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gido Junior Member Offline Posts: 23	Switched capacitor leakage at low frequencies Feb 29^th, 2012, 6:51am Hello, please can someone explain to me why there would be charge/ current leakage in switched capacitor circuits at very low frequencies? I read it somewhere but didn't get any explanation. Thanks. Gideon
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RobG

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Bozeman, MT

Re: Switched capacitor leakage at low frequencies
Reply #1 - Feb 29^th, 2012, 7:32am

The leakage will come from the switching transistors - either from the pn junction (e.g. drain to substrate) or from the drain-source "off" current. Ask the modeler how accurately they measured the difference between these two effects since sometimes they just throw in estimates.

It doubles every few degrees C, so for 20 second time constants, I hope your environment is not hot...

rg

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Switched capacitor leakage at low frequencies Reply #2 - Feb 29^th, 2012, 2:45pm Frequently it is not even in the model Also it will be poorly modeled due to the difficulty of measurement.
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carlgrace

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Berkeley, CA

Re: Switched capacitor leakage at low frequencies
Reply #3 - Feb 29^th, 2012, 4:18pm

gido wrote on Feb 29^th, 2012, 6:51am:

Hello,
please can someone explain to me why there would be charge/ current leakage in switched capacitor circuits at very low frequencies?
I read it somewhere but didn't get any explanation.
Thanks.
Gideon

You can also have trouble with the input transistors in the OTA used with the switched capacitor. This problem gets worse and worse in deep submicron technologies and puts a lower limit on the frequency of operation of many practical circuits, such as Pipelined ADCs. Also, when you make the caps small to go fast, you make the problem worse.

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Dan Clement

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Salt Lake City, Utah, USA

Re: Switched capacitor leakage at low frequencies
Reply #4 - Feb 29^th, 2012, 6:06pm

I recently tried to do a SC SDM in 0.18um and the thin gate devices were much too leaky. And I was only trying to run at about ten or twenty kHz. My high temp limit was only 85C. My caps were huge too, multiple pFs.

I can't imagine what the poor 32nm guys do... Maybe the run their sample clock in the high MHz range

.

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loose-electron

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Re: Switched capacitor leakage at low frequencies
Reply #5 - Feb 29^th, 2012, 7:18pm

Dan Clement wrote on Feb 29^th, 2012, 6:06pm:

I recently tried to do a SC SDM in 0.18um and the thin gate devices were much too leaky. And I was only trying to run at about ten or twenty kHz. My high temp limit was only 85C. �My caps were huge too, multiple pFs.

I can't imagine what the poor 32nm guys do... �Maybe the run their sample clock in the high MHz range

.

With gate oxides 4 molecules thick, what would you expect?

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Jerry Twomey
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Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical

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Dan Clement Community Member Offline Posts: 95 Salt Lake City, Utah, USA	Re: Switched capacitor leakage at low frequencies Reply #6 - Feb 29^th, 2012, 7:18pm :)
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