Andrew Beckett
Senior Fellow
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Life, don't talk to me about Life...
Posts: 1742
Bracknell, UK
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Rather than using 1 Farad capacitors, you'd be better off using spectre's switch component (spt1switch in analogLib) which can be in different positions in different analyses. 1 Farad capacitors can cause trouble, even in a small signal analysis.
Anyway, a better solution still is to use spectre's "stb" (stability) analysis. For a differential circuit, you would use the cmdmprobe component from analogLib (two inputs, two outputs), and set the CMDM parameter to either 1 (to measure common mode stability) or -1 (to measure differential loop stability). Then in the stb analysis form, use the Select button next to the probe instance definition, and select the instance of cmdmprobe (it will automatically append /vinj to the instance name, because it actually needs to probe a source within the cmdmprobe schematic).
The stability analysis has the advantage of being able to keep the loop closed during simulation, which means that the DC is happy, and also the loading is handled correctly.
Regards,
Andrew.
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