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OPA2170: How to measure Input Impedance with IN+ and IN- pin of the OPA

Brian Wang0928
Genius 12910 points
Part Number: OPA2170
Other Parts Discussed in Thread: TINA-TI

We're wondering to know how to measure the input impedance as below table shown. 

Customer would like to verify it during design phase. thanks.

  • 0
    TI__Genius 12910 points

    One more question. how to measure the Ro value? Is that component related to the stability or Phase margin? Except Ro, what component inside device related to P.M?

    Thanks. 

    Regards

    Brian

  • 0 in reply to Brian Wang0928
    TI__Guru**** 152366 points

    Hello Brian,

    Are these planned measurements with real equipment or in a simulation? 

    Ro determines loop gain or phase lost due to output load. 

    Phase margin is primarily set by the external components. See figure 19 in data sheet for unloaded gain and phase (margin).

  • 0 in reply to Ron Michallick
    TI__Genius 12910 points

    It's for AC inverter real equipment.

    How is the Ro measured? In the datasheet it says the typical is 900 Ohm.

    Also, there is another competition part from MC34074VDR2G. It says the |Z0| is 30W. I've a little confused about the unit? do you know why it's in W unit?

    Regards

    Brian

  • 0 in reply to Brian Wang0928
    Guru 140200 points

    Hi Brian,

    I'm sure that the "W" is a typo and should be "R".

    If you are interested in the phase margin and have stability questions you should watch this TI's training video series on stability:

    https://training.ti.com/node/1138805

    The best way to carry out a stability analysis is to run a phase stability analysis in TINA-TI (or a similar Spice simulator).

    I would not carry out any measurmentts by myself to find out the input capacitances or the open loop output impedance of an OPAmp. I would simply trust the datasheet specifications and the TI's Spice models. Carrying out the measurements is so complicated and sophisticated that the chance to end in huge measuring errors is extremely high. Keep in mind that you need lots of very expensive measurement devices and lots of experience to perform this measurements properly.

    Kai

  • 0 in reply to kai klaas69
    Guru 48395 points

    And also, if you want to get a start on simulated I/O impedance measurements, one approach is in this article for the inputs, (and actually, the models are often not matching the datasheets - this article was how to fix this assuming the datasheets are right)

    file:///C:/Users/Michael%20Steffes/Downloads/5008.Input%20Impedance%20Extraction%20and%20Application%20for%20High%20Speed%20Amplifiers%20May29%20(5).pdf

    The open loop output impedance model has evolved quite a bit over the last few years. Used to be a simple R, but that is not very accurate for RRout devices over frequency, 

    That is an easy simulation also, 2011 model, may need updating as this is just 10ohm over a wide span. RRout designs have much higher DC and more involved Zol curves. 

    OPA170 open loop Zol.TSC

  • +1 in reply to Michael Steffes
    TI__Mastermind 19607 points

    Hey Brian, 

    Was this issue resolved? I'll mark Michael's response as "TI Thinks Resolved," please let us know if the issue is not resolved and how we may assist further. 

    All the best,
    Carolina