lvds driver - how can I increase its speed?

[marxelus]

gb lvds

hi,
I'm designing an lvds driver like the figure below. But it have to go in 1Gb and the outup load is killing it.
I've a cutting freq. of 500 MHz aprox. and my output load is two 100 Ohms resistances and two capacitor of 10 pF.
Can you explain me how can I increase its speed?

 

[AdvaRes]

lvds speed

Hi man,

Signal speed is related to voltage and current and since the LVDS is low voltage you can only increase the current to get higher frequency.

 

[marxelus]

Re: increase lvds speed

first of all, thanks for the answer. About it, I can't increase my current, It have to be about 3.5 mA to accomplish the standard, doesn't it?

 

[AdvaRes]

Re: increase lvds speed

first of all, thanks for the answer. About it, I can't increase my current, It have to be about 3.5 mA to accomplish the standard, doesn't it?

In that case you have to modify your IO.

 

[marxelus]

Re: increase lvds speed

Are you talking about some predriving? Have you some papers about it?

because the output load is fixed from the bonding

 

[PaloAlto]

increase lvds speed

uumm, it seems to me that maybe it is the cmfb that is limiting your speed

why don't you remove the cmfb and put an ideal block to see if then you can get it, later on you can add another cmfb circuit

BTW you should be able to have less than 10p on each brand before getting to the transmission line, but you should model the bonding wires and the transmission line, they should be your final limiting factor for speed. you may need to match the impedance at the output of the lvds to get as high as 1ghz

BTW2 Are you sure you need two 100 ohms resistors in parallel? Shouldn't it be only one?

 

[marxelus]

Re: increase lvds speed

referring to the two resistors. I'm reading some papers that said that putting it, you have to doble the current of the system and have better results that with only one. But if it's better for speed purposes, it can be only one.
About the output load of two capacitors and the 100 ohms resistor, they are an especification and I can't change it.

When you are talking about matching my output load, what can it be made? with predriving or...
I'm new in analog design. I'm sorry.

 

[PaloAlto]

increase lvds speed

OK, that's what I meant with matching the impedance at both sides of the tl, of course you need double current.

And forget about the predriver. Right now it is ideal, so it cannot get better than that. Once you include it, of course that the comparator will limit also your speed and you'll need a fast one. But that's another history

Now that I am thinking again about it, I have removed the line about the impedance matching. I'm not sure where is the point from which you need to match the output impedance of the driver

 

[marxelus]

Re: increase lvds speed

and then, have you some ideas to increase the speed to 1 Gb?
Thank you very very much for the answer

 

[PaloAlto]

increase lvds speed

Go step by step:

1 get rid of the cmfb
2 make the output transistors as wide as needed for the load
3 make the predriver fast and strong enough for those transistors
4 design the cmfb (I cannot help you much there, I'm not sure if you need to go up to 500Mhz, I think that it could be enough with less)

I think that in all this you need the pair of resistors and don't forget to model your pads and lines. It really is critical

 

[shanmei]

Re: increase lvds speed

hi,
I'm designing an lvds driver like the figure below. But it have to go in 1Gb and the outup load is killing it.
I've a cutting freq. of 500 MHz aprox. and my output load is two 100 Ohms resistances and two capacitor of 10 pF.
Can you explain me how can I increase its speed?[/quote


Hi,

If you want the output is IG, which means the time t=1/f=1.0e-9. however, your output RC constant is R*C=100*10*1.0e-12=1.0e-9. so it is clear that you cannot achieve this . Unless your cap is around 2p. So the RC constant time now is 0.2e-9. NOW your lvds can drive it .

 

[marxelus]

Re: increase lvds speed

The problem is that the output impedance is a model of link and I can't control it. Are there any kind of technique to rise the speed of this lvds?
I've seen a lot of papers for 1-2 Gb LVDS using this kind of schema.

I've enclosed one pictore of ac analisys. As you can see, I've a resonance freq of 1 Gb