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Common-mode voltage on USB on STM32F103

Question asked by gregson.peter on Feb 13, 2017
Latest reply on Feb 14, 2017 by T J

Hello,

 

I hope someone can clarify this. How can the USB common mode range be less than 3 volts if  D  and D- pulled down with 15k, differential resistance is 90 ohms, and pullup is 1.5k to 3.3 volts?  Using simple network theory, the voltage must be

             3.6 x (15k || (15k + 90))  / (1.5k+(15k || (15k+90))) = 3.0 volts 

 

The STM32F103C6 data sheet specifies the characteristic impedance is 90 ohms, and states in table 44 on page 69 that the USB common-mode voltage range is 0.8 volts minimum to 2.5 volts maximum, measured from the local ground.

 

How can this be?  What am I missing?

 

Thanks

 

Peter

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