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Old 24th May 2004, 12:26 PM   #1
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Default Chip amps - referencing input to negative rail?

I was looking at the internal cct of an LM3875 today and got to thinking about the fact that the positive supply rail has somewhat better supply rejection than the negative rail. A look at the cct shows that there is a whole pile of things referenced to the neg rail. Some go through current sources at least while others go through just a resistor. This being the case, it is no wonder that the neg rail has more of an influence on the internal goings-on.

Then I thought - how about an amplifier that has the input signal referenced to the neg rail instead of zero volts? Of course it would mean the output would have to be capacitively coupled to the load, but it would also mean that the internal circuitry would have the chance to operate in a more "pure" manner. The neg rail could jump around all it likes, but provided it is connected to the input signal earth it will not function as an unintended "input" to the chip. What do others think?
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Old 24th May 2004, 01:58 PM   #2
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Or maybe it would help to just do the decoupling "right", but
I suppose you are already familiar with the following app note
http://www.analog.com/UploadedFiles/...8865AN-202.pdf
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Old 26th May 2004, 11:08 AM   #3
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Quote:
Originally posted by Christer
[B]Or maybe it would help to just do the decoupling "right"
Ah, yes. Better get me a pair of 1000uF caps then.

Thanks for the link, Christer.
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Old 27th May 2004, 12:21 PM   #4
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Default Tab connected to negative rail?

Well, then. I'm going to go ahead with this project even if no-one else is excited about it. First question concerns the tab of a non-isolated LM3875 - the datasheet says that if the tab is connected directly to a heatsink (i.e. no insulation) then the heatsink should be connected to the negative rail. What I want to know is this - is the tab actually a direct connection to the negative rail pin of the package? It would take 5 seconds to test with a multimeter but all 3 of mine are the isolated type package. Could someone fill me in here?

If the tab *is* in fact a direct path to the negative supply connection on the die, then it would have to be the shortest, most direct, lowest inductance path possible. If the tab were bolted and /or soldered to a copper groundplane + heatsink, with a surface mount ceramic cap bypassing the positive rail down to the groundplane right where the lead comes out of the plastic case then we have the possibility of some seriously good earthing.

Everyone talks about short feedback paths for getting the best out of a gainclone. I even read somewhere about cutting away part of the plastic case to make the connection shorter still! Seeing the supply rail rejection on the negative rail is only fairly good, then perhaps there is just a little to be gained ( ) by using a similarly meticulous approach to earthing and bypassing.

I've decided to go for the bridge approach because I couldn't stomach the idea of an output coupling capacitor. I didn't want to be 'straining out gnats but gulping down camels'.
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Old 27th May 2004, 12:54 PM   #5
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Default Re: Tab connected to negative rail?

Quote:
Originally posted by Circlotron


I've decided to go for the bridge approach because I couldn't stomach the idea of an output coupling capacitor. I didn't want to be 'straining out gnats but gulping down camels'.
That sounds like a good idea. You ought to have some DC
protection circuitry with a relay in series with the speaker
though. It could be a bit hot for the speaker if one of the
amps fail or just decides to do a thermal shutdown or
something. Always a good idea to include anyway, I guess,
but it seems more necessary here.
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Old 29th May 2004, 11:12 AM   #6
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Default Re: Tab connected to negative rail?

Quote:
Originally posted by Circlotron
First question concerns the tab of a non-isolated LM3875 - the datasheet says that if the tab is connected directly to a heatsink (i.e. no insulation) then the heatsink should be connected to the negative rail. What I want to know is this - is the tab actually a direct connection to the negative rail pin of the package? It would take 5 seconds to test with a multimeter but all 3 of mine are the isolated type package. Could someone fill me in here?
Could they, please?
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Old 29th May 2004, 12:29 PM   #7
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Default Re: Tab connected to negative rail?

Quote:
Originally posted by Circlotron
If the tab *is* in fact a direct path to the negative supply connection on the die, then it would have to be the shortest, most direct, lowest inductance path possible. .
I seem to remenber that the datasheet notes that you shouldn't use the negative tab as a supply rail, might be worth checking...
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Old 30th May 2004, 11:17 AM   #8
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Yeah, I did look at the data sheet, but nowhere did it tell me what I really wanted to know, and that is - is the negative supply rail pin a direct connection to the tab???
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Old 30th May 2004, 11:19 AM   #9
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Quote:
Originally posted by Circlotron
Yeah, I did look at the data sheet, but nowhere did it tell me what I really wanted to know, and that is - is the negative supply rail pin a direct connection to the tab???

Might be worth dropping a line to National's tech support then, I might be confusing it with the OPA548/9s
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Old 31st May 2004, 01:11 PM   #10
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Aw, come on you blokes! Can't one of you just do a little measurement with their multimeter? Huh? Huh? Huh?
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