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Question regarding closed loop S/H architecture

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ammar_kurd

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Questoin regarding closed loop S/H architecture

in the book of "Data Conversion System Design" by "Razavi", the author said that including the sampling switch inside a feedback loop will help suppress the pedestal errors in S/H, the reason for that is that the switch will experience swings much smaller than the input output swings.

I understand that the pedestal error is related to the input voltage, but what I couldn't understand is:

How the feedback is effecting the swings seen by the switch.

I hope someone could clear the whole concept behind this that is (closed loop <==> swings).


 

Re: Questoin regarding closed loop S/H architecture

Inside the loop You are working on error signal which is opamp openloop gain smaller than in/out ones.
 

Re: Questoin regarding closed loop S/H architecture

I really didn't get what you said, the opamp is in a unity gain configuration , right? the first stage is a transconductance amplifier OTA, now the switch is in the outer loop that is the loop from the output to the noninverting input of the OTA. in this frame how is the error suppressed exactly?
 
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Re: Questoin regarding closed loop S/H architecture

I don't believe, offhand, that putting the switch in any
particular place will eliminate pedestal errors. These
come in right at the time where you are breaking the
loop (in a THA) and they relate to gate charge imbalance
(and, where the gate charge displacement currents go,
as you choke off the channels). Things like the amount
of gate turnoff overdrive and how it relates to common
mode position, as well. None of these are taken out by
the outer loop because you're busy breaking it at the
time that it matters.

However the facing impedance also matters (where the
current spikes go, and how much voltahe excursion that
causes). Especially if you have a high-Z source, a buf
amp may greatly reduce kicks in both reverse and
forward (your pedestal error) directions by taking more
of the charge in the buf-amp output than either the
THA / SHA input, or the source signal, would.

You have to "peer review" the claims in papers for
yourself, you will find things said which don't really hold
up - often misconstruals of what's observed, or some
generalizations that only really apply to the specific
case and conditions that were really looked at. The
"publish or perish" paradigm leads to some desperate
conjectures, sometimes.
 
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