I have a device (Device 1) that is bus-powered via Full-Speed USB and that is drawing up to 400 mA from a laptop.
Device 1 has a DAC that can output an analog voltage to another device (Device 2) through a jack connector, Device 2 being powered externally.
The problem is that if the laptop connected to Device 1 and Device 2 are connected to a different outlet, it creates a ground loop and its characteristic "hum". If the laptop runs on battery, there is no issue.
edit : The ground loop occurs when both devices are connected to the same laptop
I first thought about using a USB isolator such as the ADuM4160 which works great. Unfortunately it is too expensive for my application. Another solution that works fine is to use a USB Y cable, splitting data lines and power lines. However, I would prefer a solution that is fully transparent for the user.
As there is no problem with the USB Y cable when the USB data lines are not isolated, I was wondering if it was possible to use a similar principle used in ADuM4160 ((1:1) transformer) to isolate only the power lines from USB, leaving the data lines untouched. I haven't found such a component so I'm not sure about this.
I also had other ideas to perform an isolation at another place (i.e. between the DAC and the output) but it does not look convenient at first sight.
Do you have any recommandation or any other solution to avoid ground loops in this kind of setup ?