fully differential opamp testbenches

[jimito13]

Hello,

I have two questions about setting up a couple of testbenches in cadence for my fully differential opamp.

1.How to configure the testbench for settling time measurement (is it done like a single-ended opamp with buffer connection and how the two output terminals are connected?) ?

2.How can i run a transient testbench to make sure that my circuit does not oscillate?

Thanks in advance.

 

[erikl]

1.How to configure the testbench for settling time measurement (is it done like a single-ended opamp with buffer connection and how the two output terminals are connected?) ?

**broken link removed** from the Search function!

2.How can i run a transient testbench to make sure that my circuit does not oscillate?

Stimulate the input with 2 fast edges (1 rectangular pulse) which just create an output response close to the max. output swing range (i.e. don't overdrive the output). Then note how the output settles and estimate the stability, see p. 322 from the Allen/Holberg book below.

The following output reaction indicates an unstable circuit behaviour:

 

[jimito13]

erikl thanks for your answers.i have to say something although....When somebody posts a question here doesn't mean that he has not performed the relevant search in the forum (here) or anywhere else that deals with his problem.My questions are related to time domain (first) and stability (second).The link you gave me for the first question has to do with freq domain simulation,so how would you expect that it would match with my question?!I checked it although and my question still remains unanswered...i would appreciate if you showed me a more detailed buffer connection for a fully differential opamp.

As fas as the stability,i am covered.I also thought this : To connect the (out+) of my opamp with (in-),then apply a step input to the (in+) and finally check the (out-) for existence or not of oscillations.Is it right or needs any modification?

Regards

 

[erikl]

The link you gave me for the first question has to do with freq domain simulation,so how would you expect that it would match with my question?!I checked it although and my question still remains unanswered...i would appreciate if you showed me a more detailed buffer connection for a fully differential opamp.

jimeece, I'm a bit puzzled that you didn't see that the configuration is important, and that it doesn't matter if you want to run an ac or a transient analysis. I didn't want to redraw the figure just for the tiny changes which are necessary for a transient analysis configuration: put the fully diff. amp. into a gain=1 configuration, and add the transient stimulus either between both inputs, or between one of the inputs to GND (the other one has to be ac-grounded then, of course). If you really need to find a fully detailed 1:1 schematic for timing analysis, I suggest you go on searching.

As fas as the stability,i am covered.I also thought this : To connect the (out+) of my opamp with (in-),then apply a step input to the (in+) and finally check the (out-) for existence or not of oscillations.Is it right or needs any modification?

This is the most thorough method, because the loopGain=1 configuration is of course the arrangement most prone to oscillation. Also, don't forget to add a realistic output load, because its pole affects the behaviour, too.