Linear and Switched-Mode Supply Common Shared

Question

We have a mixed mode PCB system with both digital and analogue systems. Due to high speed they all share the same ground on the PCB, but use logical spatial separation to control noise. SO PCB input of {+5 V DIG, 0 V GND and +12 V ANA}.

We are box building a modular power supply, 12 V analogue from linear power supply (8 A total) and 5V digital from switched mode supply (5 A total).

The cabling to our units from the PSU is 3 wire, as multi-core is not an option in terms of $/m (we are operating in a remote, distributed setup).

My question:

The mains input has a GND green/yellow conductor to be connected to the rack mount chassis (Earth / Protective Ground / 0 V). Both the linear and switched mode PSUs have V+ and Common outputs. As I don't need stacked voltages or negative voltages, and that the supplies use the same ground on the PCB, I was planning on connecting both Commons together and out to the PCBs.

1. Is connecting common together a reasonable strategy when cable ohm/m is taken into account? Is switched-mode noise likely to negate the usefulness of the linear supply?

2. To ensure this shared common is 0 V, should I also connect the common output to Earth (Chassis Ground)

• Also, all supplies are re-regulated and decoupled on the PCB.

Answer 1

I was planning on connecting both Commons together and out to the PCBs

Not a good idea - high speed digital current pulses will cause small spiky volt drops end-to-end on the "common 0V" cable and this will look like the clean analogue supply has got small spikes superimposed on it relative to the 0V ground reference point on the PCB.

This may or may not upset your sensitive analogue circuits but is it a risk worth taking? Probably not.