I need to connect +/-5V and +/-12V supplies to a PCB and want to know the best way to design the connecting cable. Should I tie the GNDs together at the supplies and then bring a single ground wire to the PCB? I'm thinking about ground loops. The power cables also need to be shielded so should I just connect one end to ground at the power supply?
Electronic – how to connect multiple voltage supplies to a PCB
groundvoltage
Related Solutions
This is a very complex issue, since it deals with EMI/RFI, ESD, and safety stuff. As you've noticed, there are many ways do handle chassis and digital grounds-- everybody has an opinion and everybody thinks that the other people are wrong. Just so you know, they are all wrong and I'm right. Honest! :)
I've done it several ways, but the way that seems to work best for me is the same way that PC motherboards do it. Every mounting hole on the PCB connects signal gnd (a.k.a. digital ground) directly to the metal chassis through a screw and metal stand-off.
For connectors with a shield, that shield is connected to the metal chassis through as short of a connection as possible. Ideally the connector shield would be touching the chassis, otherwise there would be a mounting screw on the PCB as close to the connector as possible. The idea here is that any noise or static discharge would stay on the shield/chassis and never make it inside the box or onto the PCB. Sometimes that's not possible, so if it does make it to the PCB you want to get it off of the PCB as quickly as possible.
Let me make this clear: For a PCB with connectors, signal GND is connected to the metal case using mounting holes. Chassis GND is connected to the metal case using mounting holes. Chassis GND and Signal GND are NOT connected together on the PCB, but instead use the metal case for that connection.
The metal chassis is then eventually connected to the GND pin on the 3-prong AC power connector, NOT the neutral pin. There are more safety issues when we're talking about 2-prong AC power connectors-- and you'll have to look those up as I'm not as well versed in those regulations/laws.
Tie them together at a single point with a 0 Ohm resistor near the power supply
Don't do that. Doing this would assure that any noise on the cable has to travel THROUGH your circuit to get to GND. This could disrupt your circuit. The reason for the 0-Ohm resistor is because this doesn't always work and having the resistor there gives you an easy way to remove the connection or replace the resistor with a cap.
Tie them together with a single 0.01uF/2kV capacitor at near the power supply
Don't do that. This is a variation of the 0-ohm resistor thing. Same idea, but the thought is that the cap will allow AC signals to pass but not DC. Seems silly to me, as you want DC (or at least 60 Hz) signals to pass so that the circuit breaker will pop if there was a bad failure.
Tie them together with a 1M resistor and a 0.1uF capacitor in parallel
Don't do that. The problem with the previous "solution" is that the chassis is now floating, relative to GND, and could collect a charge enough to cause minor issues. The 1M ohm resistor is supposed to prevent that. Otherwise this is identical to the previous solution.
Short them together with a 0 Ohm resistor and a 0.1uF capacitor in parallel
Don't do that. If there is a 0 Ohm resistor, why bother with the cap? This is just a variation on the others, but with more things on the PCB to allow you to change things up until it works.
Tie them together with multiple 0.01uF capacitors in parallel near the I/O
Closer. Near the I/O is better than near the power connector, as noise wouldn't travel through the circuit. Multiple caps are used to reduce the impedance and to connect things where it counts. But this is not as good as what I do.
Short them together directly via the mounting holes on the PCB
As mentioned, I like this approach. Very low impedance, everywhere.
Tie them together with capacitors between digital GND and the mounting holes
Not as good as just shorting them together, since the impedance is higher and you're blocking DC.
Tie them together via multiple low inductance connections near the I/O connectors
Variations on the same thing. Might as well call the "multiple low inductance connections" things like "ground planes" and "mounting holes"
Leave them totally isolated (not connected together anywhere)
This is basically what is done when you don't have a metal chassis (like, an all plastic enclosure). This gets tricky and requires careful circuit design and PCB layout to do right, and still pass all EMI regulatory testing. It can be done, but as I said, it's tricky.
No, I would not tie the grounds together for such a distance. That does make the data connection more tricky though. In your case, the two grounds shouldn't be all that different since presumably they are tied together back at the breaker box in your house. In that case you could use differential signalling, knowing that the common mode offset should stay bounded to a few volts at most.
However, I think real isolation of the data signal is a better idea. If you can use ethernet, then you don't have to do anything further since that is already isolated. Otherwise, it's probably easier to use a opto-coupler at the receiver. The transmitter would send pulses of a few mA, which is enough to light the LED in the opto at the other end.
One simple scheme is to use UARTS, with the line idle level being no current. Cheap optos can be surprisingly slow, so either get fast optos or run the UARTs at a slow baud rate. Even a really slow opto can support 9600 baud well enough, and you can go much faster with the right choice of opto. If you need high data rate, then ethernet starts to sound good.
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Best Answer
By "tie the GNDs together" I assume you mean tying the DC Common terminals of the power supplies together. So, yes in generalit is better to tie the grounds together as close to the power supplies as you can manage.
There are two reasons for doing this. First, you won't be relying on the PCB connections to connect the supplies. So, for example, if one of the ground connections fails at the PCB, the supply outputs will not float with respect to each other, potentially causing a problem in the connected circuitry.
Second, you may introduce voltage offsets between the two supply voltage pairs due to IR drops in separate connection geometries. This could also cause problems or accuracy issues in the connected circuitry.
Having said this, it is also possible under certain circumstances that you would want to tie the grounds together on the PCB for a specific reason of performance, accuracy, IR wire drop isolation, and similar. However, in this case you would still be susceptible to the first issue I described above.
You would tie only the one end of the power supply cable shield to the shared DC common connection at the power supplies to avoid a ground loop issue. If earth ground is available you might want to connect the shield to it instead of DC Common. It depends on the purpose of the shield. If, for instance, you run into EMI problems, you may want to try the earth ground connection to see if it helps the EMI situation. If you need to connect earth ground to some aspect of your internal circuitry (e.g. a shield around a component or circuit section), you may be forced to connect to both ends of the shield and live with the potential of an AC-coupled ground loop of some sort.