GND (analog-gnd, digital-gnd, earth-gnd) MUST = 0V?

Do you think that, when we refer to GND (ground) point, the voltage at the point MUST be ZERO?

I thought what is more right on the definition of GND is that GND provide a way through which current goes back to the source. The ground also has the inductor and capacity.

Hi,nicleo

You see, GND is a reference voltage for others, i.e. we need (Vothers - GND), analogous to Op Amp's differencial input.

The GND is not 0V all the time, for example, caused by ground bounce. So GND design is very difficult for us.

Cheers,

Davy Zhu

voltage is a relative quantity. there is no such thing as absolute voltage. so there is no such thing as absolute 0V or absolute ground. maybe what u are calling 0V, is infact 100V with respect to another system.

u can take any terminal or node in a circuit and call it Ground. now that would be the reference terminal. so when u will measure the voltage at some other node u will be measuring the voltage between that node and the GND node.

so actually voltage is the difference of potential between two points.

You connect gnd pin from all your digital components to Digital GND, and ground from all your analog components to AGND.
AGND and DGND can be realized as two planes that are connected in one point, or one plane. In both cases they are at the same potential 0V.
In some applications for AGND can be named voltage where there is no input AC signal(PSoC microcontrollers).In this case potential of AGND and DGND does not have to be same. For example AGND can be at VCC/2 referred to its DGND.

I see... great answers from all of you.

So, the conclusion is that the statement "V(earth ground) = 0V" is WRONG, agree?

For voltage is only important difference between potentials of signal and ground, not absolute value of their potentials.
The answer is that you can take, also some other value for voltage reference, but it is used to be 0.
Then you measure all other voltages considering that references.

A true 0V is where would have a material where you would have equal numbers of protons and electrons. The earth ground may not necessarely have, at a specific point, for a specific moment, exactly this condition, but is very close.

When you see things like Analog GND, Digital GND and other GND, this is marely to distinguish between different GND path, which should be electrically treated independently. For example, in a digital circuit, you can see analog and digital GND. The digital circuitry will create a lot of noise, and the current flowing through the GND traces or plane will be very noisy. As a result, any chips connected to that GND line will pick-up the noise. The farther you get on the GND lines from the main entry capacitor, the greatted the noise sum will be. On digital chips, this usually doesn't cause too much problems, but if you also have analog circuits, for example an analog to digital chip, where you would sample voltage beween 0 and 4.095V, with 4096 different steps (12 bit ADC), then the noise on GND line will definitely affect the readings.

This is why you must use a GND signal that is clean. It is still 0V refference, but must be a different GND than the noisy digital GND. So, analog GND is a GND signal, that is independent from Digital GND, and is connected to the same point as Digital GND *only* at the input capactor, where the voltage is regulated. You mau also see Analog VCC (or AVCC), in a similar way as for GND. It is not rare to also see choke coils or little ferite beads to make sure there's no feedback from the digital circuitry.

When a circuit reffer to earth GND, it often reffer to the chassis GND (the metal box and supporting frame), where the chassis is also connected to the power outlet GND line. This is more common in power-supplies, and in home appliances.

Though, exceptions exists, and older circuits schematics sometimes mixed the GND symbols.

usually, I saw the gnd symbol in schematic represented by "triangle" and by "3 parallel horizontal line with diminishing length".

so which represent which (virtual gnd, analog gnd, real gnd)?
thx

It depend on your definition. You can define 3 GND with 3 netname, for instance, DGND, AGND and ChasisGND. So you could deal with three kind of GND with different method separately. The symbol is only a symbol. The key lies in the net name.

"triangle" maybe the GND on the Circuit．

"3 parallel horizontal line with diminishing length"　maybe the real earth.

Davy Zhu

Ground not necessery 0 V. It is relative.

For example; if you have a two rail power supply, +12V, 0V and -12V. The 0V point "normally" goes/attach to GND. It is 0V relative to the +12V, but it is actually +12V relative to the -12V point. That mean, if your gadget require +12V supply, you can connect to 0V, +12V or -12V, 0V (in order). In the later case, the -12V become the ground for your gadget and the 0V is the 12V. But under this situation, you can not mix the ground within same gadget.

In most circuit, to simplify the design, 0V goes to GND and the supply line goes to separate rail lines

Triangle is AGND, tree lines is DGND.

Earth ground is usually represented by an horizontal line, with 3 parallel diagonal lines unthe the horizontal lines.

Something like:

so we can conclude that : triangle is for Agnd, horizontal+3parallel diagonal line is for earth gnd, and three diminishing line is for Dgnd. am i rite?
thx

It depend, some schematic use mainly the triangle. This is the most standard. If a 2nd GND is needed, it usually is the diminishing lines. But then, they can be anything (AGND or DGND). It depend on the mood of the designer!