LVDS

Short answer: yes.

Long answer:

I'm not quite clear about whether you mean length matching between the various bits coming out of the ADC, or length matching between the P and N signals in an LVDS pair of a single bit.

1. Assume that you are referring to the length match between the bits within the bus.

Divide 1mm by the speed of light in epoxy to get the skew between the signals. My mental ALU says this works out to be about 5ps. Note that this is *skew*. You still have to ensure adequate timing margin for each signal in the bus. In this sense, the skew reduces your timing margin, however, if you have to worry about 5ps when your clock has a 4ns period, you have much bigger problems on your hands.

1. Assume you are referring to the length match between the P and N signals in an LVDS pair of an individual bit.

Normally for data signals this would be treated as in case 1, however an ADC is sensitive to noise, and a length mismatch may result in a small amount of differential mode to common mode conversion. You might want to do some spice modelling here, however I don't expect you'll have a problem.

Also bear in mind that 1mm is probably better than the length match inside the BGA package. (I'm referring to the tracks between the solder balls and the die pads.) If you get really keen, you can determine these lengths, and adjust the the lengths of the traces on your pcb to compensate. I did this once for a couple of 16 bit 622MHz buses. It was a PITA to do, and probably didn't make much real difference (it might have changed the eye width from 73 to 74% or something like that). The PCB guy decided to change to software programming after that project. Since then, we've changed to Expedition PCB from Mentor, and the (new) PCB guy here says length matching including offsets is really easy to do.

Another caveat: the velocity factor of a trace differs depending on the layer inside the PCB. Outer layers propagate signals faster due to the lower effective permittivity! This can cause problems with length matching if you are not careful. A good PCB tool will allow you to specify the velocity factor independently for each layer.

Regards, Allan

Yes, the apr. 6 pS skew because of the 1mm should be negligible. In fact, I would say you'd still be safe with

Dimiter

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Thanks for everyones help, very usefull info.

Jon