How to avoid a ground loop between pcbs

Question

There is an enclosure with 2 separate pcbs. Pcb A amplifies a mV level signal (10Hz to 100kHz) coming from an external sensor.

The amplified analog signal is sent to pcb B, which digitizes it and processes it, and contains lots of digital circuitry (fpga, microcontroller, ethernet PHYs, radio transceiver, etc).

Both Pcbs get power from a common DC input (star configuration).

There was the preliminary fear that the ground loop created by the analog output reference would allow return AC currents to flow from Pcb B back to the power input via Pcb A and that those currents would inject noise at the amplifier's input stage (the sensor is referenced to ground), even though this would be considered when laying out the ground plane in Pcb A, and pcb B has plenty of bulk & bypass capacitance.

For this reason the analog output was made (pseudo) differential and goes into a differential ADC in Pcb B. Common mode voltage is kept constant at mid-voltage. Because the system ground is the same, and the ground return current is theoretically minimal, the ground wire was not included, breaking the ground loop.

Now it is all built and it turns out that it works fine (very high SNR) with or without the ground wire, it makes no practical difference.

My question is what the consensus is of whether this technique of breaking the ground loop is sound or not, even though it made no difference in this particular case.

Answer 1

It seems the important point was making the analog signal between the boards differfential. If the boards are close enough to each other, which it seems they are, then a star ground as you have should be good.

The danger with a ground loop is that you don't know what path the power return currents of a noisy board will take. You want them going directly back to the power supply via the same connection that the power comes in, but they could flow back thru the other board. That could cause offsets in the other board, which could get onto the signal.

I think your non-looped ground and differential analog signal were both correct choices. Just because it may work with a ground loop doesn't make what you did a bad idea.