seperation of grounds

What is meant by seperation of grounds when we say seperate the grounds of control signal and power electronic circuit? But we still use the same ground even after using optocouplers. So, how are the grounds seperated. Would that be correct to say there is only one ground in a circuit?

There may be only one "real ground" but a plethora of "false
grounds" (or, not ideal). What these get up to in nonideal
or fault conditions, is the question.

Using a common ground is your choice. There can be large
voltage isolation. Or sometimes isolation capability is merely
spec'd so that somebody has the final say, for sure, over
where the current loops and zero reference lie.

Depending on the type of optical isolator used, it shoul be noted that optical isolator can operate on two seperate voltage source. In the case of optical isolated Triac, the input is controlled with a dc while the output is being powered by an ac. In this case the ground of the two supplies should be seperated because they are not the same as the dc can be an ordinary battery. Note that ground simply means return path.

Actually I was suppose to drive a MOSFET circuit. As I read from internet that there should be different grounds for microcontroller and MOSFET otherwise it will damage the microcontroller. From what I did I just come to know that the seperation of ground mean inserting an optocoupler so that no leakage current can flow to the microcontroller. But still I used the same ground for the MOSFET and optocoupler, then how the grounds are seperated. And if the grounds are not same then from what reference the controller will MOSFET will consider a high or low.

When I used different grounds(I mean literally i didn't make the ground of MOSFET and microcontroller same) then a high from microcontroller was around 8 volts and low was around 4 volts.

Also please explain your previous post in a simple manner I felt I couldn't understood it completely.
Thanx!

When a device is supposed to have a "separate ground" I believe it means that the ground is supposed to be "quiet". In other words, the ground voltage should not change very much. You might ask, "what would cause the ground voltage to change?" The answer, "current will cause the voltage to change".
So, if you have a microprocessor connected to ground (0V) and some other thing connected to the same ground that is dumping lots of current, then maybe your ground isn't really 0V like you would have hoped (V=IR). In this case, you need to make sure to hook things up in a way to minimize ground moving around. In other words, you need to "separate the grounds."

Hi 100k,

ground separation for sensitive analog circuits and power cricuits (with high current and fast transitions) is always recommended. Each ground line has an inductance and fast transitions causing a voltage drop. If this flows through the ground connection of other circuits(like analog) it sees a ground shift. The solution to this is a ground plane with short connections, or separeted ground line which are connected to the ground from the power supply (star connections).

Enjoy your design work!

Remember that optical isolator transfer electrical energy by the utilization of light wave, in this case there is no direct connection between the controlled circuit and the controller. If you are still confused let me konw.

Hi,

I have got 4 grounds in my layout. AGND,DGND and SERVO_GND, CHASS_GND. AGND,DGND,SERVO_GND are spread all over the board and I can't able to group the respective grounds together. I am using 8 layers. Could you please tell me if the following PCB stack up is ok for layout and there wont be any EMC issues.

--------------------TOP
--------------------DGND
--------------------SGINAL 1
--------------------POWER
--------------------AGND
--------------------SIGNAL 2
--------------------SERVO_GND
--------------------BOTTOM

Iam using ferrite bead to separate grounds. I can use the ferrite bead only in the top layer. how do i connect the ferrite bead with two different grounds on two different layers. (Layer 2 and layer 5). Using via from ferrite bead to the appropriate layer