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GROUND for two different voltages

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Hello

I have always been told that if we are using a circuit with two different voltages (say 5V and 12V) we should as much as possible separate the grounds.

What happens if instead of two power sources we use only one (say 12V) and some circuit involving a regulator to generate the other (5V) (something like a DC-DC)

say:

Code:
12V--------------------------[78L05]-----------------------5V
            |      |             |          |             |
           ==      ==            |          ==          == (capacitors)
            |      |             |          |             |
GND---------------------------------------------------------------
Is it ok now to use just ONE GROUND connection for both 5V and 12V circuits???

Thanks in Advance

Kansai
 

MikeMl

Well-Known Member
Most Helpful Member
If all you have to work with is a THREE TERMINAL regulator, then you have no choice, there MUST be a common connection. I think you are asking a different question. That is, how do you route wiring or a PC board such that RIPPLE currents that flow in the INPUT side of the regulator do not cause an IR voltage drop across any of the traces that carry only the regulated OUTPUT from the regulator?
 
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Mikebits

Well-Known Member
For noise performance, it is common to separate analog and digital grounds. Such as Analog gnd for audio circuit, and digital gnd for digital A/D circuit comes to mind. At some point the two grounds are tied together.

See image.
figure37lg.gif

See this article. Analog Devices: Analog Dialogue: Ask The Applications Engineer - 12
 

Ubergeek63

Well-Known Member
it is not always necesary to separate the grounds. it is actually better to have a solid ground plane as long as you can route noisy stuff such that they are away from the sensative analog stuff.

in fact, in rare cases it can be detrimental.... i had a couple VERY rare cases...

1. a static hit into a membrane keyboard static shield charged a FET gate turning on the system

2. EMI emissions test failure at 22MHz: display board running at 12MHz and power board at 11MHz... the structure was acting as a resonant cavity causing the entire dispay board to act as an antenna even after converted to 4 layer and grounded to the frame!

on the other hand, it is ALWAYS adventagious to isolate the power rail...

dan
 
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