Ive build a few boards now wih the PIC18F4550 and using USB, and have never 
used any external resistors with the internal usb transciever, had no 
issues with it yet. The only thing to be careful with is to ensure that the 
tracks are as short as possible between the PIC and usb connector to 
prevent interference. Have a look at the usb article on www.piccoder.co.uk 
for the circuit diagram I typically use, (also a brief usb getting started 
guide for the microchip firmware, just a simple how to get it up and 
running one, more coming shortly!).

Is anyone else working on ADC sampling through the USB currently? We are 
currently trying to figure out the best way to go about this,(aiming for a 
few 100kHz-1Mhz if possible), any suggests or comments would be most 
welcome.

Mat



On Apr 22 2006, cdb wrote:

>According to Jan Axelson's book a 36 ohm resistor is required in 
>series at each drivers output, to act as source terminations that 
>reduce reflections when the outputs switch. She goes on to say that 
>these resistors may be on or off chip.
>
>:: Looking around, I see some applications of micros and USB with
>::
>:: 22 to 27 ohm resistors in series with D+ and D-.
>
>
>Colin
>
>
> -- 
> cdb, colin@btech-online.co.uk on 22/04/2006
>
> Web presence: www.btech-online.co.uk  
>
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>
> I don't have to approve of things that I am willing to tolerate.
>
>
>
>
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