transmission gate sizing

[analog2003]

Hi,

I am unable to decide how to size a transmission gate. I sized them in the ratio of their unCox. The delay is not a huge concern, because the sampling capacitance is very small. Is that it, or some other factor like channel charge injection has to be considered?

Thanks for your help.

 

[erikl]

nsmission gate

I am unable to decide how to size a transmission gate. I sized them in the ratio of their unCox.

In most cases, equal w/l ratios for both p- and nfets are used. But sizing according to their µCox-ratio is also fine, I think.

The delay is not a huge concern, because the sampling capacitance is very small.

Make the w/l as small as possible, just big enough for your max. delay requirement. If 2 or more transmission gates have to match, consider the necessary w*l product for your matching requirement.

Is that it, or some other factor like channel charge injection has to be considered?

Channel charge injection very often is a problem. Consider at least the simplest compensation method with (1/2)*(w/l) anti-phase-switched drain-to-source-shorted compensation fets.