Why is there different ground in a circuit?

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marco antonio

Joined: 28 Jul 2004
Posts: 52

07 Jan 2008 20:46 Why is there different ground in a circuit?

When I observed some schematics of different circuits I noted that exist different grounds, could you explain the reason
rgds
marco antonio

nitin_ndg

Joined: 24 Sep 2007
Posts: 23
Helped: 1

07 Jan 2008 21:35 Re: Why is there different ground in a circuit?

Hi marco antonio
There can diffrent grounds in your circuit. its all depend on the verity of component's VCC supply.
Say for Example If you use a chip that works on 3.3v then circuit will have a groung reference of 3.3v as all the cureent will return to this referrence only.
like wise if design have ADC or DAC or OPAMP then there can more grounds say +5v digital supply ground, +/- 15v supply ground reference.

Hope its clear if not you are free to ask
Nitin

House_Cat

Joined: 21 Feb 2002
Posts: 1507
Helped: 307
Location: USA

07 Jan 2008 21:44 Re: Why is there different ground in a circuit?

In some circuits, the ground returns for analog signals are kept separate from the ground returns for digital signals to limit the possibility for coupling noise from the digital signals into the analog signals. Additionally, in some circuits the chassis, or enclosure, ground may be connected to the electrical system ground in your home or office for electrical safety reasons. Each of these different grounds may be represented by a different ground symbol.

rikie_rizza

Joined: 05 Aug 2006
Posts: 334
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Location: Bikini Bottom, between a rock and a pineapple

08 Jan 2008 3:54 Re: Why is there different ground in a circuit?

Quote:

its all depend on the verity of component's VCC supply.

Any VCC, 5 12 18 24V, they share the same ground. Mixed voltage system has a single ground. Why ground should be different is based on the category of the return signal. I always split 3 ground for my project, Power GND, AGND, and DGND.

Power ground only for power converter system ground such as regulator gnd, capacitor gnd etc.

AGND is an analog signal ground for sensor, amplifier, analog part of ADC, etc.

DGND is the diginal ground for digital signal like MCU, communication IC, memory, etc.

Some system with ADC can support splitting AGND and DGND totally because it's united inside the ADC device. Some system that doesn support this, ground should be joint in a single point (star)... usually at the GND input from the power source (eg GND part of the power terminal).

The gnd system can improved overall system performance, especially in mixed system (analog and digital).

avdrummerboy

Joined: 10 Nov 2006
Posts: 47
Helped: 1

08 Jan 2008 7:08 Why is there different ground in a circuit?

Ground basically means the negative supply line, so each different positive line in has to have somewhere to go, the negative, aka ground.

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marco antonio

Joined: 28 Jul 2004
Posts: 52

08 Jan 2008 9:23 Re: Why is there different ground in a circuit?

Very interesting, a basic notion but I think of it is not well understand for most of us so that is the reason because in the switched mode power supply we have a hot side with a diffrent ground. really?
I appreciated for your response that pointed the poit.
rgds
marco antonio

sandeep_torgal

Joined: 02 Jan 2008
Posts: 121
Helped: 7

08 Jan 2008 11:26 Re: Why is there different ground in a circuit?

Usually the power is star connected in a circuit. The kelvin connection starts from the VSS pad. If any noise is generated from different supplies will all be sinked in the pad reducing the coupling of this noise to other blocks in the branches. The VSS connection is strong near the pad is taken into account.

Let me know if there are any other views.

zyd912

Joined: 21 Nov 2007
Posts: 3

08 Jan 2008 11:38 Why is there different ground in a circuit?

I think rikie_rizza what he said is right.

cydi

Joined: 12 Dec 2006
Posts: 169
Helped: 11

09 Jan 2008 7:37 Why is there different ground in a circuit?

Different supply voltages generally hav different sources... so different grounds are there

Khabree

Joined: 03 Dec 2007
Posts: 130
Helped: 16

09 Jan 2008 7:50 Re: Why is there different ground in a circuit?

avdrummerboy wrote:

Ground basically means the negative supply line, so each different positive line in has to have somewhere to go, the negative, aka ground.

I don't think, this is right. A ground is positive for negative supply. For example, in a dual supply circuits, for negative rail, the ground is positive. And so for the all the circuit that uses VCC negative, ground will be positive.

avdrummerboy

Joined: 10 Nov 2006
Posts: 47
Helped: 1

09 Jan 2008 7:55 Why is there different ground in a circuit?

Okay, let me elaborate, "ground" is whatever is opposite of the "main" output voltage polarity of the supply. Meaning "ground" in reference to positive is the negative rail/ terminal/ etc. "Ground" for a negative supply is the positive rail/ terminal/ etc. "Ground" just being the opposite polarity of whatever went into the circuitry.

ehsanelahimirza

Joined: 24 Feb 2006
Posts: 430
Helped: 33

09 Jan 2008 8:09 Why is there different ground in a circuit?

some times we need complete isolation of electrical parts, some ICs provide isolation like optocoupler does and then if we use the same ground, there will be no isolation between our components which we desired.

so multi ground circuits are mainly used in electrically isolated circuit.

cyberrat

Joined: 19 Jun 2001
Posts: 966
Helped: 40
Location: In the sewers of the U.K.

09 Jan 2008 11:01 Why is there different ground in a circuit?

The Ground is the stuff on the floor & outside of my window!

As ehs.... says, I have just been doing boards that have several different common rails because certain sections needed to be floating.
With several you have to start getting clever on what to name them all but they are the same basic thing, a common rail.

Although you have to be very careful which parts of the circuit use which common so that they get a good return & supply feed.

Livingston

Joined: 26 Nov 2007
Posts: 121
Helped: 2
Location: Chennai-India

22 Jan 2008 14:22 Re: Why is there different ground in a circuit?

And all three types of ground(power gnd, Analog Gnd, Digital Gnd) is seperated from power ground by using 0E resistor.

sandeep_torgal

Joined: 02 Jan 2008
Posts: 121
Helped: 7

23 Jan 2008 12:06 Why is there different ground in a circuit?

Check this :
http://www.analog.com/en/content/0,2886,761%255F795%255F97529%255F0,00.html

subharpe

Joined: 09 Jan 2008
Posts: 244
Helped: 34
Location: Bangalore,India

24 Jan 2008 8:46 Re: Why is there different ground in a circuit?

[quote="marco antonio"]

Apart from the different grounds for digital and analog signals care need to be taken in isolating the ESD ground from the electrical ground. The base of the setup table is usually connected to the ESD ground in most cases and the electrical ground should be isolated from it using high resistance (typically less than 1Meg).
The resson is the static discharge may damage the device through the electrical ground pin return path.