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AD8675 and AD8597/99 Input impedance?

Hi all - hope you're able to help me with this.

I've been unable to find the input impedance and Cin for both AD8675 and AD8597/99 in the datasheet(s).
- I apologize if I've missed the obvius....

Thx in advance

Kind regards


  • Kasper,

      Both of the parts are very low voltage noise, so they are normally used with impedances less than 2k-5k, so input resistance is

    not generally important.  The AD8597/8599 d.s. on page 3, right above "Output characteristics" does have Cdm and Ccm.

    If we have some recent date code material in stock, we can measure the AD8675 this week.  If not, I'll have to order some

    samples from SEA, so give me a few weeks on that.

      BTW, AN-940 has some good tables in the back.  Whenever we introduce a low noise part, we update the tables.


  • Thanks to both of you for your quick answers.

    How precise is the macro spice models - can rely on thoose to get both the impedance and and Cin?

    Kiind regards

  • Kasper,

    For the AD8675, Ccm is ~3.8 pF and Cdm is ~9.6 pF.


  • Kasper,

      Spice macro models hardly ever have Ccm and Cdm modeled.  You have to be careful just adding them as the actual value,

    because if the macro model used an actual transistor diff pair, then you will have C-pi also, so the Ccm and Cdm will be

    higher than they should be.  Over the past two years, we have had more requests for Ccm and Cdm (and also current noise),

    so we are doing a better job on recently released models.


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