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Design >> RF Design >> Minimum cap
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Message started by Tareeq on Dec 19^th, 2008, 6:17am

Title: Minimum cap
Post by Tareeq on Dec 19^th, 2008, 6:17am

Hi,

I am traying to design a digital varactor, is it possible to to have a minimum capacitance of 11fF by putting in serie 3 mim cap without layout issues ?

Thanks.

Tareeq

Title: Re: Minimum cap
Post by vivkr on Dec 30^th, 2008, 1:07am

Hi Tareeq,

I would only be worried about how you set the DC levels of the intermediate nodes
in this series connection. There may also be antenna error issues for the same reason as any charge deposited on the intermediate nodes has no path through which it can flow out.

Moreover, 11 fF is a very small cap value. You would be very sensitive to interconnect
parasitic, although the bottom-plate parasitic of MIM may not be an issue.

Regards,

Vivek

Title: Re: Minimum cap
Post by barry on Dec 31^st, 2008, 2:07pm

Hi Tareeq,

A standard approach for a differential oscillator is to have two varactors connected in series between the two terminals of the differential inductor. The top plates face the inductor. The bottom caps are the control voltage.

You should not need three capacitors in series to achieve 11fF. In fact, Staezewski et. al. used a 38aF unit capacitor in their DCO published in JSSC Dec. 2004. The PDK varactors are typically much larger, but you can layout a smaller one.