What does the analog ground mean?

[Guest]

Can anybody explain me why we should split the digital ground from analog ground? Is it necessary to sperate the analog voltage from digital one? and every analog ic such as OPAMP should be considered as a analog part, for example a voltage follower?
I used an OPAMP to divided the voltage, I supplied the opamp's voltage by digital VCC and the GND of OPAMP was connected to digital's GND. Now, because I am not clear about the reason of grounding (I know the type of grounding like star ,etc.) so I am not sure that I should consider it as an analog part or not (becuase I think we should sperate the analog GND from ANLOG GND when the analog part is amplifying a signal, but here I am dividing the 3.3 voltage to 1.65 v by an opamp )?
Is it right or not?

[IgorStojanovic]

In analog, each voltage level is represents information. So, if you change it a little (i.e. by neighboring tracks radiation) you also disturb the information. Therefore, it is necessary to protect and keep the analog part as clean as possible. In digital, there are only two voltage levels for representing information (high voltage and low voltage) with relatively wide ranges that can be recognized as low and high (depends on supply voltage). Thus, digital signals are much immune to disturbances. However, digital signals are power source of radiation because of steep rising and falling edges. This can make substantial analog signal corruption if the digital lines are close to the analog. The second source of disturbance is non proper GND design. Many people imagine GND as something neutral that does not conduct the current. GND is return path for the current in signal/power line. So exact the same current flows in signal/power line and GND. If the GND path is large, it resistance would be high so the return current will cause voltage drop on GND. As in digital circuits currents changes rapidly (following the changes of digital signals), the GND will fluctuate. If the analog circuitry is connected to that GND, the output signal will be disturbed. That is why there is a general rule to separate analog and digital GND and connect it to the star point. I think (but this is only my opinion) it is important to make the GND return paths as short as possible and practical and then try to have all analog GNDs isolated and connected to single point (however there are many advice on this topic). If you use decoupling capacitors properly, ensure the recommended minimum clearance between the critical tracks and keep the GND return paths short (of course keep the other design rules from the datasheet), you won't have problems. I hope I somehow helped you.
Hope, Robert will also give you the answer.

[Guest]

In analog, each voltage level is represents information. So, if you change it a little (i.e. by neighboring tracks radiation) you also disturb the information. Therefore, it is necessary to protect and keep the analog part as clean as possible. In digital, there are only two voltage levels for representing information (high voltage and low voltage) with relatively wide ranges that can be recognized as low and high (depends on supply voltage). Thus, digital signals are much immune to disturbances. However, digital signals are power source of radiation because of steep rising and falling edges. This can make substantial analog signal corruption if the digital lines are close to the analog. The second source of disturbance is non proper GND design. Many people imagine GND as something neutral that does not conduct the current. GND is return path for the current in signal/power line. So exact the same current flows in signal/power line and GND. If the GND path is large, it resistance would be high so the return current will cause voltage drop on GND. As in digital circuits currents changes rapidly (following the changes of digital signals), the GND will fluctuate. If the analog circuitry is connected to that GND, the output signal will be disturbed. That is why there is a general rule to separate analog and digital GND and connect it to the star point. I think (but this is only my opinion) it is more important to make the GND return paths as short as possible and practical then try to have all analog GNDs isolated and connected to single point. If you use decoupling capacitors properly, ensure the recommended minimum clearance between the critical tracks and keep the GND return paths short (of course keep the other design rules from the datasheet), you won't have problems. I hope I somehow helped you.
Hope, Robert will also give you the answer.

Thank you for your response.
I agree with your explanations, but there are 2 questions.
I used a single rail opamp which is supplied by 3.3V. 3.3V is also connected to VCC of microcontroller. I used this opamp as a voltage divider. so the output is 1.65V. Now the questions are:
Should I consider it as a analog component while it does not deal with small signal. the input is 3.3v and out put is 1.65v?
I connected the Vcc to 3.3v which is the same with microcontroller. if it is considered as an anlaog part should I use the same VCC for both analog and digital?

[IgorStojanovic]

If you want formal classification, it is the analog part because you use it in a circuit for voltage division. Any disturbance from neighbor will reflect to the output voltage fluctuation. Now it is the question how much stable voltage you need at the output of the opamp. If you want to be sure, you can use separated VCC track for the opamp that you will connect to the star point. My personal advice is (lets Robert/others say what is the best): use wide power supply track (if you don't have separate power layer) with branches to MCU and opamp, use proper ferrite bead and decoupling capacitors in opamp power rail (also for MCU supply). In case of MCU, ferrite bead won't only filter incoming voltage, it will also attenuate voltage fluctuations in reverse direction (from MCU to the rest of power rail). Of course, it should be properly selected. I suppose you use high resistance resistors for dividing the voltage to minimize the leakage current, so you use the opamp as voltage follower. If it is the case, take care of the fact that high resistance resistors (> 1MEG) are prone to the noise. You can also shield the resistor network with proper polygon (like it is often used for the crystals) attached to GND.
Again, I hope some other will give additional answer.

[robertferanec]

, is it possible to post screenshot of the analogue part here on forum? So we have a better idea.

PS: I am not expert for Analog, but I had discussion about it in different threat, have a look here: http://www.fedevel.com/designhelp/fo...multilayer-pcb

[Guest]

here is what i am saying.
one more question: when and where we must not use the ground plane under the components? for example I know we should not put ground under the mechanical relay. what about transformer, transducers, usb port etc???

[robertferanec]

I have used similar circuits before. I kept the analogue parts close together and only used one common ground - no problems.

PS: For example I do not use GND planes if there is possibility, that a high voltage can jump to my plane and damage the board. Also, it is very important to watch what isolation gap is required for certain voltages ... e.g. for higher voltages you can not have internal planes, because PCB is very thin and if isolation requires 5mm distance, anything on 1.6mm thick PCB would be closer (of course depends on materials and other factors - this is just to give you an idea).

You can check Saturn PCB to give you some ideas about Conductor spacing