# Opamp characterization at system level

1. ## Opamp characterization at system level

hi friends,

i got one problem.
i have already one opamp, for which i checked gain, ugb and phase margin. everything is fine because i gave 1V ac signal to one input (DC bias voltage at two inputs) and did ac analysis.
now my opamp is used in a big circuit having different blocks like bias, loads, and extra circuitry.
can i do ac analysis of my opamp at this system level? if so, where should i give my ac 1V input ( because the two input nodes are already connected to somewhere in the big circuit)

Added after 3 hours 40 minutes:

Hi, i got the solution like this.

i calculated the overall capacitance the output node experienced.
i put the same as load capacitance and test it as individual opamp.

i hope the procedure is correct

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2. ## Re: Opamp characterization at system level

Originally Posted by analog_prodigy
hi friends,

i got one problem.
i have already one opamp, for which i checked gain, ugb and phase margin. everything is fine because i gave 1V ac signal to one input (DC bias voltage at two inputs) and did ac analysis.
now my opamp is used in a big circuit having different blocks like bias, loads, and extra circuitry.
can i do ac analysis of my opamp at this system level? if so, where should i give my ac 1V input ( because the two input nodes are already connected to somewhere in the big circuit)

Added after 3 hours 40 minutes:

Hi, i got the solution like this.

i calculated the overall capacitance the output node experienced.
i put the same as load capacitance and test it as individual opamp.

i hope the procedure is correct

after the ac analysis, you can use the transient analysis to double check it.

the overall capacitance the output node experienced can be used as the load.

but the transient analysis is still neccessary.

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3. ## Opamp characterization at system level

you can cut the loop at your op input then add a great r(about 1GOhm) connect them, then add a great c(about 1F) to the op input, give your ac1v to the other side of the cap, then you can run your ac simulation

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4. ## Opamp characterization at system level

I think put all capacitors at the output is sufficient but not necessary in many cases.

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