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Need help for capacitor layout

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meghna

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Hi,
I am desiging a sigma-delta modulator in 0.35um technology having 5-bit coarse quantization. For the switch capacitor integrator, I have chosen 32 sampling capacitor (Cs) of value 0.3pF each. And the value of integrating capacitor (Ci) is 32*0.3=9.6pf.
Please suggest me some good layout technique for capacitor of this much large value (10pf), where maching is important between Ci and Cs and & Ci+ and Ci-
I think poly1-poly2 cap is the best choice. Should I use just 2 square planes of poly1-poly2 or other techniques can be used.

Thanks and Regards
~Meghna
 

would you like to use mimcap?
you can use pcell in cadence to generate mimcap.
 

read design rule first
 

Yes. the PIP capacitor is the best. even it is expensive.
Place them in symmetrical, and surroud them by dummy PIP cell.
 

why not reduce ur total capacitance ?
why did u choose 9.6pF for ur integrator ? for noise issue, or matching issue?
by the way, what do u mean "5-bit coarse quantization", I have not heard about that term from books, is it a new technique ?
 

Hi,
Thanks for your replies. By coarse quantization, I meant internal quantization i.e. 5-bit flash ADC (that needs 32 comparators) and 5-bit DAC (that needs 32 sampling capacitors with switches).
I have chosen 0.3pF for each sampling capacitor, so total value of sampling cap is 32*0.3pF=9.6pF. And that should be the value of integrating capacitor also for a gain of 1 (Cs /Ci).
yeah...PIP capacitor will be the best. But somehow I am not getting a good idea about how to place it.

Regards
Meghna
 

I think u should better add double guardring for these caps
 

0.3pf is all right?
what about the parasitic capcitance?
it can reach 0.x pf sometime.
 

Yeah..sometimes paracitic capacitance reaches in some 0.xpf. But then what can be the minimun capacitor value, I can safely choose in 0.35um Technology.
(please keep in mind that I will have to multiply this value by 32, and that multiplied value will be the load capacitance for the OTA, so I am avoiding to use large values)
 

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