# What value series resistor should I use for USB data lines when two ICs require different impedance values?

I'm using a USB hub (TUSB2036VFR) that communicates with an ATMEGA8U2 and an FTDI chip. The incoming USB signal comes through an isolated USB communications chip ADUM3160BRWZ. The TUSB2036VFR requires a 90 ohm differential impedance, while the other devices require 22-24ohm.

Should I go with the lower impedance values or use 90 ohm for all of them? Selecting a different USB hub controller isn't really an option because of shortages.

The information for the TUSB2036VFR is on page 19, the ATMEGA8U2 is on page 265 and the ADUM3160BRWZ is on page 10.

The TUSB tells you to use a 90 Ω impedance for the differential pair of tracks.

The ADuM tells you to use series 24 Ω resistors from the chip to connect to the tracks.

Both are correct. The numbers are different, but they're numbers defining different things.

Although the ADuM calls the resistors 'termination resistors', they will not be providing the whole of the line terminating resistance. They don't tell you what the impedance of the chip interface is, and neither should they need to, if you follow the application information.

The notes to fig 9 on page 17 of the TUSB data say All USB DP, DM signal pairs require series resistors of approximately 27 ohms to ensure proper termination.

So both chips require essentially the same interface to the 90 Ω line between them.

• Thank you for the helpful answer. It seems my issue was using the series resistance and impedance numbers interchangeably which is incorrect because they are not the same. Sep 7 '21 at 13:23
• The ADuM is a FS/LS device. Usually the design of FS drivers does not bother much with impedance control, so the typical simple FS driver has about 20 Ω impedance. That's why people need another ~25 Ω to match the characteristic impedance of USB, 45+45 = 90. (20+25 = 45). this is a simple math. Sep 8 '21 at 3:37
• @Ale..chenski 20+25=45 is simple math. Knowing that the internal impedance is about 20 is knowing. Sep 8 '21 at 14:09