Test jig probing a USB connection - Page 1
Forum section: electronics / projects

author: nali
author_rank: Frequent Contributor
timestamp: April 11, 2016, 09:25:55 am
content: So I have my first PCB design that will be manufactured in a reasonable volume and as such needs a test jig made. Most of the test signals are DC or slow speed (DIP switch setting, power, RS232) but I also have a USB2 connection to test. The USB device is a FT234XD which is USB 2.0 Full Speed (12Mbps).Is it OK to add a couple of test pads on the data lines under the connector on the bottom layer? This will in effect create a couple of short stubs with the extra track/via/pads, and the routing isn't ideal thanks to the lines having to be swapped although the total track length from chip to connector is only about 6mm.The alternative is probing the 0603 components connected to the data lines but that's less reliable due to variances in component positioning after reflow I'm told.Layout below: Red=top Green=bottom, middle layers omitted but are just groundplane fill under this area.I've read that USB 12Mbps routing isn't that critical so maybe I'm being paranoid. Any advice or critique welcome!Edit: Image doesn't seem to be showing, it's

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 11, 2016, 11:44:44 am
content: Pad usually on top side,component side.anywhere on track route, hence less affecting congestion.

author: tggzzz
author_rank: Super Contributor
timestamp: April 11, 2016, 11:54:25 am
content: The frequency is irrelevant, only the transition time and sampling instant matters.All probing will affect the UUT. The severity and significance of the effect depends on the details.Probing is an art and science. Some scope probes cost >$10000 and the latest Tek probetipscost $5 every time theytoucha circuit

author: stmdude
author_rank: Frequent Contributor
timestamp: April 11, 2016, 03:14:26 pm
content: Pads are okay for up to USB2 High-Speed (480Mbps) in my experience. They might become an issue on USB3.0, but I have no experience with it.The problems we've had (with 480Mbps) occurred during production, as the pads were okay, but with the probes+wires connected, it caused issues sometimes.Keep the probe+cable as short as possible, and in some cases, individually shielded cables helps a bit.

author: 51mmz0rz
author_rank: Newbie
timestamp: April 12, 2016, 11:08:32 pm
content: You would be totally fine in my opinion. At that speed it shouldn't be very sensitive.

author: Apollyon25_
author_rank: Regular Contributor
timestamp: April 27, 2016, 03:20:14 am
content: Can do this for up to USB 2 as has been stated.You can avoid the stub effect by simply routing the traces apart a bit to enter and then exit from the other side of the TP pad. Then have the traces reconverging back into a diff. pair.Don't stub-connect them, this will cause reflections and you'll potentially kill the eye-diagram and your BER.The TPs look like step-changes (capacitors) in impedance from the nominal 90ohm diff/45ohm SE, so its a simple matter to pull back the GND plane copper under the TP pads to lessen the effective capacitance.You also need to ensure that the TPs occur at the same electrical lengths in each of the pair. This prevents DM->CM conversion and an increase in your EM signature.