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fran2k5

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Posts: 29

how to use cmdmprobe
Mar 28^th, 2010, 11:24am

Hi everybody,

I have been trying to understand how to use the cmdmprobe component in Spectre together with the stb analysis. Although cited some times in the forum it is still not completely clear to me how to use it. In particular:
- Is the stb analysis a single-ended analysis?
- how can it be used in a differential circuit? Do I need to use back-to-back baluns?
- Should it be used in conjunction with ac input sources?

Thank you very much for your help.

Best regards,

fran2k5

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sheldon

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Re: how to use cmdmprobe
Reply #1 - Mar 28^th, 2010, 7:02pm

Greetings,

� The cmdmprobe is intended for use with differential input, differential
output operational amplifiers and OTAs. See the following append
for an example of how to use this component in a testbench,

http://www.designers-guide.org/Forum/YaBB.pl?num=1268207804

The example testbench also includes the setup for the input signal
sources. There is one additional point to keep in mind, to test the
common-mode loop characteristics set the probe value to 1, to
test the differential-mode characteristics set the probe value to -1.

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Best Regards,

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Sheldon

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fran2k5

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Posts: 29

Re: how to use cmdmprobe
Reply #2 - Apr 1^st, 2010, 3:12am

Thank you Sheldon,

I have simulated the AC response of a fully differential amplifier (continuous time with CMFB) with these 2 setups (the opamp is closed in feedback with very large resistors):

1. AC analysis in performed with the input signals injected through 2 big capacitors

2. The input caps are disconnected and the cmdmprobe is used inside the feedback loop (between output opamp and feedback resistors)

The 2 simulations provide a different result. In particular while for the AC analysis the opamp is stable for the stb analysis the loop phase margin is really low.
The low frequency behavior is the same in the AC and stb analysis.

How to interpret these results? Is the difference in frequency response due to the CMFB.
In conclusion, is the opamp stable or not?

Any comment would be appreciated.

Thank you,

fran2k5

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Andrew Beckett Senior Fellow Offline Life, don't talk to me about Life... Posts: 1742 Bracknell, UK	Re: how to use cmdmprobe Reply #3 - Apr 7^th, 2010, 6:39am It's not obvious to me how your first simulation has checked the stability. Perhaps you can show show pictures of the schematics in both configurations, with more detail on how the simulations have been set up? Regards, Andrew.
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