Ground loop?

[FredFl]

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[village_idiot]

Probably not. If they all share a common power supply, there should be no voltage differential between the grounds.

[jeffs555]

To avoid a ground loop, you need to maintain constant control of the rudder during landing.

Seriously, if the ground on the GPS is a DC ground, you wouldn't need caps between the two grounds. If anything, you would want to connect the grounds with a heavy braid. Since the only signals running from the GPS will be RS-232 or TTL levels you shouldn't have too many problems.

[JohnMuchow]

A capacitor won't prevent a ground loop. It might shunt electrical noise to "ground" depending on how it's used though.

You need to physically manage the current flow to prevent unwanted ground current. Connecting the ground of the two boards at only one very robust, low impedance point helps prevent conducted noise from one board getting onto another. The noise will go into the pwer supply ground instead of into the other board. Depending on the circuit, this may or may not be a good idea. Some devices use multiple low impedance connections between boards.

For your setup, without a LOT more info, I think that a single connecting of the grounds where the power supply enters the circuit boards would probably be OK.

But...
Be aware that depending on the amount of current flowing through the grounds that there is most definitely a voltage gradient across each ground and a voltage difference between different grounds. The creatiion of the term "ground" was one of the biggest mistakes in the history of electronics. It makes it sound like every point on each ground is at the same voltage and that's just not possible. Each ground is only at earth ground right where it's connected to earth ground. Anywhere else, current flow causes the "ground's" voltage to rise in relation to the power supply's negative terminal where it's attached to the circuit board.

But at low currents, and that means without high-current flow due to logic circuitry switching or FET-drivers firing, you can often ignore the voltage gradient across the grounds.

Check out any application notes you can find from the manufacturers of the chips used in your circuit. See how they lay out the grounds and connect them together (if separated). A lot of manufacturers have app notes just on power and ground layout. Also check out the app notes for noise-sensitive devices like analog-to-digital converters and operational amplifiers. The layout of their power and ground traces/planes are critical and you can learn a lot from reading any recommendations the manufacturer might have.

John