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I'm trying to run USB D+/D- signals and some unrelated 19V power between two PCBs, on the daughter PCB the USB D+/D- will terminate to a USB-A-F jack. I have no problem maintaining 90 ohm differential impedance on the actual PCB, but how important is it to maintain that differential impedance on the cable connecting the two PCBs?

I'd like to use a flexible flat cable, but can't seem to find any that are 90-ohm. Is there an easier way to accomplish this?

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  • \$\begingroup\$ How long is the separation? \$\endgroup\$ – W5VO Feb 5 '14 at 18:48
  • \$\begingroup\$ 60-120mm is an approximate length \$\endgroup\$ – Kevin Feb 5 '14 at 18:56
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The preferred approach for a commercial product would be to contract some cable company to make exactly what you want. There are lots of vendors that do this.

You could try this: 90 Ohm FFC. But I don't know much about the vendor.

You could also look at off the shelf LVDS FFCs or shielded FFCs. There are quite a few out there with 100 ohm differential impedance.

Another option would be to build your own flex PCB with controlled impedance.

...all that being said...

If you can't find impedance controlled FFC and this is for a non-commercial product, you'll probably be okay just using standard FFC. Here's an ... interesting article ... about the subject from Microchip. I couldn't quite figure out what the bottom line was.

Consider that on FR4, a 120mm distance would have a propagation delay of about 1ns, so anything with a rise time of 5ns+ would probably be okay. Full speed USB would probably be okay here at 12Mbps (83ns /bit), but high speed USB would be really stretching it at just 2 ns/bit. Your propagation speed in an FFC will be different from FR4, but this is just an estimate. You are also probably going to have some nasty EMI if you have impedance discontinuities so it may not be an acceptable approach for a commercial product.

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  • \$\begingroup\$ Thanks! You raised some good points re: timing, and I'm going to try to talk to that vendor. This is for a commercial product so I may end up getting a cable made, I thought there would be an easier solution. In the mean time I'll try some LVDS cables. \$\endgroup\$ – Kevin Feb 6 '14 at 20:42
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This isn't a precise answer, but I have a device on my bench that runs USB over that length of standard 0.5mm FFC (after a 1m normal USB cable), and it works fine. I've not tested it in detail or with a max length cable or in a noisy environment.

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  • \$\begingroup\$ Thanks, that's reassuring. Did you use a SMT connector for the FFC cable and is there any shielding? \$\endgroup\$ – Kevin Feb 5 '14 at 21:08
  • \$\begingroup\$ Yes SMT, and no shield. \$\endgroup\$ – pjc50 Feb 6 '14 at 8:14

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