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Mysterious leakage current (Read 4282 times)
Lieutenant Columbo
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Mysterious leakage current
Dec 20th, 2002, 11:41pm
 
I am simulating a high-gain amplifier that uses a switched-capacitor auto-zero circuit. It exhibits a long term drift that higher that I would expect given the leakage currents fhat are present. Could this drift be an artifact of simulator errors? If so, is there anything that I can do to reduce the errors?
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Ken Kundert
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Re: Mysterious leakage current
Reply #1 - Dec 23rd, 2002, 11:59am
 
There are three things that might lead to this kind of drift in a charge storage circuit.
  • gmin
  • capacitive-based models
  • KCL tolerances
gmin is a very small valued conductor (large valued resistor) that shunts most semiconductor devices. Its main purpose is to prevent nodes from floating when surrounded by devices that have completely switched off. Floating nodes cause DC analysis to fail because there is no unique solution. It is usually possible to set gmin=0 (gmin is a simulator option) in order to eliminate this source of error, however this will often cause DC analysis to fail, especially with charge storage circuits. You can run a transient analysis if you use initial conditions to pin the voltage for each floating node. Spectre is unique in that it will generally warn you during the DC analysis if gmin is noticeably affecting the accuracy of the results.

It was learned long ago that capacitive-based models cause charge conservation problems. At that time, there was a concerted effort to switch to charge-based models. All of the main semiconductor models (BSIM, etc.) are now charge based. However, it is still common for the less mainstream models to be formulated in terms of capacitance rather than charge. This is really problematic for charge-storage circuits. If you are using capacitive-based models, you will need to reformulate them in terms of charge (see article on varactors in www.designers-guide.com/Modeling).

Finally, on every time step the simulator works to find the node voltage values that result in Kirchhoff Current Law being satisfied. Since computers have only finite precision, it cannot fully satisfy KCL. There is always a small amount of charge that is either being created or annihilated at each node on each time step. Most simulators do not explicitly control this error, and so it can sometimes be large. With these simulators, you can tighten tolerances (reltol, abstol) in the hope that the error will be reduced, but there is no guarantee. Spectre is one of the few, and perhaps the only, simulator that always explicitly monitors and controls the error in KCL, so tightening reltol and abstol are assured to reduce this error.

All of these issues are discussed in more depth in my book, which can be found at www.designers-guide.com/books.html.
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macquigg
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Re: Mysterious leakage current
Reply #2 - Jul 28th, 2003, 8:20am
 
In addition to the simulator inaccuracies that Ken discusses, there are some causes of long-term drift in real switched-capacitor circuits that may or may not be modeled correctly in your simulation.  In general, there are four causes of such drift
a) Leakage currents
b) Op-amp offset
c) Switch asymmetry
d) Surface states in the MOS switches
My guess is you are seeing the effects of (c) asymmetry in the capacitance from gate-to-source and gate-to-drain.  A good simulator, with charge-conserving device models, will simulate these effects.
For an in-depth discussion of these issues see "Drift in Switched-Capacitor Integrators", D. MacQuigg & T. Somerville, Microelectronics J., vol.15, pp.38-49 (1984).
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