
USB Impedance Spec Explained
• What does the spec say?
• High-Speed Zo
– Z
o – cable
= 90 +/-15 %
– Z
HSTERM
= 80 to 100 ohms differential
– Z
HSTHRU
= 70 to 110 ohms differential
• FS Driver Impedance
– Not HS capable: Zdrv = 28-44 ohms
– HS Capable: Zdrv = 40.5-49.5 ohms
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