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.
C1 is your first line of defense against differential noise coupled onto your wires, and your last line of defense against your device sending signals out into the world on those wires. Twisting the two wires tightly will help with differential noise as well.
If your buck supplies 4 A, it does not necessarily draw 4 A (as a linear regulator would!). If you have a specification on its efficiency, and you have good knowledge of its operating point, you might get away with less. But 4 A should certainly be enough, so that is a good 1st point. If you can't get one you like rated at 4 A, then sharpen your pencil (so to speak) and figure out exactly how much current will be needed.
Regarding the frequency for the common mode choke, the best option is to build it without one and measure the noise. Then select one that is best suited for where noise is the worst. If that is not feasible, then just try to get it below your circuit operating frequencies.
Do you have a separate chassis / earth ground, or is your negative coming in grounded?
Best Answer
Current flows in loops, always, and power ground is often the other half of that loop.
When a screened cable is used and the screen is just that (It is not part of the signal path) then the screen should be bonded to chassis right at the connector (Ideally with a 360 degree bond), the idea being to direct the current thru the chassis and thus away from the electronics. If you were to connect the screen via a long trace across the PCB then that trace could re radiate interference inside the enclosure.
There is one subtlety here, in that if your electronics has a limited common mode range (RS485 for example) you will want to connect the electronics board ground to chassis, but this must be done at ONE point so as to ensure that interference currents flowing in the chassis cannot flow in the electronics reference plane.
The very worst way to screw this up is to violate BOTH of the above, and connect the screen to the electronics 0V reference net which then connects to chassis down at the other side of the board, as this gets you voltage drops in the reference net due to the screen current as well as the radiation from the screen current.
It took the audio game far longer then it should have to figure this out, see AES-48 for some good explanation of the details.
Usually when you have a remote sensor the wiring will have a foil or wire screen surrounding the signal pairs, which is bonded to chassis at both ends and thus forms a loop with the power safety ground connections at each end, into which common mode currents are driven by either external fields or by momentary differences in potential due to earth faults or other sources of earth current. I was addressing what to do with this screen connection, but looking at it I think you may be looking more at the fact that the PCB should have a single connection between the electronics reference plane and the chassis ground so as to avoid circulating currents in the board.
In the case that your cables are unscreened or otherwise single ended then everything should terminate onto the same edge of the PCB with all the grounds bonded together at that point and with a single connection from there to the internal reference on the board, again the objective is to keep circulating noise current loops as local as possible.
Regards, Dan.