Ground and the USB 8451-> What should it reference?

[Doughboy]

I am using a usb 8451 board attached to a PCB that has its own power supply with seperate analog and digital grounds. When I attach the 8451 to the ground at the power supply strange results occur. Is it ok to leave the grounds seperate and unconnected?

Message Edited by Doughboy on 07-14-2009 11:07 AM

[Buechsenschuetz · Accepted solution]

USB devices usually get their power supply (+5V and GND) from the device they are connected to. There is no need for additional GND connections. Making such extra connections may result in "ground loops" which may pick up noise and cause errors. - The "front end" of a properly designed USB device is isolated from the power supply. So if you connect anything to the inputs of the USB device, use the GND or reference terminal of the input, NOT and NEVER the GND terminal of the board which supplies the USB device. 

[AnalogKid2DigitalMan]

What strang results were occuring, more details here may assist in determining what may be going on.

-AK2DM

[baloojay]

I had a similar question about whether the USB ground from the PC was isolated from the signal ground attached to my device.  The application engineer said the ground from the USB was connected to the signal ground--no isolation.  This opens up the possibility of a ground loop.  Should I leave the ground connections on the I2C and I/O disconnected from my device which has it's own ground connection?

[Hassan_Atassi]

Hi baloojay,

I think that you would still have to have the grounds connected in order to make sure they are on the same reference.  For more information I would check out Figure 8 in the User Guide and Specifications

[baloojay]

Thanks, for the reply.  Using Figure 8 for discussion, my slave module has a ground connection of it's own (not shown in Figure 😎 back to the same ground as the USB ground.  If I then connect the 8451 ground to the slave, I've created a ground loop--which I don't want.

[Hassan_Atassi]

Hi baloojay,

Have you tried connecting it? Are you seeing any symptoms of a ground loop?  If so, can you describe them so we can get a clearer picture of what is happening?

[baloojay]

Sorry, I don't actually have an 8451 yet.  I'm placing the order this week.  The purpose of the investigation was to avoid issues in my test design.  I'll let you know how it all works out in the coming months when I get it all put together.