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Design >> Analog Design >> Decap size
https://designers-guide.org/forum/YaBB.pl?num=1287063847

Message started by newic on Oct 14^th, 2010, 6:44am

Title: Decap size
Post by newic on Oct 14^th, 2010, 6:44am

I use transistor to form decouple cap (decap) for power supply vdd.

what are the differences between a single large decap with size 50u x 50u (width x length) and multple decap with smaller size but same area in total, suh as 25 10u x 10u decap. Which one is better?

The differences from aspect like frequency response / speed, ESR, Rg, leakage etc..

Title: Re: Decap size
Post by rfmagic on Oct 14^th, 2010, 7:46am

The only limitation is set by the PDK design rules that you use. usually we use the maximum WxL of the MIM and MOSCAP for decoupling

Title: Re: Decap size
Post by RobG on Oct 14^th, 2010, 8:47am

Shorter lengths will be faster, but you might want to lower the Q with longer lengths. I asked the same question myself and finally got it figured out with some help. Click here for the thread.

Title: Re: Decap size
Post by newic on Oct 14^th, 2010, 6:10pm

How does the model looks like?? There must be a Rg & Rs/d as ESR of the cap.

for a 50u x 50u (max size), the worst case resistance Rg & Rs/d would be at the centre of the cap. is it true ??

would check on the freq response and see...