Measuring Impedance Using LTspice

Introduction: Measuring Impedance Using LTspice

About: College student perusing a degree in electrical engineering

Hey everyone this is going to be a simple introduction to generating an AC sweep of a circuit and finding the impedance at any given point, this came up several times in my courses and it was very difficult for me to find any way to do it online so hopefully this will help everyone especially those people (like me) trying to find answers at 3am.

Step 1: How to Set Up the Simulation

To do this its very simple the first step is to make your desired circuit (I will do another instructable on how to actually do this) but leave the voltage source blank.

The next step is to right click on the voltage source and select advanced you will see several options and on the right there will be small signal AC analysis, you can set that to anything however I do 1v at 0 degrees.

Then you will want to set up the sweep since this is an ac analysis you will do AC Analysis as the sweep type then select decade and use 101 points per decade you can change this to match your needs but I have yet to experience a problem with this method, and then set the desired frequency range.

Finally you will want to label the input node as you can see it says V1 in the schematic above the voltage source, of course this can be at whatever point you are measuring the impedance at.

Step 2: Results of the First Simulation

After you simulate and plot the results you will notice they dont look very conducive to take them and find the impedance, the plots images here are the voltage and current at the battery of course you can select them anywhere in the circuit and you will find a variety of results.

Step 3: Converting From Current and Voltage to Impedance

Just a defining relationship impedance Z=V/I (phasors) so to get that plotted its rather simple you will want to right click on the label of the voltage plot it should be V(v1) or whatever node was used and in the window that pops up you will simply change it from having V(v1) to V(v1)/I(V1) then hit ok. When you change this area you can make a more complex expression like (V(v1)-V(v2))/(I(v1)-I(v3)) ... the goal is to just vet V/I.

This will change the graph but the units are still going to be in decibels so you will want to right click on the Y axis and change it to linear then hit ok and the units will now be in Ohms .

Step 4: Reading the Results

After you have converted to impedance it still may be hard to read the graph and a simple fix is to right click on the label of the graph and select a cursor under Attached Cursor I used 1 and 2 so I could measure at multiple points as you can see in the image with a results window showing up.

Thanks for reading feel free to ask any questions and if this goes well I will try to create more like it. :D

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    5 Discussions

    0
    FrediM
    FrediM

    10 months ago on Step 4

    Thanks a lot for this! It helped me a lot.
    If I did everything right I guess the attached image should be showing the correct result for this buffer's input impedance (around 320k). Is it correct?
    Thanks again!

    Buffer input impedance.png
    0
    transistor2
    transistor2

    Reply 10 months ago

    Where is the I(v2) taken at? If you have your voltage and current at two different places your result will change, depending on how the voltage and current change.

    0
    FrediM
    FrediM

    Reply 10 months ago

    I(V2) is taken from AC 1, its reference name is V2. At least that's my understanding. I tried I(Vin) but is says it has no current.

    0
    transistor2
    transistor2

    Reply 10 months ago

    oh yeah then it should be right, and as a sort of sanity check the impedance should drop because you only have capacitors in there and the impedance for them is 1/jwc.

    0
    FrediM
    FrediM

    Reply 10 months ago

    Thanks for you reply! :D
    Is there an easy way to measure output impedance of a circuit with opamp?