF decoupling caps on the REFC_x pins. I would recommend the capacitor used with the ADS8555EVM-PDK REFC_x pins: CL21A106KAFN3NE (CAP, CERM, 10 uF, 25 V, +/- 10%, X5R, 0805). If using a smaller capacitor size like 0603, you would need to use one with a larger voltage rating to account for the steeper derating curve.

    Best,

    Samiha

  • 0 in reply to Samiha Sharif
    Prodigy 225 points

    Hello Samiha,

    I am reviewing a prior design and the temperature and characteristic range of X5R is not within the specifications. The preference is X7R. Although in cases where X7R is unavailable or not need the X5R would be a good fit. I have an extremely tight design where it would be nice to reduce the package size down from 1206 to 0805 and preferably 0603.

    1. Can the 10uF value be increased so that the drop will not go below a certain value?

    2. What is the capacitance recommended range for the REFC_x pins? 

    3. What is the acceptable C drop over DC BIAS and TEMP RANGE to meet the operating conditions for the ADS8556.

    4. How was the 10uF capacitor value derived for best performance?

    5. Does ESL of 1206 package versus 0603, as well as loop area have any significant impact.

    I would like to use smaller components and those with better TEMP and DC BIAS, and hopefully industry keep progressing in discreet component development.

    In designing complex systems, I believe that having full design criteria inevitably makes our decisions easier. As design engineers we never live in a perfect or ideal world however we can design as best we can with available components and make the logical tradeoffs for best fit and performance.

    I greatly appreciate your insights and look forward to hearing from you.

    Sincerely,

    David

  • +1 in reply to DAVID SABALESKY
    TI__Expert 8625 points

    Hi David,

    Thanks for the details. The decoupling capacitor value is decided by the switching current caused by the ADC sampling capacitor and the internal reference buffer. The minimum effective capacitance you may use at the REFC_x pins is shown below, as 4.7uF, so the 0603 capacitor you referenced using before should be fine. That is the only parameter the datasheet defines about the required decoupling capacitor.

    I hope that helps.

    Best regards,

    Samiha