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From: email@example.com (Ken Smith)
Subject: Re: mitigating charge injection in switched capacitor systems...
Date: Fri, 13 Dec 2002 16:29:55 +0000 (UTC)
Organization: a2i network
NNTP-Posting-Date: Fri, 13 Dec 2002 16:29:55 +0000 (UTC)
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Originator: firstname.lastname@example.org (Ken Smith)
In article ,
Josh Model wrote:
>From my understanding, at least one cause of charge injection is the
>source-gate capacitance of a FET switch, and a sort of fast switching time
Yes. what happens is: a charge is capacitively coupled in from the gate to
the source when the gate moves. The channel then becomes a very high
resistance and traps this charge as the source of the FET. If you are
making your own gate driver for the fet, you can control the shape of the
switching wave form to reduce this effect quite a bit.
Another way to solve the problem is to cancel the injected charge by
having another FET. Take this example:
/ ! !
------ 0--------!- \ !
---------!+ / Good op-amp
S2 \ --- C2
If C1=C2 and the two switches are the same circuit, the same charge and
hence the same voltage ends up on C1 and C2. Also the bias current one
the op-amp makes the same ramp on the two capacitors. The result is no
step or ramp on the output of the op-amp.
Well actually: You do see some ramp because of the offset current and
there is a funny looking sort of a one cycle of a sine wave glitch when
the switching happens.
Another option is to figure out a way to put the switch inside a closed
loop so that the step on the output gets servoed out. I have done this in
a sample and hold circuit and it works but it requires great care to keep
the op-amps acting linearly at all times.
email@example.com forging knowledge
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