# Understanding USB Differential and Single Ended Impedance Requirements

The USB IF spec mentions the following for USB 3.0:

This has me scratching my head. In my experience, $Z_{diff} < 2 Z_0$ -- typically around 1.6-1.8. So how is it possible to meet both the single and double ended requirements if the single ended requirement needs to be around 50$\Omega$ (readily achievable) to get the 90$\Omega$ differential impedance?

Some of the resources I've found even explicitly say 50$\Omega$, such as this Toradex high speed layout guide (PDF):

And this TI USB 3.0 hub reference design, which uses ~4.5mil trace, 5 mil space on 1oz cu, 3.7mil dielectric thickness also does 50/90:

...but this is straight from the horse's mouth.

Most information I find seems to emphasize the 90$\Omega$ requirement more than the 45 ohm requirement. Is it really important to get 45$\Omega$ or is 50$\Omega$ preferred? Can someone set the record straight for me?

Single ended impedance is the trace impedance with reference to ground.

Differential Impedance is the impedance between two differential pair signal traces.

So I think both needs to be matched if you want to work at rated high frequency. Also need to be within tolerance range as in USB case it is 15%.

Reference: http://www.ti.com/lit/an/slla414/slla414.pdf (Page # 6)

I think they're talking about two different sets of wires in different generations of the standard.

First sentence refers to SDP, the two Shielded Differential Pairs introduced in USB 3.0 for 5 Gbps throughput. These are usually shielded twisted pairs. Their $Z_{diff} < 2 Z_o$.

Second sentence is talking about Enhanced Superspeed, the 10 Gbps signalling introduced with USB 3.1. These can be (high-grade) shielded twisted pairs, but they can also be individual micro-coax wires - one each for the direct and inverted signal of each differential pair. In this latter case because there is little or no mutual coupling between the + and - signals, $Z_{diff} \approx 2 Z_o$.

• As a caveat: I'm only somewhat confident in this answer. I'm not familiar with USB 3.x. Jun 16 '17 at 15:25

SE impedance is controlled by trace width and spacing to the plane mostly.

Diff impedance for far apart pairs is twice the single ended value. As you move the two traces closer, the diff impedance will drop until it's fine.

So choose trace width and prepreg thickness to match the SE impedance, then choose pair spacing to match diff impedance.

A nominal factor of 2 demands quite some gap. I would aim to reach within the specced tolerance. But I also remember some ppl saying ( eg. Rick hartley) that routing the two wires on different areas of the board causes no issues if done right.. so maybe just follow length matching and don't make a diff pair at all.

My recommendation is to use "loosely-coupled" pairs for this. If you have the space, route 2 SE traces with 45 ohm Z0 using the same path. Keep them 3 or so trace widths apart. This will result in approximately 90 ohm (Zdiff). It is best if you run these traces next to each other (with consistent spacing) and match their lengths. This gets you the benefit of aggressor signals being coupled to both (and therefore largely ignored by the differential receiver).