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[SOLVED] : what's a switch capacitor in analog circuit design?

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stackprogramer

stackprogramer

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i want to create a slide conference about switched capacitor.this topic is created that i shared my result soon.
thanks
 

It could be several things but my guess is you are refering to switched capacitors in filter applications. The concept is to use fixed value capacitors because they are easy to fabricate, either on silicon or as discrete devices, then switch them in and out of circuit at high speed to make their value appear to be smaller. If you are creating a conference about them you should read up on "switched capacitor filters" for more information. The topic is too big to fully explain in a message system.

Switched capacitors can also be a reference to voltage multiplier circuits, AC power regulators and motor speed control systems.

Brian.
 
Back and forth (load/unload) switched capacitors can be used to replace resistors in analog systems (e.g. filters).
 
National semiconductors came out with the first monolithic switched capacitor filters in the late 70s or early 80s.

They actually published a book, Switched Capacitor Handbook, which had lots of theory and practical information.

Nevertheless, if you perform an Amazon or Google search, there are many available books and online topics.
 
The reason for using switched-capacitors in an analog IC is that it's much easier to fabricate accurate capacitors than accurate resistors, so a switched-capacitor can be (and is) used to emulate a resistor.
For higher value resistances the switched-capacitor is also smaller than the resistor.
 
crutschow said:
The reason for using switched-capacitors in an analog IC is that it's much easier to fabricate accurate capacitors than accurate resistors, so a switched-capacitor can be (and is) used to emulate a resistor.
For higher value resistances the switched-capacitor is also smaller than the resistor.
Click to expand...

Yes - and more than that, it is even much easier to realize capacitor RATIOS (which only matter in S/C technology) in IC technology.
 

Switched capacitors are also useful for storing / compensating
offset voltage, any "zero drift" op amp front end is doing this.
 

All very true, and switch capacitors team up very well with analog switches to perform many very useful functions.
Filters have already been mentioned, but sample and hold and auto zeroing are other applications, along with various charge pump circuits.
 

thanks all,for participating in our post
.i will use your reply in creating my slide.only i can talk thanks very much.
 

in book razavi cmos:
when we use resistors R1,R2 in closed feedback opamp,we don't have accuracy gain:
Untitled.png
so the idea is we use capacitor to have a gain constant vs former.we uses switch capacitor:
Untitled.png
in this state is gain open-loop is infinite;
 

No - I don`t think so because of the switched capacitor in the feedback path.
Click to expand...
yes,when we sue switched capacitor ,the loop gain is not closed and we will have deal with infinite gain.
according sampling and amplifying when we have calculate the gain it is (according Razavi cmos book)
Untitled.png
that is more accuracy vs former circuits.
 

stackprogramer said:
yes,when we sue switched capacitor ,the loop gain is not closed and we will have deal with infinite gain.
Click to expand...

No - I repeat: The loop is closed continuously.
Please, can you explain why and under which conditions you would think that the loop is open?
 

No - I repeat: The loop is closed continuously.
Please, can you explain why and under which conditions you would think that the loop is open?
Click to expand...
because i study in razavi book,we have two mode:sampling,amplifying
so when is mode sampling c3 open,c2 short
in amplifying mode c3 short,c2 open :so it is a discrete time circuit i think because in reference for filters use
Z transform.may be i am wrong,this is my understanding.this image describe both sampling and amplifying
Untitled.png

-------------------------------------
new part ;
on other hand we for switching we need high impedance ,cmos technology is best selection for us,
bipolar tech. has base current ,and it cause in accuracy in output voltage gin.
this citation is razavi book:
 

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stackprogramer said:
...we have two mode:sampling,amplifying
so when is mode sampling c3 open,c2 short
in amplifying mode c3 short,c2 open :
Click to expand...

So, in both cases, the feedback path is always closed - either with a short or with a capacitive path.
 
so we know switching in this here is very importtant:
we select mosfet as switch:
parameter a good switch:
razavi book
  1. speed consideration
  2. precision consideration
  3. charge injection cancellation
  4. and ......
 

for more speed switch we use a pmos and nmos that are parallel.
Untitled.png
the on and off clock circuit is:
Untitled.png

- - - Updated - - -

if we W/L increase the capacitor sampling decrease and speed rise,but this method increasing speed make inaccuracy
and this way is not practical.
Untitled.png
and equation
Untitled.png
 

this S.C use for designing all types of filters,Common mode feedback,modulators ................
that all of them that use s.c have more accuracy.
picture a filter first order is designed with S.C
Untitled.png

high Biquad filter:
Untitled.png

my question from experts is:
this subject Switch capacitor can reason we should go to using digital circuit?????
 
Last edited:

in replying my question,i concluded that yes,this is a gate that guides us to using digital circuits.
thanks from all for contribution.good luck
 

stackprogramer said:
in replying my question,i concluded that yes,this is a gate that guides us to using digital circuits.
thanks from all for contribution.good luck
Click to expand...

Perhaps the following info is helpful:

S/C circuits are something "between analog and digital".
In particular, they are classified as "analog sampled data systems" (analog but not continuous in time).
 
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