LM3886 Pin7(gnd) current? - diyAudio
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Old 17th July 2007, 02:10 AM   #1
Arx is offline Arx  Canada
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Default LM3886 Pin7(gnd) current?

What is the purpose of the pin 7 ground on a 3886? Is it strictly there for the muting circuit?

Any special PCB layout requirements for it. I'm guessing it doesn't dump much current.

-Nick
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Old 17th July 2007, 06:03 AM   #2
glennb is offline glennb  Australia
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My web page at http://home.pacific.net.au/~gnb/audi...0.html#gndpins has some words to say about the GND pin on the LM4780 (the dual LM3886). I recommend that it be connected to the power supply star earth, and not the input signal earth.
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Old 17th July 2007, 07:27 AM   #3
Dxvideo is offline Dxvideo  Turkey
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I use that PCB layout for 4 different project. They work very robust and noiseless. I recommend you to connect pin7 to signal gnd. Because It doesnt carry anykind of current. Its for op-amp reference point. I guess!
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Old 17th July 2007, 10:20 AM   #4
glennb is offline glennb  Australia
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Quote:
Originally posted by Dxvideo
I use that PCB layout for 4 different project. They work very robust and noiseless. I recommend you to connect pin7 to signal gnd. Because It doesnt carry anykind of current. Its for op-amp reference point. I guess!
On such a tight layout where the ground traces are so short, it wouldn't really matter where the GND pin is located on the trace.

The GND pin has to carry some type of current, otherwise it wouldn't be necessary! Its of the order of 1mA, which won't add any measurable distortion on traces which are a few MicroOhms long.
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Old 17th July 2007, 10:49 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi Dxv,
I like the location of CS1 & 2, with the ground pins very close together. A short and effective HF loop with minimal inductance.

Have you located both these caps on the reverse side?
Have you tried connecting a rail to rail cap across pins 4 to 5?

Have you snipped off the unused pins?
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Old 17th July 2007, 07:36 PM   #6
Mick_F is offline Mick_F  Germany
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Of course, the ground pin of the LM should be connected to signal ground, not to power ground.

Look at the reference board of the LM4780! Very useful

Mick
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Old 17th July 2007, 10:14 PM   #7
KSTR is offline KSTR  Germany
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From the datasheet and internal schematic of the LM3886 it can be seen that the GND pin has several functions.

1 - its voltage vs. the V- pin is monitored, keeping the chip in mute mode when it is less than 9V.

2 - it defines the output potential in mute/startup mode. Note the 10k on the left side of the inner diff amplifier. This diff provides a closed feedback loop in mute mode with the right hand 10k and the clamping diodes. These diodes go to ground, thus in normal operation there is a current path from the output to GND, via the 10k, about 0.1mA per Volt of output voltage.

3 - it's the reference potential for the mute logic. When current is drawn from the mute pin (towards V-) in the ON mode, the startup diff's current source gets turned off by the grounded base transistor on the left. The GND pin provides the base current for it.

4 - there must be something missing here, because from the single-supply schematic I realize that the GND pin shall only sink current in normal operation. OTOH, that schem might be wrong/flawed.


Especially point #2 indicates that the current into/out of the GND pin should not be routed to signal GND, in theory, as this nonlinear current might contaminate the signal GND. In fact it's so small (well below 10mA) that this is not a problem in a proper design/layout. Likewise, if this GND pin carries some noise, it won't affect operation -- but avoid excessive HF noise, of course.

So, my 2 cts:
If you have the option, don't bond it to signal GND. But don't sacrfice a good layout just for that tidbit.

Regards, Klaus
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Old 17th July 2007, 10:14 PM   #8
KSTR is offline KSTR  Germany
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Arrgh, I should have read Glenn's excellent comments on his site before posting...
At last, we come to the same conclusion...
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Old 18th July 2007, 06:49 AM   #9
glennb is offline glennb  Australia
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Quote:
Originally posted by Mick_F
Of course, the ground pin of the LM should be connected to signal ground, not to power ground.

Look at the reference board of the LM4780! Very useful

Mick
I agree with KSTR's comments. The schematic does not show everything. NS do not want to give away proprietary design information !

Just because the reference / evaluation board does something it a certain way, it doesn't mean that they have researched it to be the absolute best way ! It may have just suited them at the time.

It would be interesting to add a 10 Ohm resistor between the clean signal earth and the dirty power supply earth, and then measure the output THD etc when connecting the GND pin to either the clean or dirty earth. It would prove the point one way or the other and provide a magnitude of the effect. It may vindicate or eradicate my assumptions !
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Old 18th July 2007, 07:20 AM   #10
Mick_F is offline Mick_F  Germany
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Quote:
Originally posted by KSTR
....
Regards, Klaus [/B]

Nice to see you here Klaus. The Forum will benefit from your knowledge!

Quote:
Originally posted by glennb


It would be interesting to add a 10 Ohm resistor between the clean signal earth and the dirty power supply earth, and then measure the output THD etc when connecting the GND pin to either the clean or dirty earth. It would prove the point one way or the other and provide a magnitude of the effect.
That's a very good idea.

Mick
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