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ADS7825: About capacitors connected to pins CAP and REF

Masanori Tachibana
Genius 5350 points
Part Number: ADS7825

Hello support team,

Our customer is using ADS7825 now.
We have gotten questions from the customer regarding the capacitors which should be connected to the pins of CAP and REF.

In the PIN ASSIGNMENTS described in the data sheet, the capacitor connected to the pins of CAP and REF is specified 2.2 μF of "tantalum" capacitors.
Is it possible to replace these capacitors with "ceramic" capacitors?
Also, could you teach us the recommended values and the characteristics to be noted if those are possible to change to ceramic capacitors.

Sincerely,
M. Tachibana

  • 0
    TI__Mastermind 25020 points
    Hello,

    This device is quite old and was characterized with tantalum capacitors, thus it does require tantalum capcaitors.

    If the customer does not want to use tantalum capacitors, they can use ceramic capacitors, of the same farad value, but they will need to add a resistor in series with the ceramic capacitor.
    The concerning difference is that tantalum capacitors have a much higher ESR, in range of tens of Ohms, while ceramic capacitor's ESR tends to be in the low milliohms.
    I would recommond a 10 to 15 ohm series resistor if using a ceramic capacitor.

    Regards
    Cynthia
  • 0 in reply to Cynthia
    Genius 5350 points

    Hello Cynthia-san,

    Thank you for teaching me.

    I understand that it becomes equivalent to a tantalum capacitor by inserting a resistor corresponding to ESR in series with a ceramic capacitor.
    Since the capacitor connected to the pin of CAP is a buffer output, I think that the phase margin is concerned and ESR equivalent resistance is necessary.
    However, regarding the capacitor connected to the pin of REF, I think that ESR may not affect. So is the series resistance necessary?

    Sincerely,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Mastermind 25020 points
    No problem.

    This device was release when ceramic capacitors were more expensive and not as good performing as tantalum, therefor the device was characterized using tantalum capacitors. We have looked at other devices previously released characterized with tantalum, and they have shown to work properly with a ceramic in the ref pin, but this device is much older and unfortunately cannot we cannot evaluate it.
    The inside structure does not seem as it would be greatly affected from the capacitor change, but we cannot guarantee performance.

    Regards
    Cynthia
  • 0 in reply to Cynthia
    Genius 5350 points

    Hello Cynthia-san,

    Thank you for teaching additionally.
    I understood.

    I'll ask recommend our customer to add resistance equivalent to ESR of tantalum capacitor.

    Best regards,
    M. Tachibana