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Design >> Analog Design >> How to calculate Mismatch of current Mirrors
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Message started by xwj623 on Dec 27^th, 2007, 6:13am

Title: How to calculate Mismatch of current Mirrors
Post by xwj623 on Dec 27^th, 2007, 6:13am

how to calculate or predict the Mismatch of current mirrors in practice design?
we know two formulas in mirrors: delta(I)/I=delta(W/L)/(W/L)+2*delta(Vth)/(Vgs-Vth)
delta(Vth)=0.1*tox / [ (WL)^0.5]

I don't know the Mismatch of delta(W/L)/(W/L) in practice MOS transistor, I didn't find the answer in kit's design rule.
What are the rules of thumb on this term?

As a roughly predict, I calculate the Mismatch like this:
delta(I)/I=2*delta(Vth)/(Vgs-Vth)
delta(Vth)=0.1*tox / [ (WL)^0.5]
tox=0.35um/50=70A ( for 0.35um technology )
Vgs-Vth=Vdsat=200mv
So i get the following roughly formula to predict the Mistach:
delta(I)/I=14mV/{[ (WL)^0.5]*Vdsat}
=14mV/{[ (WL)^0.5]*200mV}

Did my couclusion correct or reasonable?
Are there any rules of thumb about the Mismatch of current mirrors?
Thanks for help. :)

Title: Re: How to calculate Mismatch of current Mirrors
Post by ywguo on Dec 28^th, 2007, 2:57am

Hi,

I don't know how you derive the formula delta(Vth)=0.1*tox / [ (WL)^0.5]. There is a paper titled matching properties of MOS transistors, by M. J. Pelgrom, in Oct. 1989 issue of JSSC. It guides the mismatch analysis in circuit design. Most foundries provides the measured matching data based on the theory and formulas in that paper.

Yawei

Title: Re: How to calculate Mismatch of current Mirrors
Post by xwj623 on Dec 28^th, 2007, 8:11am

Hi

I derive the formula " delta(Vth)=0.1*tox/[ (wl)^0.5 ] mV " from "Design of analog cmos IC" by razavi,2001. In chapter 13, page 465, equation 13.62. :)

It seems that razavi also referenced the paper you adivsed. :)

Thanks.