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Design >> Analog Design >> What should be criteria (Voltages, Currents )
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Message started by bharat on Nov 12^th, 2006, 9:54pm

Title: What should be criteria (Voltages, Currents )
Post by bharat on Nov 12^th, 2006, 9:54pm

I am stuck up here, please help me.

I am designing a high speed comparator with very stringent rise/fall specs. By using the minimum channel length of the process technology, I was able to achieve that specs. After that I realised that min channel length is no good for offset of the comparator so I increased the channel length but I am not able to get the same rise/fall mismtach data. Now I am comparing my previous design (with min channel length) to new design just to get the feel as what is making the difference in performance. I started with matching the currents of the two designs in individual transistors, the rise/fall mismatch is still far off. I gave the reason that though the currents may be same, now the capacitance values have increased in later design and it is taking more time to charge and discharge the capacitances.
So I realized that it shouldn't be current rather some other parameter.
As I incresed the channel length, to compensate it with same current, I incresed the Width of the device also. Resulting this, I am having different gm, Vgs so on.
Also I have deep submicron process narrow channel length so don't know the second and third order of effects resulting this mismatch like Vth variation with device dimension.

This is the background of my question. Now the question is, if I have 2 same designs with different dimensions and I want the same performance, what should be matched in first order assumption. Is it current, gm, Vgs Vds ??

Please get back to me, if it is not clear..

Thanks
-Bharat

Title: Re: What should be criteria (Voltages, Currents )
Post by bharat on Nov 12^th, 2006, 10:48pm

In continuation of my previous email, I do understand that its difficult to precisely say that what parameters should be matched. It is not an attempt to overly simplify such a great design work. Also, designer's discretion is required to make a call. I am just expecting some guiding rules where I can give a shot.

Title: Re: What should be criteria (Voltages, Currents )
Post by loose-electron on Nov 13^th, 2006, 8:51am

In actuality you may have answered the question yourself.

You are trying to match the performance of the device.

Since a number of different things interact with each other in the process of doing the design, you can not really say that a particular parameter needs to be matched.

As for the geometry vs. speed issue, you may want to consider techniques to calibrate out the offset as mismatch gets worse, AC coupling of stages to eliminate mismatch, or something similar.

Jerry

Title: Re: What should be criteria (Voltages, Currents )
Post by Croaker on Nov 21^st, 2006, 8:04pm

You should plot Vth vs. L to get an idea of how much Vth is varying with L. If you care about mismatch, pick an L that will give a fairly constant Vth. For short-channel devices, Vth is fairly sensitive with min L.

A larger Vgs is going to help reduce the Id variation as Vth varies. Also, a larger overdrive is going to give you higher speed, and will help compensate for the reduction in speed caused by a non-min L. One thing to watch for is increasing W/L, thinking you are going to get a speed increase. If you keep Vgs constant, current goes up, but so does capacitance, so you waste power. And, if you keep Id constant, Vgs has to drop, lowering speed and making mismatch worse.

For short-channel devices, I'd say that the variations in Vds are pretty important for matching. Even if you have matched Vgs and Vth and W/L, that's not going to help much for short-channel devices with very different Vds. Of course, longer L will decrease channel-length modulation and Vds mismatches won't matter as much.

In summary:
large L
larger Vgs
match Vds

Marc

bharat wrote on Nov 12^th, 2006, 9:54pm:

Also I have deep submicron process narrow channel length so don't know the second and third order of effects resulting this mismatch like Vth variation with device dimension.

This is the background of my question. Now the question is, if I have 2 same designs with different dimensions and I want the same performance, what should be matched in first order assumption. Is it current, gm, Vgs Vds ??

Thanks
-Bharat

Title: Re: What should be criteria (Voltages, Currents )
Post by avlsi on Mar 26^th, 2007, 4:13am

I think if offset was problem, does any CDS technique solve your problem?

Title: Re: What should be criteria (Voltages, Currents )
Post by chase.ng on Mar 26^th, 2007, 6:41pm

Hi,

I guess the more efficient way to solve the prob is by architecture rather than by tuning the devices. Like wat suggested by avlsi, CDS is good, or try putting a preamplifier in front.

chase