In the spice model of the AD8310, the input impedance is very low, less than 1 ohm. Not on the real chip and the specs. Is there a reason ?

I use the AD8310 like in the specs page 14, fig 27 as a wide band detector. I use Multim to simulate the whole circuit, with an input wide band filter in front of the detector AD8310. Thereal  circuit works perfectly, but the simulation is wrong, because the input impedance of the AD8310 crush the output of the filter.

Have an idea ?

  • 0
    •  Analog Employees 
    on Jul 12, 2018 7:00 PM

    The SPICE model should not do that.  Please check how you have the AD8310 subcircuit connected.  

    Input impedance is set by R1 and R2, and each is 500 Ohms, which creates a 1k Ohm input impedance.  I'm guessing you have a low impedance path to ground somewhere in your Multisim circuit, external to the AD8310. 

  • Thank you for your answer, but I don't see any low impedance path outside the subcircuit.And the result :

    As you see, the input seems very capacitive (-3dB @ 1.3 MHz), lowering the input impedance at the frequency of interest.

  • 0
    •  Analog Employees 
    on Jul 17, 2018 8:56 PM

    There is a 1.4pF cap across the input pins(C1 in the model) that is internal to the part and models the differential capacitance.

    The model was tested to 440MHz with a CW source, and matches the measured typical data quite well.  I would recommend running transient simulations with a CW source first, before doing anything else, to make sure the part is working in your setup.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:06 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin