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The Designer's Guide Community Forum › Design › Mixed-Signal Design › What is drawback to use minimum-sized transistors for analog switch?

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What is drawback to use minimum-sized transistors for analog switch? (Read 19097 times)

tony_taoyh New Member Offline Posts: 7	What is drawback to use minimum-sized transistors for analog switch? Sep 16^th, 2009, 4:11am To reduce the charge injection and clock feed-through for analog switch, minimum sized device can be used if Ron is not a issue. What is the drawback to use the minimum sized device? Thanks a lot.
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Berti Community Fellow Offline Posts: 356	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #1 - Sep 16^th, 2009, 4:16am If settling (Ron, also taking into account process variations) is still ok I don't see an issue. Other opinions? Regards
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #2 - Sep 17^th, 2009, 12:13am hi, what is the meaning of no issue Ron? means if you have any value is it ok or even though it changes with time or PVT? because if you put smaller switch you will see larger PVT variations Thanks, Rajasekhar.
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Mayank Community Fellow Offline Posts: 334	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #3 - Sep 17^th, 2009, 1:16am hi, minimum sized device(or length) will introduce more flicker noise....That might cause problems. regards, Mayank.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #4 - Sep 17^th, 2009, 3:53am hi, ya,thats true but generally in high speed sample and hold circuits flicker noise will not come due to its low frequency spectrum. By the where you are using switch? Thanks, Rajasekhar.
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RobG

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Re: What is drawback to use minimum-sized transistors for analog switch?
Reply #5 - Sep 17^th, 2009, 8:39am

I think it is one of those rules of thumb that has been followed so long nobody can remember why. The reason I was given was that you are pushing the edge of the process, which might give some leakage when off, but I'm not sure how true this is. I used min gate lengths for my last design.

You may already know this, but contrary to intuition, differential charge injection is minimized with minimal switches since you are subtracting the mismatch, not averaging the mismatch. So that is another reason to use as small as possible switches.

rg

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #6 - Sep 17^th, 2009, 10:05pm hi rg, i didn't understand last para in your post (average mismatch....etc)...if you decrease the device size all secondary effects will decrease vth and leakage will increase. And i feel differential configuration would be help full to cancel this one. Thanks, Rajasekhar.
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Berti Community Fellow Offline Posts: 356	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #7 - Sep 18^th, 2009, 12:11am rg, I don't understand your post, too. What "rules of thumb" are you talking about? Rajasehkhar, "decrease the device size" means decreasing L or W? Why decreases vth with smaller W? Cheers
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tony_taoyh New Member Offline Posts: 7	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #8 - Sep 18^th, 2009, 12:54am Thanks a lot. Guys. With short channel effect, the Vth is roll-off when L is reduced.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #9 - Sep 18^th, 2009, 1:30am hi berthi, i am sorry, type mistake decreasing w will increase Vth due narrow width effect Thanks, Rajasekhar.
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Berti Community Fellow Offline Posts: 356	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #10 - Sep 18^th, 2009, 5:10am Rajasekhar, sorry, I think you original post was generally correct (in fact I think it depends on the technology, but I am not so familiar with this topic).
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rajdeep

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Re: What is drawback to use minimum-sized transistors for analog switch?
Reply #11 - Sep 18^th, 2009, 7:29am

Hi all,

I also think it is due to more PVT variation at min length. The models that we use for simulaiton may not be that accurate at that limit. It is always safe to use 4*Lmin if possible. So, if speed is ok and feedthrough problem doesnt increase much I'll tend to use longer length just to be in the safer side.

Not sure whether the switch will be more leaky or not in OFF state when using Lmin for the same W/L ratio...... Lips Sealed

Thanks,
Rajdeep

Design is fun, verification is a requirement.

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RobG

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Re: What is drawback to use minimum-sized transistors for analog switch?
Reply #12 - Sep 18^th, 2009, 8:08am

Berti wrote on Sep 18^th, 2009, 12:11am:

rg, I don't understand your post, too. What "rules of thumb" are you talking about?

I was speaking about the "rule of thumb" that says not to use minimum sized devices for switches. Throughout my career I've heard it from many people to use channel lengths a bit longer than minimum, but nobody ever knew exactly why.

As dimensions decrease the Vt starts changing pretty rapidly with small ΔL and ΔW implying more mismatch potential. Perhaps that is a practical consideration.

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RobG

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Re: What is drawback to use minimum-sized transistors for analog switch?
Reply #13 - Sep 18^th, 2009, 8:30am

raja.cedt wrote on Sep 17^th, 2009, 10:05pm:

hi rg,
�i didn't understand last para in your post (average mismatch....etc)...if you decrease the device size all secondary effects will decrease vth and leakage will increase. And i feel differential configuration would be help full to cancel this one.

Guess I'm confusing everyone...

I was talking talking about charge injection in differential circuits. As you know, if it is matched the injected charge is common mode and not an issue.

On the other hand, if there is a mismatch in the injected charge you will get an error signal. It follows that you would like to match the charge injections as well as possible. Most people assume that larger switches will match better, but this is not true for matching charge injection. The matching improves as sqrt(area), but the amount of charge increase proportional to area.

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: What is drawback to use minimum-sized transistors for analog switch? Reply #14 - Sep 19^th, 2009, 4:17pm Common mode effects are greatly reduced, agreed, but not eliminated. Don't get into binary thinking with analog systems. From my experience, min geometry for switches is generally the way to go. For everything else, the matching concerns and PVT variance come out to bite you.
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