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I have a use case where I use JTAG debugger connected to the circuit under test. Circuit is powered from a desk PSU and measured via desk MSO. This would normally create a ground loop, so I use an USB isolator ADUM4160 based cheapo device.

Measuring resistance between un-isolated USB "ground" and PSU ground I get around 50+ MΩ. When USB-isolator is connected I am measuring 0Ω with my digital multimeter. Everything works fine, as far as my knowledge reaches.

Now, I want to throw two more USB grounded devices into a loop. Another JTAG debugger for second circuit that will be connected to the first one over serial port (sharing same PSU). Second USB device will be logic analyzer.

  • Can I connect USB hub after isolator and connect 3 devices into a hub (that will probably reduce the bandwidth substantially and wouldn't really work)?
  • Or do I need USB isolator per device ?

Also, I am running out of USB connections on the PC, so I will need to use the hub.

  • Is there any issues connecting multiple isolators to the hub?
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    \$\begingroup\$ What does "With usb Isolator, DMM gives zero" tell you or mean? \$\endgroup\$ – Andy aka Jun 10 at 14:32
  • \$\begingroup\$ Sorry, I was unclear. I updated the text. I was referring to the digital multimeter and measuring ground between usb isolator and PSU. \$\endgroup\$ – Gossamer Jun 10 at 14:36
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    \$\begingroup\$ OK, to spell it out: if the resistance is 0Ω, the two parts are not isolated, thus your "isolator" is not actually an isolator. \$\endgroup\$ – Marcus Müller Jun 10 at 14:41
  • \$\begingroup\$ I was running continuity test and I didnt got the "beep". I would say it is working. \$\endgroup\$ – Gossamer Jun 10 at 14:49
  • \$\begingroup\$ continuity is literally the opposite of isolation. It is clearly NOT working as isolator. \$\endgroup\$ – Marcus Müller Jun 10 at 15:14
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Can I connect USB hub after isolator and connect 3 devices into a hub (that will probably reduce the bandwidth substantially and wouldn't really work)?

Yes. I was worried about a similar problem, so I bought a USB 2.0 isolator. The problem is they are pricey (over 200$) because no one makes a USB 2.0 isolator IC. I then plugged the hub into the USB 2.0 isolator and then to the computer. Because the devices were adjacent to each other on a metal chassis, I was more concerned about the ground loop between the hub and the comptuer than the devices.

It depends on what currents are on the shields of the USB cables. If you have a really nice volt meter that measures uA, sometimes you can dissconect the cables and measure the current or voltage and see if it's worth it to throw an isolator on the cable.

Is there any issues connecting multiple isolators to the hub?

Shouldn't be, I've done it and I was fine. Isolators will introduce a slight bit of latency.

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  • \$\begingroup\$ Most of the cheapo isolators are using this IC: analog.com/media/en/technical-documentation/data-sheets/…. Only thing is that they might be slowish for JTAG & LA. On the other hand if I have JTAG per isolator and connected signals from one board to another (sharing ground), would that cause any problems ? \$\endgroup\$ – Gossamer Jun 10 at 15:19
  • \$\begingroup\$ It really depends on the setup, normally no, if you have a lot of noise or large currents then it could become a problem. There was a company that briefly made a 2.0 isolator, then pulled it from the market. Most 2.0 implementations require FPGA's and regular isolators. \$\endgroup\$ – Voltage Spike Jun 10 at 15:25
  • \$\begingroup\$ My setup is quite simple. I just want to debug two boards which are powered by PSU and be able to check the signals on the oscilloscope and logic analyzer. Its all low current digital communication, with speed bellow 5Mhz. Only complication is the bandwidth of the JTAG's and LA. I could make my life simpler and just use the laptop on the battery, but again USB ports are problem. Not that many on the laptop and dont really want to risk the motherboard if I have short somewhere :) \$\endgroup\$ – Gossamer Jun 10 at 15:31

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