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The Designer's Guide Community Forum › Design › Analog Design › justification to use RFCMOS vs normal digital CMOS

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justification to use RFCMOS vs normal digital CMOS (Read 6332 times)

supermoment

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justification to use RFCMOS vs normal digital CMOS
Mar 11^th, 2010, 9:20pm

What is the justification to use RFCMOS vs normal digital CMOS if I doing 10GHz CDR clock and data recovery circuit. (processing digital NRZ signal in the phase detector)

Could anyone point out the reason of using RFCMOS.

I think the RFCMOS model is more accurate than the normal one but it takes much more simulation time.

what else? Should I go for RFCMOS or normal digital CMOS?

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Berti Community Fellow Offline Posts: 356	Re: justification to use RFCMOS vs normal digital CMOS Reply #1 - Mar 11^th, 2010, 11:34pm Hi Casual, I think you should be more specific in order to start a nice discussion: What the difference between "RFCMOS and digital CMOS" you are referring to? Is it just the models or are you referring also to process options (MIM caps, thick top metal for inductors etc.)?
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supermoment

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Re: justification to use RFCMOS vs normal digital CMOS
Reply #2 - Mar 11^th, 2010, 11:45pm

I have an option to use RF & digital library or both.

In the RF library, I could use RFCMOS (with RF model). The layout format is come with guard ring + double gate contact etc

In the digital library, I could use normal cmos (with ps pd as ad). The layout is without guard ring and stuff. I know I could add them in layout.

I just want to know which library should I go for for 10GHz CML latch for CDR application.

I think RFCMOS is more accurate but i dont need noise performance since it processing digital signal.

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Berti

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Re: justification to use RFCMOS vs normal digital CMOS
Reply #3 - Mar 12^th, 2010, 3:28am

Generally it is better you use a transistor model which has been characterized for RF frequencies.

However, in my experience many fabs don't have the knowledge to do this characterization properly, but do just some blind fitting of S-parameters (sometimes not even properly de-embeded. So be careful.
This special RF layout I never found useful.

But that's my experience. Probably the PDK you have is better than what I had to use.

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supermoment Senior Member Offline Posts: 167	Re: justification to use RFCMOS vs normal digital CMOS Reply #4 - Mar 12^th, 2010, 3:38am why did you say special layout technique is not useful since running at high frequency??? Could you share out your experience? I use Chartered PDK. Now it is global foundry.
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love_analog

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Re: justification to use RFCMOS vs normal digital CMOS
Reply #5 - Apr 7^th, 2010, 9:41pm

If you use digital CMOS transistors, their models will typically not be accurate for analog parameters - such as gds, noise etc.
In addition most digital centeric process don't talk too much about matching (or at least don't characterize it well enuf)

I would go with Rfcmos unless cost is an issue and exacting performance is not desired.

loveanalog.blogspot.com
The Power of Analog

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: justification to use RFCMOS vs normal digital CMOS Reply #6 - Apr 9^th, 2010, 4:08pm Specifics Needed: What foundry, What Geometry What Design kits What one foundry calls RFCMOS is not the same thing at another foundy
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supermoment

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Re: justification to use RFCMOS vs normal digital CMOS
Reply #7 - Apr 9^th, 2010, 6:10pm

I use global foundry (ex Chartered semiconductor). It has two options, normal PDK (digital), and the other one is RFCMOS PDK.

The digital PDK has a calculated PD, PS, AD, AS in the netlist whereas RFCMOS does not have this in the netlist since parasitic has embedded somewhere. Besides this, the parameter for RFCMOS has longer and need long simulation time. In term of layout, it has fixed structure like guardring, double gate contact etc..

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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: justification to use RFCMOS vs normal digital CMOS Reply #8 - Apr 11^th, 2010, 8:12am Casual, I don't know those 2 PDKs you are using. But the first thing you need to do is asking the foundry. Yawei
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RFICDUDE

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Re: justification to use RFCMOS vs normal digital CMOS
Reply #9 - Apr 12^th, 2010, 6:40pm

You really need to read the design and modeling documentation between the digital and RFCMOS PDK to decide if you need to use the RFCMOS version of the PDK or not.

The RFCMOS version may (or may not) have better noise and high frequency modeling, but this model may be accurate for limited layout geometry which may not be optimal for digital designs. If noise and high frequency analog performance is important to you then the RFCMOS version may be useful, but only you can determine that (for your application/design) after reading the information in the manual.

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