Difference between USB Single ended impedance and common mode impedance

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Hi Team,

I was learning about USB2.0 layout guidelines from this material[USB Layout Guidelines].

In page 2 you can see the statement that says a single-ended impedance is not critical as differential impedance.A range of 42 to 78 ohms is acceptable(equivalently common-mode impedance must be between 21ohms to 39ohms).

As per my knowledge, here single-ended impedance(my understanding about single-ended impedance is the impedance of D+ or D- line measured w.r.t GND) and its value must be 45 Ω ±10%. Please correct me if I am wrong.

May I know about which single-ended impedance this document says a range of 42ohms to 78Ohms is acceptable?

I don't have any idea about the common-mode impedance they mentioned in the document. Could you please explain about this common-mode impedance also?

Regards
Hari
 

Delta Prime

Joined Nov 15, 2019
1,196
Hello there! :)
a single-ended impedance is not critical as differential impedance.A range of 42 to 78 ohms is acceptable(equivalently common-mode impedance must be between 21ohms to 39ohms)
Keep in mind 42 is double 21 and 78 is double 39 impedance! ohms! Common mode.
Is easier for me to explain the physical layout of a PCB design then you can attack the single ended impedance or differential impedance. There are advantages and disadvantages in either design.
two traces (differential signals are identical)with like sources and terminations & cannot be run with the same amount of wiring as one Trace , the trace lengths have to match for differential signals.
One Trace(single ended) Noise sensitivity can be an issue, especially common-mode noise which typically appears on the ground or return, the ground plane which is directly underneath the traces or should be. Remember this is a four-layer board). the specific trace parameters are dependent on the type of reference used to process and measure the signal, and that is under the preference of Me the designer. Now obviously two traces are going to have twice the impedance as one Trace which is absolutely dependent on the type of signal used.In this case with respect to the ground plane. This is a very general explanation to paint a picture in your mind.
 
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