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Simulation with dynamic comparator problem

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leokaven

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cmos dynamic comparators

HI! friends

I'm simulation with a dynamic comparator, but meet a problem i can't resolve it.
There are two simulation result, fig 1 is my simulation result, fig 2 is i want to get result.

The simuation parameter are following
Vref+ =Vref1 =1.9V
Vref- =Vref2 =1.1V
Vin+ =Vin1 =DC 1.5V ,5MHz
Vin- =Vin2 =1.3V
VDD=3V
The comparator threshold leve are 0.2V and 0.4V
Result represent by (Vout+) - (Vout-)

I don't know how to resolve this problem anybody can help me, please.
Sorry!my English is poor
Thanks for your help
 

maxwellequ

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Well, to start the comparator schematic is wrong: you should apply Vin+/Vref+ to one differential pair and Vin-/Vref- to the other one.

Then, your input must be differential (it seems to me that your Vin- is fixed and that is NOT the idea).

Rgds
 

ezt

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maxwellequ said:
Well, to start the comparator schematic is wrong: you should apply Vin+/Vref+ to one differential pair and Vin-/Vref- to the other one.

Then, your input must be differential (it seems to me that your Vin- is fixed and that is NOT the idea).

Rgds

Hi.
first of all this schematic is completely right. Actually I think it's the exact picture from Sumanen Thesis, they call it "Differential Pair Comparator". Am I right leokaven? And this circuit became a paper

L. Sumanen, M. Waltari, K. Halonen, “A Mismatch Insensitive CMOS Dynamic
Comparator for Pipeline A/D Converters,” in Proceedings of the IEEE International
Conference on Circuits and Systems (ICECS’00), Dec. 2000, pp. I-32–35.

But a better and more complete paper would be

L. Sumanen, M.Waltari, K. Halonen, “CMOS Dynamic Comparators for Pipeline
A/D Converters,” in Proceedings of the IEEE Int. Symposium on Circuits and
Systems (ISCAS’02), May 2002, pp. V-157–160.

And why you can't get a right output? As I remember this circuit have a complex equation to set its trip point. Maybe you didn't make it right. Besides, to make sure your circuit works properly a better test input would be a ramp which can be produced by PWL instruction in HSpice. This ramp will start from 0 to Vdd. When it crosses the trip point and be larger than that the output will be high and vice versa. Sometimes a problem like yours may occur because of not a suitable CM level. you can change your input CM levels to 1.6 for Vin+ and 1.4 for Vin-.

Good luck.
Regards,
EZT
 

maxwellequ

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Dear ezt,

In fact this seems the Fig. 4.2 of Sumanen's Thesis. But I mantain what I said: THIS IS WRONG.

If (VREF+)-(VREF-) is larger than sqrt(2).Vovd (input saturation voltage of a diff pair), one of the transistors in the differential pair, the tail current flows entirelly in one of the transistors -> circuit becomes insensitive to the values of the VREFs. Therefore this circuit only works if Vref+ is not very far from Vref- (which does not seem to be the case)

See, for example:
Y. Wang and B. Razavi, "An 8-bit 150-MHz CMOS A/D Converter", JSSC, pp. 308-317, March 2000 (Fig. 12)

Rgds
 

arsenal

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Dear maxwellequ,

So long as the diff pair of vref doesnt go out of the linearity region, the structure will be ok.
 

tecsiun

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Design techniques for high-speed, high-resolution comparators
 

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