Common ground for 2 eval boards with different powering

[raff5184]

Hi,
I have two eval boards. One for an FPGA and the other for an ADC. The two devices communicate via SPI. The FPGA board is powered from a PC to which it is connected via USB; the ADC board is powered with a DC voltage supply.

1. I think that the two different grounds might create a problem since the SPI signals are not referenced to the same GND.  How should I connect the two boards and reference them to the same ground?

2. The FPGA has an analog GND and a digital GND, and I saw that they are at the same potential. So I deduced that they are connected together. So is grounding either the AGND or the DGND the same?

Thanks

[sokoloff]

I’d power the ADC board from a floating battery-based supply.

[raff5184]

Thanks, why is it better?

[sokoloff]

It requires the least amount of thought, testing, and precaution to provide a ground-potential-free system.

Without looking at the boards or knowing anything about them, you know that an isolated supply will float to whatever the PC/USB ground is. You can then safely tie the grounds together and not have two different earth-references that might be pulling their respective ground to a different reference. (Now, in all likelihood, you'd be fine to just naively hook up any two old supplies and tie the grounds together, but by using a battery, you take one variable out of the equation entirely.)