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RLC series model in MEMS gyroscope

FEDERICO GIACC
Hi all! I am now designing an oscillator for MEMS gyroscopes. The MEMS has an equivalent RLC series model; so i put it in a positive loop and make it oscillate. But my problem is: the equivalent inductance is of 2MH! and the equivalent capacitance is of 40aF. I see that Cadence does not simulate correctly the transient response of this RLC. I am very inexperienced about Cadence: Is there any simulation parameter to be changed in order to get a correct simulation? Thank you all for your answers

  • The Forum Guidelines ask you not to append new questions onto existing threads - it makes it very confusing when there are two topics going on in the same thread (even more so when one is still active) and makes it very hard to follow. So I split this off from http://community.cadence.com/cadence_technology_forums/f/33/t/32224

    The Forum Guidelines also ask you to state what tools you are using and the version. Cadence is the name of the company, not the tool - and Cadence provides a large number of tools in different domains. So please make it clear which tools (and version) that you are using, and ideally what the circuit looks like and what simulation options you've chosen. Otherwise  your question is so open-ended that it's virtually impossible to answer.

    Kind Regards,

    Andrew.

  • Hi,
    ok, I'm using Virtuoso 6.1.4-64b. 
    In particular, I'm trying to simulate the RLC series circuit shown in figure. Here down there are also the tolerance options I've choosen. I run a transient, with accuracy defaults-->liberal. The current response in time domain to the sinusoidal excitation  (at the resonance frequency) does not fit with theory: I see that changing reltol from 100e-6 to 0.001e-6 changes the shape of the response, and never seems to get to an amplitude value equal to Vsin(amplitude of sin excitation) / R. I suppose it is because the RLC values are very unusual, as they are: R=15Mohm, C=40aF, L=1.49MH.  
    Thank you a lot for your kind answer!!
    Federico

  • Circuit simulators are designed to simulate realistic circuits. There are so many things that are so out of the normal range here that the chances of getting anything reasonable are very small. Your capacitances are miniscule and the inductances huge - these will end up with impedances that are beyond the normal range (small impedances are bad for convergence, and large impedances end up with the circuit virtually floating which also can lead to convergence issues).

    A reltol of 1e-9 is leaving you with very little numerical margin (there is only numerical resolution of around 1e-14 due to the limit of double precision floating point numbers), and your abstols are also way off.

    I suspect this is not a very good model - maybe you should use a VerilogA model with appropriate disciplines being used with a better indication of the ranges of the values involved, rather than trying to emulate this with some artificial electrical components?

    Regards,

    Andrew.

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