USB to UUT isolation

[NivagSwerdna]

I'm embarking on a gadget that on one side is connected to a PC via a FTDI chip etc and on the other side is connected to a UUT which is a vintage microprocessor board with its own power supply.

One side of the circuit is 3V3 whilst the other is 5V derived from the UUT.  I'm planning some kind of TI voltage level translation between the two.

But that got me to thinking... can I really connect the GND together between the 3V3 and the 5V side?  The 3V3 side is relative to PC ground; the 5V side is relative to UUT ground?

(This is for my poor man's Fluke 9010a project)

Thanks in advance

[NivagSwerdna]

Looking at the original Fluke it appears that the Power Supply is derived from mains and that the different power signals are derived from various secondaries on the transformer so it is not ground referenced.

I guess my scenario applies to all test gear that has a PC on one side but a UUT on the other

[Ian.M]

For many applications it isn't a problem to have a common ground shared with the PC, but a common ground is undesirable for test equipment due to the risk of ground loops.   You can easily optoisolate the FTDI chip on its I/O pin side, with an isolated DC-DC converter fed by USB Vbus to power the isolated section.

For logic level conversion look at 74LVC1T45 or 2T45 buffers.  They support partial power down operation so if you power the DUT side from the DUT, they'll tristate all their outputs if either the DUT is powered off or the USB interface is powered down or disconnected.    Powering from the DUT also auto-sets the logic level.   Add 50mA polyfuses for robustness. 

[NivagSwerdna]

Thanks.  I was just looking at Murata CRE1 as a 5V:5V isolated supply, looks feasible.

For the USB data it is differential... do I really need to opto isolate it?

[Ian.M]

USB isn't pure differential - it uses single ended signalling for framing - and has poor tolerance for ground offsets.

*DONT* isolate the USB side of the FTDI chip - that's a royal PITA to do properly, and even if you use a dedicated USB isolator chipset it makes it impossible to support USB speed negotiation or to support USB speeds faster than 12 Mbit/s.  Its far easier to isolate the FTDI chip's serial or parallel I/O side.