Grounds in electronics?

Question

When a schematic shows multiple grounds, do the grounds need to be electrically isolated from each other?

Also, I've seen implementations where the ground is really just wire connected to the chassis. Why does this work when leaving the wire unconnected doesn't work? Does it have something to do with capacitance?

Thanks to those who have answered so far, but you seem to have missed the point of the second part of my question:

The only difference between connecting the chassis to the circuit, and leaving the circuit unconnected, is that when the circuit is connected, there's an extra chunk of metal at the connection point. Instead of just a solder point, there's a wire, with a chunk of metal, the chassis, at the end of that wire. Why should it matter that there's an extra chunk of metal there? What is the physical mechanism that makes the difference? Is it a capacitive effect? Inductive, maybe? What is the minimum necessary size?

I mean, the circuit won't work if you just connect the ground points together without any sort of ground, will it? What if only one ground is necessary? It won't work to just leave the circuit disconnected from a ground (if the schematic calls for it), right? Then how massive a hunk of metal does the ground need to be?

Answer 1

Multiple ground symbols may or may not be connected together, depending on the application.

Where I work, there are a couple of ways to do things. We have 'analog' grounds and 'digital' grounds that are isolated on separate 'planes' of copper inside multi-layer printed circuit boards. The 2 separate grounds are connected together underneath any IC that has both analog and digital circuitry (analog-to-digital converters, and digital-to-analog converters). We also keep a separate 'chassis' ground that is isolated from the other 2 grounds. keeping the grounds separate (except at a few strategic points) isolates all the switching noise in the digital circuitry from the analog circuitry -- which is generally low noise to begin with.

Some customers want all 3 grounds to be electrically the same potential (at DC), so we have connection points that allow all 3 ground to be connected together. Other customers require isolation, so we place capacitors in those connection points (they act as noise filters in those cases).

It's mainly about signal integrity, and keeping the circuit from radiating EMI. As long as every signal has a well defined return path (one that follows the signal trace line on a single ground plane back to the source), radiated and conducted EMI is minimized.

Automobiles are good examples of a single ground system, where the chassis *is* the return path for any current flowing (to light bulbs, window motors, horns, radio, etc.). The battery 'minus' cable is connected to a point on the motor block, which is also part of the chassis. Leaving a chassis wire unconnected will stop the current from flowing through the device (light bulb, radio, etc.).

.

Answer 2

No, the point is that all the grounds ARE connected together via the chassis. Showing multiple grounds can simplify a schematic enormously. It is telling you "This connection goes to the common earth line or chassis" without cluttering up the diagram with lines connecting all those points together .

The chassis is in fact completing a circuit.

Answer 3

what you call grounds is usually just one end of your power supply and you need to connect components of your schematic to it in order to close the circuit.

you do not need to connect the chassis to ground in order to make your circuit works, unless the end of your power supply is actually connected to the chassis and from there to your circuit.

Normally the chassis is connected to ground for two reasons:

safety-- if something goes wrong in your circuit and a high voltage is connected to the chassis (which an user can touch)
then the chassis makes a short circuit (fuse should blow) protecting the user.

electric noise--the chassis behaves as an antenna "grounding" all electric noise of your enviroment and it does not allow electric noise to get your circuit.

Answer 4

Grounds in electronics is a big problem
Generally speaking ground is reference zero for all voltages
and also for protect our devices
When we have analogue and digital circuits we have
to separate their respectives grounds
Sometimes we have to separate by a shunt simply the
mechanical and electrical grounds
As already told in cars or planes the chassis is sufficient
for playing that function

Answer 5

"grounds connected to chassis?"
ground is also usually the negative supply