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Matching of capacitors in a DAC

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maalma

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Hello,

I'm layouting a charge distribution DAC for a 12 bits SAR-ADC, the DAC uses 9 capacitors : 1fF, 2fF, 4fF, 8fF, 16fF, 32fF, 64fF, 128fF, 256fF.
I need to match these capacitors, :idea:my first idea is to use a matrix of unitary capacitance (1fF) and to dispatch the 511 capacitors as in show in the attachement, is it a good idea??
(I must be carefull toward parasitic capacitance)
do you have any other ideas??

Excuse me for my english !!
Thanks for your help :D
 

1) What is a good idea is to set a unit cell that allow you to create all the set of caps...don't run away from this choice, it's the correct one.

2) The problem is the huge leap between your smallest and your biggest cap, there's no alternative, your unit cell MUST be 1fF...

3) From what I see in the picture, the distribution, at first sight, seems to be okay. For sure you'll have an interesting routing job...

4) Something I would add is POLY (you're using POLY caps, right?) all around teh complete structure and between the caps (it will create a POLY GRID, with the caps inside every single grid square).
This is oriented to avoid process etching issues when processing the part.

Hope this helps.
 

    M

    maalma

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i'm trying to use fringe capacitor with multiple metalic layers, is that ok??
 

So, what I understand from your post is that you'll be using MiM caps (Metal-Insulator-Metal), in that case, the grounded grid I'm suggesting should be done also with metal...
 

    M

    maalma

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hi master,

what r the types of capacitors used in analog layout, in which cases they r used, differences between them, layout techniques individually for them. pls tell me.
 

hello master,

i didn't understand what you mean by the grounded grid, what i'm trying to do is to use to comb of the same metallic layer (metal 1 for example), so i have only to pins (plus and minus) i have no grid!!
can you explain your suggestion plz
thank you
 

hello,
why you don't use the 2fF or 4fF as unit capa?
In this case you can use 2 capa to obtain 1fF, but you can reduce the number of capa and so you can have more simple routing.

Bye
 

    M

    maalma

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hi climber 73

nice idea , thank you, it will definitely help me in routing, let me try, then i'll tell you if your idea suits me, thank you once again

have a nice evening
maalma
 

hello,
why you don't use the 2fF or 4fF as unit capa?
In this case you can use 2 capa to obtain 1fF, but you can reduce the number of capa and so you can have more simple routing.


At that time what about 1fF capacitor, if we use 2fF or 4fF as unit capacitor
pls clarify
 

hello gksivas,

if we choose to use 2 fF as unit capacitor, we'll use 2 * 2 fF capacitors connected in series to obtain one 1fF capacitor it's simple & if we choose 4fF as unit capacitor, we'll use 4 * 4fF capacitors connected in series to obtain one 1fF capacitor , 2 * 4fF capacitors connected in series to obtain one 2fF capacitor. that's it :)

maalma
 

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