spice impedance - input and output

[Anonymous]

Is there a way to use spice to get the input and output impedance of an analog circuit?

[jorge_garcia]

Hello @Anonymous,

I hope you're doing well. I don't know of a good way to simulate output impedance in SPICE. However, input impedance is pretty easy. Use and AC current source as the input to your circuit and set its value to 1A. The voltage across that current source is your input impedance. If you have your signals AC coupled you may have to set the RSHUNT option in the simulation setup to on.

Let me know if there's anything else I can do for you.

Best Regards,

[Anonymous]

Thanks for the reply. I do indeed have AC coupling and was getting errors and unable to simulate. I will try turning RSHUNT on and try again. Thanks again for responding.

[Anonymous]

My circuit is AC coupled and I followed you advice and turned on RSHUNT. The simulation now runs but the voltage at the output of the current source is near zero. I have attached my simple schematic and the simulation setup screen. Thanks for your help.

[mtl_asm]

Hi,

I believe "operating point" will only give DC values and dump them on your schematic.

You need to do an ac sweep, with probes to measure the voltage, current and phase, then you can calculate the impedance at each frequency in the sweep once you get teh bode plot (remember impedance is frequency dependent) 

you can google "how to measure impedance with spice" or something like that, there are many examples.

hope this helps.

s

[Anonymous]

Excellent, thanks for the response. I will try it this evening.

[Anonymous · Accepted solution]

I tried doing an AC sweep and it worked. To summarize, the missing key was that RSHUNT needs to be set to "ON" in the configuration in order for the simulation to run. It also to be an AC sweep. I have attached images of the schematic, configuration, and graph. Thanks for all the help from everyone.