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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> how to use cmdmprobe https://designers-guide.org/forum/YaBB.pl?num=1269800680 Message started by fran2k5 on Mar 28th, 2010, 11:24am |
Title: how to use cmdmprobe Post by fran2k5 on Mar 28th, 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 |
Title: Re: how to use cmdmprobe Post by sheldon on Mar 28th, 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 |
Title: Re: how to use cmdmprobe Post by fran2k5 on Apr 1st, 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 |
Title: Re: how to use cmdmprobe Post by Andrew Beckett on Apr 7th, 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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