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# How to analyze Vout / Vin in S/H circuit ?

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#### mpig09

##### Full Member level 4
Dear all :

The attached file has two sample and hold circuit,
I try to analyze the Vout / Vin, but the result always
Vout / Vin =1.

Could you help me how to analyze the circuit ?

What parameter you want analysis? And what model of opamp you use?
For begining you can add input votage offset and changing sampling frequency. And what about pedestal error?

Hi
edited see below
regards

Dear all :

First circuit : C0=C5=1.2p
C1=C3=0.8p

Second circuit : C1=C2=1.2p
C0=C3=0.8p

Could anyone describe how to analyze ?
(analyze principle / formula)

mpig

Hi
sorry
I took a mistake.

draw half model of fully diff S/H.
charges in sampleing phase:
Q0=Vcm.C0
Q1=Vin.C1

charges in holding phase:
Qout=Q0+Q1=Vcm.C0+Vin.C1
Vout = (Vcm.C0+Vin.C1)/C0 = C1.Vin/C0+Vcm

VOdiff=C1.VINdiff/C0
C1=1.2p, C0=0.8p
VOdiff=1.5 VINdiff

regards

Dear hr_rezaee :

For second circuit :
Sample phase
Q0=Vin*C0, Q1=DAC*C1

Hold phase
Vout=Vin + Qout / C0
Qout=Q0+Q1
so, Vout=Vin+ (Vin*C0+DAC*C1) / C0
=Vin + Vin + DAC*(C1/C0)
=2*Vin + DAC*(C1/C0)

Is it right ?

mpig

Added after 1 hours 48 minutes:

Dear hr_rezaee :

Do we need to care +/- sign of capacitor charge in the first and second circuit ?

mpig

Hi
I think there is a mistake in second S/H.
I think path from input to output is extra.
If we remove this path we have:

Sample phase :
Q0=Vin.C0, Q1=DAC.C1

Vout=Q0/C0=Vin

I'm not shure this circuit is true.
regards

Dear hr_rezaee :

I use hspice to verify the gain result, Vout approaches 2*Vin - DAC*(C1/C0)
, I try to calculate this result, but the result is still not correct.

Could you have any suggest ?

mpig

Hi
could you attach an image that shows full circuit?
all switches and nodes.
regards

Dear hr_rezaee :

I can't show detail schematic.
I hope it is enough to help.

mpig

Hi
Sorry
I can't Understand.
If you can, draw better schematic
Please explain where you see this S/H?
regards

Dear hr_rezaee :

It is my project.

mpig

Hi
Ok, but you must have more detail about it.
such as type of S/H or references.
regards

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