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Old 1st December 2005, 07:10 AM   #1
Wynand is offline Wynand  South Africa
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Default Multiple Grounds

Hi, I'm busy drawing a schematic for a 6 channel amplifier using the LM4780 chips. This will be controlled with a ATMEGA163 MCU and some PGA2310 chips.

The problem I've got is that on the 4780's eval board there is a resistor between power Gnd and signal Gnd.

Do I need to keep each amps signal ground Isolated?

I'll end up with Digital ground(for +5v), Ground for +-15v(PGA2310), Ground for the power amplifier V supply AND then Each chips signal ground.

This is 6 different grounds.
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Old 1st December 2005, 07:44 AM   #2
Nordic is offline Nordic  South Africa
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Hi Wynand, lekker dag vandag nč.
Sorry I don't have an answer, but would like to add a question to your list for when the smart people wake up... if you'd be so kind.

Anyway on basic inverted GC the signal ground and power ground are to be connected, could I use a 1 ohm resistor between these 2 points, I don't have any suitable thin wire left...
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Old 1st December 2005, 08:20 AM   #3
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Default Re: Multiple Grounds

Quote:
Originally posted by Wynand
[BDo I need to keep each amps signal ground Isolated?...This is 6 different grounds. [/B]
Yes, then bring them all back to a central star ground. And you also want to keep power grounds separate from signal grounds to as near as the star point as you can get away with, so you will end up with more.
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Old 1st December 2005, 08:59 AM   #4
Wynand is offline Wynand  South Africa
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Lekker dag ja!!

Ek tik maar engels vir almal...

Why dont you cut the leads off a resistor and use that in stead of a 1R resistor?.

PS Ek woon in Brackenfell, jy?
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Old 1st December 2005, 09:09 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
my basic grounding philosophy is simple.

Design your system with three separate grounds (for Wynand you need four).

Safety ground (chassis), Power ground (dirty), Digital power ground (spiky?), Signal ground (clean).

Keep these three/four grounds completely separate throughout the design and construction. Keep in mind that PCB decoupling is considered as dirty ground and so too are the Speaker return and Zobel/Thiel network at the output of Power Amps. The input connector ground is Signal ground and should be isolated from chassis.

Decision time;-
option A, connect all the grounds to a central star ground (CSG).
option B, connect Safety to network then to CSG. Connect P, S & D to CSG.
option C, connect Safety, D & P together, then network and then to CSG. S to CSG.
In B & C there is a network between Safety and S. This usually keeps hum and buzzing out of the output.

The Network can be any single or combination of the following:- resistor//capacitor//diode bridge//snubber//shorting switch. Closing the switch converts B & C to A. Opening the switch is the equivalent to lifting the ground (commonly referred to in commercial amps). You can experiment with other combinations but as Pinkmouse said keep the different grounds separate until they reach CSG.

The PSU common (on bipolar supplies) should connect 0Volts to smoothing capacitor common to transformer centre tap. Make a wired connection from 0V to CSG. Do not use the cap common as a CSG.

Really important safety rule. Always connect the Safety ground permanently to chassis. NEVER switch this connection and NEVER run the equipment from the mains with this connection temporarily disconnected.
The safest way to achieve this is welding the mains safety earth to chassis, next best is bolt this connection directly to chassis with it's own nut then add the other grounds/connecting wire above this first nut and use a second nut for experimenting with different grounding arrangements.
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Old 1st December 2005, 09:31 AM   #6
Nordic is offline Nordic  South Africa
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Bit of a ghetto project, with oll the odds and ends left over from other tests.... the wires I used so far is pretty thin, I like using a thinner wire between PGS and SGS than the rest of the wires, which with the dental floss thin stuff I used anyway, would mean I need a hair or something made of copper... PSU and 2 channels all on one recyled piece of veroboard... one benifit of being poor is, you learn to build things so you can reuse as mutch of it as possible... even veroboard...

thats why I thought a one ohm would do the job, to connect the PGS and SGS but still be more "resistive" than the rest of the connections from PGS, keeping the noise frome there, away from the signal ground, yet still allowing a grounding path for the signal ground.

Milnerton hier
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Old 1st December 2005, 09:49 AM   #7
Wynand is offline Wynand  South Africa
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On the LM4780 eval board there is a 2R7 resistor from the Power Ground to the SGS.
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Old 1st December 2005, 09:51 AM   #8
Wynand is offline Wynand  South Africa
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Thank you very much AndrewT. Thats an exelent explanation!!
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Old 1st December 2005, 11:40 AM   #9
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Quote:
Originally posted by Wynand
Thank you very much AndrewT. Thats an exelent explanation!!
And not the first time he's written about this > maybe time for a wiki page Andrew?
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Old 1st December 2005, 12:07 PM   #10
Wynand is offline Wynand  South Africa
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The problem with this type of question is that you get 7810 responses when doing a search for "ground" so finding it would be a problem...
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