Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

USB differential pair on 2 layer board

Status
Not open for further replies.

Jester

Full Member level 6
Joined
Aug 18, 2012
Messages
377
Helped
7
Reputation
14
Reaction score
7
Trophy points
1,298
Location
.
Activity points
4,754
I realize a 2 layer board is less than ideal for this, however that's what were stuck with for this application.

I'm laying out a board that will make use of an open source Arduino board for the uC section, because the code is already written and tested on the MkrZero board. I imported the PCB files and when I perused the USB differential pair, first thoughts are this is not right. Trace width is 0.25mm (9.84 mils), and space is 0.2mm (7.87 mils). This is a 1.6mm board. Assuming FR-4

Viewed as an edge coupled external plane, Zdiff =148 \$\Omega\$

However, both the top an bottom are flooded with ground plane as shown.

Viewed as a coplanar waveguide, Zo is close to 90 \$\Omega\$, not sure about Zdiff?

I guess I need a more versatile impedance calculator.

Looks like they made some compromises on this board WRT the USB routing? Or does the top flooded plane significantly lower Zdiff?

The total length is only about 37mm (1.5"), so perhaps reflections are acceptable?

PCB house tells me to send it and they will tweak as required, however for 90 \$\Omega\$ based on edge coupled, the trace width would need to be closer to 40 mils and that's not a tweak, and 40 mil traces are not going to fit. I'm also not keen on using a 0.8mm board as there are some fairly heavy components in other parts of the circuit.

90 Ohms NOT.png
 

I have no impedance calculator at hand right now, but 40 miles trace width doesn't sound far off. It depends on the minimal clearance that can be implemented in the applied technology.

What's the maximal USB speed you want to achieve?
 
  • Like
Reactions: Jester

    Jester

    Points: 2
    Helpful Answer Positive Rating
I have no impedance calculator at hand right now, but 40 miles trace width doesn't sound far off. It depends on the minimal clearance that can be implemented in the applied technology.

What's the maximal USB speed you want to achieve?

This port is only used for connection to the Arduino IDE for program downloading so 2.0 speed would be more than adequate.
 

Hi,

USB2.0 is 480MBit/s
Wavelength at the copper traces maybe is 40cm (just estimated)

And your trace length is only about 4cm .... thus the whole situation is not critical.
Don't stress yourself.

Klaus
 
  • Like
Reactions: Jester

    Jester

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top