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Design >> Analog Design >> Load capacitance for ac simulation of opamp
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Message started by adesign on Jun 30^th, 2011, 9:04pm

Title: Load capacitance for ac simulation of opamp
Post by adesign on Jun 30^th, 2011, 9:04pm

Hi all,

Consider the circuit in the figure. It is a fully differential opamp used for switched-capacitor application. Here Φ1 and Φ2 are two non-overlapping phases.
I want to do ac analysis for this opamp. Here are my queries:
1. How much load should I consider at the output?
2. Will the absolute value of C3 matters for ac simulation, if ratio of C3/C1 is intact.

Please comment.

Title: Re: Load capacitance for ac simulation of opamp
Post by raja.cedt on Jul 1^st, 2011, 1:07am

hello,
it really depends on your settling spec, from closed loop DC gain point of view it's ok.
Thanks.

Title: Re: Load capacitance for ac simulation of opamp
Post by buddypoor on Jul 1^st, 2011, 1:56am

adesign wrote on Jun 30^th, 2011, 9:04pm:

I want to do ac analysis for this opamp. Here are my queries:
1. How much load should I consider at the output?
2. Will the absolute value of C3 matters for ac simulation, if ratio of C3/C1 is intact.
Please comment.

I don't understand the background of your question "How much load...."?
When all the capacitances have the intended value the simulation program "knows" what load has to be considered.
A more important question is if your simulation package is capable to perform an ac analysis for a time-variant circuit (or if you have to create a time-continuous equivalent before).

Title: Re: Load capacitance for ac simulation of opamp
Post by adesign on Jul 1^st, 2011, 2:44am

Just to elaborate, I want to do open loop characterization for this opamp which is supposed to be used in the circuit in given figure. For open loop characterization, I need to connect some output load capacitance to see its impact on second pole, considering the pole by compensation capacitor would be the dominating one.
Now, could you please estimate the load to be connected to the output of the opamp to do open loop characterization viz. phase margin, bandwidth, gain margin etc.

Thanks

Title: Re: Load capacitance for ac simulation of opamp
Post by buddypoor on Jul 1^st, 2011, 3:19am

I like to repeat that your time-dependent circuit first must be transferred to a time continuous equivalent before you can perform ac simulations with the aim to measure loop gain, stability margin, etc.
Another option would be the simulation package SPECTRE that is capable to accept the circuit as it is - however, I am not familiar with it.
What is the simulator you intend to work with?

Title: Re: Load capacitance for ac simulation of opamp
Post by carlgrace on Jul 8^th, 2011, 10:16am

( C3 || (C1 + C2) ) + expected external load

That should be good enough for some ac analysis to look at gain and such. But be sure you do extensive transients to look for ringing.

Title: Re: Load capacitance for ac simulation of opamp
Post by thechopper on Jul 15^th, 2011, 5:57am

carlgrace wrote on Jul 8^th, 2011, 10:16am:

( C3 || (C1 + C2) ) + expected external load

That should be good enough for some ac analysis to look at gain and such. But be sure you do extensive transients to look for ringing.

Hi Carlgrace,

That assumption looks a little bit conservative to me. In my opinion C1 and C2 do not act as a capacitive load since:

1) They are connected to opamp virtual ground during phase 2 --> thus there is no AC voltage signal accross them and therefore they do not act as a load.

2) I would use the equivalent CT resistor they represent when running AC simulations

So total load for me would be C3 + external load, and an input impedance of 1/fck*(C1+C2)

Best
Tosei