Electronic – differential trace impedance for USB (90 Ohms) on 2-layer FR4 board

differentialimpedancepcbusb

I am currently working on the USB hub part for a multiport USB-RS232-converter. The current design is for Terminus Tech's FE 2.1 Hub IC. Regarding the impedance of USB signal traces the layout guidelines state:

DP, DM Differential trace impedance = [DP(45ohm) + DM(45ohm)] = 90 ohm,
and do not jump the DP DM signals that cause impedance miss match

To meet those requirements I used some online impedance calculators to estimate the required trace properties for a 2-layer FR4 PCB. However, the results I got are not consistent at all:

  • Hughes Circuits
    • input values
      • trace thickness: 1 oz/ft^2
      • substrate height: 1.6 mm
      • trace width: 1 mm
      • trace spacing: 0.15 mm
      • substrate dielectric 4.5
    • results
      • odd impedance: 45.5 ohms
      • even impedance: 119 ohms
      • common impedance: 59.3 ohms
      • differential impedance: 90.9 ohms
  • Montaro
    • input values
      • trace width: 3.35 mm
      • trace separation: 10 mm
      • trace thickness: 0.03556 mm
      • dielectric thickness: 1.6 mm
      • relative dielectric constant: 4.5
    • results
      • differential impedance: 90.035 ohms
      • single ended impedance: 45.071 ohms
  • All About Circuits
    • input values
      • trace thickness: 1 oz/ft^2
      • substrate height: 1.6 mm
      • trace width: 4.15 mm
      • trace spacing: 14.8 mm
      • substrate dielectric 4.5
    • results
      • odd impedance: 45.0 ohms
      • even impedance: 36.8 ohms
      • common impedance: 18.4 ohms
      • differential impedance: 90.0 ohms
  • Colorado Electronic Product Design
    • input values for single microstrip
      • w: 3.5
      • h: 1.6
      • t: 0.03556
      • epsilon: 4.5
    • result for single microstrip
      • impedance: 45.12 Ohms
    • input values for microstrip pair
      • s: 15
      • h: 1.6
      • Z_0: 45.12 Ohms
    • result for microstrip pair
      • Z_d: 90.23 Ohms
  • EEWeb
    • input values
      • trace thickness: 1 oz/ft^2
      • substrate height: 1.6 mm
      • trace width: 4 mm
      • trace spacing: 5.5 mm
      • substrate dielectric: 4.5
    • results
      • odd impedance: 45.0 ohms
      • even impedance: 38.7 ohms
      • common impedance: 19.4 ohms
      • differential impedance: 90.1 ohms

In addition to those calculation I found some other resources with some sample trace properties:

  • Mikrocontroller.net Forum
    • trace width: 0.22 mm
    • trace distance: 0.13 mm
    • substrate thickness 1.6 mm
    • resulting differential impedance: 100 Ohms
  • IBEX
    • 1.6 mm PCB
    • 1.48 mm FR4 thickness to GND plane
    • 35 um copper trace thickness
    • trace spacing 0.15 mm
    • trace width 1.12 mm
    • resulting differential impedance: 90.184 Ohms

Why do those results differ that much? Since I am a beginner in doing impedance controlled PCB design I do not know which resource to trust.
Any hints about dependable trace properties for designing USB on a 2-layer FR4 board are pretty much appreciated.

Best Answer

The reason the values differ so much is because different tools use different formulas to calculate impedance. Some are approximated closer than others, but finding exact impedance is extremely difficult if not impossible. The best tool I've used, and I still use today, is the EEWeb calculator (which you linked to in your post). That one seemed to have the best, most accurate results in my experience. There are also plenty of clones that use the same math as the EEWeb calculator so they will also be just as accurate.

Generally, when designing with controlled impedance as long as you get within +/-20% you will not notice much in the way of reflections and distortion, though of course this depends on frequency and switching speed. However, I try to shoot for about 10% over the target impedance. It's better for the Zd to be higher than the target than for it to be lower. 10% is pretty standard for most designs. In your case I'd shoot for a Zd around 100 ohms (basically 10% higher than your initial target).