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ADS1298

Don Hall
Prodigy 30 points

Other Parts Discussed in Thread: ADS1298

ADS1298 - I was told via phone that ADS1298 questions should be directed to the Precision Data Converters Forum. But, when I include ADS1298 as the Part Number (above), the Medical Forum is automatically invoked. So, I omitted the Part Number, above.

Hello,

I inherited a design that uses an ADS1298. The device specification indicates the input capacitance is 20 pF (typical), which seems high to me. Also, the spec does not indicate whether that input capacitance represents a common mode or differential value.

Since there is no SPICE model available that I can find for the ADS1298, I downloaded and reviewed the IBIS model. I am not an IBIS model expert, but it appears as though the IBIS model indicates that the analog inputs have a much lower input capacitance than 20pF.

So, I have two questions: (1) Does the specified input capacitance represent the common mode or differential capacitance and (2) Does the specification indicate the incorrect capacitance (or is the IBIS model incorrect or neither of those two things)? Thank you.

Regards,

Don Hall

Medtronic, Inc.

  • 0
    TI__Guru 69156 points

    Hi Don,

    Sorry that your post was automatically placed under the Medical Forum. I think this has been happening the last couple days since we made an update to our E2E forums. I'll pass the feedback along.

    The input capacitance is common-mode on the ADS1298, not differential.

    The information in the IBIS model is only pertinent to the packaging material (i.e. bond wire inductance, capacitance from bond wire to the die, etc.). It is not reflective of the design characteristics.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 30 points
    Hi Ryan,

    Thank you very much for your response; I appreciate it.

    Following my post, and after doing additional reading on IBIS models, I was pretty close to the conclusion that the parasitics in the model were packaging-only. But, your confirmation certainly helps.

    Regarding the 20pF of CM input capacitance designated in the spec, I am still puzzeled. I do not see how the design I investigating even works with that capacitance. I will continue to look at this.

    Regards,

    Don Hall