[SI-LIST] Re: 90 OHM Differential Impedance
- From: "o. laney" <olaney@xxxxxxxx>
- To: saril_k@xxxxxxxxx
- Date: Sat, 20 Mar 2010 14:02:11 -0700
Common mode impedance is a problem only for common mode signals. Design
to minimize this component of the total signal and it won't matter.
Besides matching trace lengths and the other usual things to minimize CM
amplitude, a common mode choke at the USB connector is recommended as a
barrier to common mode signals traveling in either direction. The basic
principle is to match the impedance for signals you want (diff mode) and
mismatch the ones you don't want (common mode). The mismatch reflects
the unwanted signals back where they came from, and a common mode choke
provides that function. Of course, rather than place the choke some
distance down the transmission line where it will cause CM standing
waves, it is best to place it at the source. CMC placement at the
connector is driven by the need to provide an EMC barrier. Best practice
is to put the transmitter IC close to the connector so the CMC does both
jobs well, or you can always use two.
Note that for any given DM impedance, CM impedance can never be less than
diff-Z/4, which is the special case of two single ended but uncoupled TX
lines used in tandem. Think of the instance of a pair of 50 ohm coax
cables used differentially (100 ohms) versus in parallel (25 ohms). For
coupled lines, the correct amount of coupling allows the diff-Z to be
held constant despite an arbitrary increase of impedance of the
individual lines with respect to ground.
Orin
On Sun, 21 Mar 2010 00:11:21 +0530 (IST) Saril <saril_k@xxxxxxxxx>
writes:
> Hi All,
> Â
> Basically USB signals require 45 Ω characteristic impedance and
> have a target differential impedance of 90 Ω.
> The 90 Ω Differential impedance can be achieved without having the
> single ended characteristics impedance of 45 Ω. Sometimes the
> Single ended characteristics impedance may be 50 Ω but still we
> can have the differential impedance of 90 Ω, if we tune the
> differential trace separation.
>
> How important is it to meet single ended as well as differential
> impedance requirements together, can I just neglect the single ended
> impedance and consider only the differential impedance value.
>
> Thanks in advance.
>
>
> Regards,
> Saril Kaiprath
>
>
>
> The INTERNET now has a personality. YOURS! See your Yahoo!
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