Calculating Differential Impedance for USB Differential Pair - Page 1
Forum section: electronics / beginners

author: Apocx
author_rank: Newbie
timestamp: September 14, 2015, 08:04:33 pm
content: Hi all,I'm trying to route some 90? differential pairs for some USB 2.0 High-speed data lines. To calculate the trace width/spacing, I used SaturnPCB. However, when using EEWeb's impedance calculator, I get much different values. I'm not sure which calculator to trust. Should I just go with SaturnPCB's values and pray it works? The difference seems pretty severe (10%) but I guess would fall within the 25% tolerance, so it might be OK either way--I don't know.I thought about calculating it myself manually, but when I looked at the equations I was a bit lost...The board stack (OSHPark) is as follows:1 mil solder resist1 oz copper (1.4 mil)6.7 mil prepreg0.5 oz copper (0.7 mil)47 mil core0.5 oz copper (0.7 mil)6.7 mil prepreg1 oz copper (1.4 mil)1 mil solder resistAny help is greatly appreciated!

author: Neganur
author_rank: Unknown
timestamp: September 15, 2015, 06:49:03 am
content: Ask the board manufacturer for the values. You can use all the calculators you want if you don't know the exact board make up they're using; they should be able to help you.Edit: also, think ahead if you are going to use ground pours around the traces.

author: Scrts
author_rank: Frequent Contributor
timestamp: September 15, 2015, 08:27:51 am
content: I usually send the gerbers to the PCB manufacturer and tell which differential lines needs specific impedance. They modify the gerbers themselves and send me to approve.

author: Apocx
author_rank: Newbie
timestamp: September 15, 2015, 08:30:06 am
content: Quote from: Neganur on September 15, 2015, 06:49:03 amThe board make up is listed at the bottom of my post, unless I'm misunderstanding you. Or are you saying the manufacturer would be able to supply values for track width/etc.? If so then I will ask them.This page shows the stackup and states the dielectric constant is 3.66 at 1GHz, so I used that value in the impedance calculators. As far as I know all the input values I provided the calculators are accurate. They also include the full datasheet at the bottom.http://support.oshpark.com/support/solutions/articles/122263-4-layer-board-stackup-and-specificationsQuote from: Neganur on September 15, 2015, 06:49:03 amCan you elaborate please? This is what my unfinished design looks like so far, using 11 mil traces with 8 mil gap. As I understood the guidelines, the diff. pairs (red) should not cross any plane splits, and should have a contiguous ground plane beneath it (yellow). Am I doing it wrong?Thanks for your help!

author: Apocx
author_rank: Newbie
timestamp: September 15, 2015, 08:31:09 am
content: Quote from: Scrts on September 15, 2015, 08:27:51 amOh wow, I didn't know that was something they'd do for you. I'm not sure OSHPark does that as it's basically a community batch of PCBs that is then sent to the actual manufacturer, but I can ask I suppose.Thanks!

author: Neganur
author_rank: Unknown
timestamp: September 15, 2015, 09:08:10 am
content: Exactly that, I prefer asking them for the correct values instead of letting them edit the gerber.They should also be able to give you the gap distance to ground planes in the same layer. But maybe your design does not have any ground plane in that area and so you don't need it.Edit: added a pic, not sure how well the mobile app handles the size.

author: tggzzz
author_rank: Super Contributor
timestamp: September 15, 2015, 09:51:40 am
content: Quote from: Neganur on September 15, 2015, 09:08:10 amWhat produced that picture?

author: jahonen
author_rank: Super Contributor
timestamp: September 15, 2015, 11:46:10 am
content: Looks like Polar Instruments Si8000 :http://www.polarinstruments.com/products/cits/Si8000.htmlAnd regarding to the USB impedance requirements, there is also a common mode impedance specification of 30 ohms which should be considered in addition to the differential one.Regards,Janne

author: Scrts
author_rank: Frequent Contributor
timestamp: September 17, 2015, 04:18:50 pm
content: Quote from: Neganur on September 15, 2015, 09:08:10 amTried that.. It's still not perfect in the end. I let them do the edit. Never had any issues to be honest.