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ADS1118: Input capacitance of digital signals

Ashkan Olyaei
Prodigy 40 points

Part Number: ADS1118

Can you let me know the total input capacitance value of SCLK, CS, DIN and DOUT pins (including package, pad, ESD, etc.)?

We're using it for a low-power application and I want to know how much the dynamic power would be. 

  • 0
    TI__Guru** 100775 points

    Ashkan,


    The pin capacitance for the ADS1118 digital pins is modeled in the ADS1118 IBIS model. You can find the model here:

    www.ti.com/.../sbam083

    For information about the IBIS model structure, you can find information in this document:

    www.ti.com/.../snla046.pdf

    In the model, the capacitance for each pin is shown as a combination of C_pkg and C_comp. The capacitance for the digital lines are approximately 1.5 to 2pF each. There is some difference between the DGS and RUG packages, and the RUG capacitance will be a bit smaller. I would note that the PCB trace lines often have more capacitance than the device themselves.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 40 points
    Thanks for the feedback Joseph.
    Looking at the IBIS file,
    for RUG, C_pkg=0.19pF, C_pin~0.14pF
    for DGS, C_pkg=0.32pF, C_pin~0.31pF

    So does C_comp (1.5pF for SCLK for example) have to be added to the C_pin number above, or does it already include it?
    Ctotal = C_pkg + C_pin + C_comp?
    IN the IBIS model structure, I only see C_pkg and C_comp (no C_pin).
  • 0 in reply to Ashkan Olyaei
    TI__Guru** 100775 points
    Ashkan,


    I'm not an IBIS expert, but I would add just C_comp and C_pkg together and use that as the typical value. My understanding of the model is that the C_pkg (min and max) encompasses the min and max values of the different C_pin values. This isn't entirely true for all of the pins, but it looks like the min/max values are taken from the higher and lower C_pin values.

    The parasitic resistances and inductances in the model are likely small compared to the external resistance and inductances, so for most frequencies, the sum of the capacitance will be a good approximation.

    Note that the IBIS model is used for the digital signal integrity. These models only approximate the behavior at the pins, so the capacitance model can only be considered approximate.

    If you really need more information, you should be able to ask questions about the ibis model at:
    elab_ibis@list.ti.com


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    Thanks Joseph. Appreciate it.