differential pair of 8o ohms for a usb io PCB 1

[DanielJackMorley]

I have never needed to do a differential pair myself but the schematic calls out for one. The data sheet says it needs to be 80 differential between the two traces. Reading tutorials on it tell me that the this can be determine by trace width height and permeability of the dielectric. I found two approaches on this stripline and microstrip. I am confused on the advantages of which one to use. Can some elaborate on this?

 

[IRstuff]

Regardless, you'd still need to terminate the line at the chip with a resistor with that value of resistance.

Wikipedia has the following:

http://en.wikipedia.org/wiki/Stripline

http://en.wikipedia.org/wiki/Microstrip

In your specific case, you'd want something slightly different, I think, which is that you want the characteristic impedance BETWEEN the two lines to be the value desired. This would normally entail running one of the two lines on top of the other.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MacGyverS2000]

Meh, side by side works well enough, just match line length... I've done the same for Ether without issue.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DanielJackMorley]

Like a microstrip because this board has only one ground plane. That is a brutal calculation. I am still a little confused. The schematic I am working off of tells me to differential pair map two net sets but not the whole track of the USB signal. I don't see where the termination resistor goes. I made a mistake in my original post it was 90 ohms. This signal comes for an external coldfire processor board the note was left by the manufacturer of this board. I up loaded a bit map of the schematic if that helps.

 

[pstuckey]

There are free calculators that will help you with this. I'm familiar with the tool that Rogers provides:

http://www.rogerscorp.com/acm/downloads/mwi/index.aspx

Once you get it running, go to:
Design Type > Microstrip > Edge Coupled

Pick your board material (you can input the board properties manually if you're using someone else's material). Then you can vary the height, width, spacing etc until you reach the right differential impedance.

Peter

 

[DanielJackMorley]

Another issue on this design came up. The usb connector needs to rotated 90 on a piggy back board. Is this possible on a USB design if the Piggy back board follows the micro strip rules of impedance and trace length. What about going through a via on the differential pair.

 

[MacGyverS2000]

What speed will you be running your USB connection at? Is this USB 2.0 or 3.0?

Orientation of the connector isn't an issue as long as your impedance and signal length are within spec. At high speeds, placing it on a piggy-back connector could present issues...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DanielJackMorley]

Sorry for the late response but there was some confusion. We plan on using 2.0 but 1.0 is possible.

 

[MacGyverS2000]

1.0? Not likely... 1.1, at worst.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[jimkirk]

You might also want to check out

http://www.usb.org/home