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Old 13th April 2005, 10:13 AM   #1
Nuuk is offline Nuuk  United Kingdom
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Default 3886 DC offset (again)

I have completed my second 3886 stereo GC.

On the first, I had 19mV offset on each channel. On the new one, I have 40mV on one channel but 1.2 volts on the second. All four channels are built with identical circuits, ie inverted circuit, 270K feedback, 12K input, wire from NI to ground, Carlos FM snubbered PSU.

I have read all the results from a search on this topic but couldn't find anybody who had got such a high DC offset. Is this down to the chip? I know there is quite a variation but 1.2 volts seems excessive.

I have double checked everything else. The rails are fine. All connections are good.

I tried a 270K resistor from NI to ground but the offset went up to 4.6 volts!
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Old 13th April 2005, 10:16 AM   #2
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If the circuits are truely identical, then it must be the chip. Swap it out and see what happens.
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Old 13th April 2005, 10:22 AM   #3
Nuuk is offline Nuuk  United Kingdom
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Quote:
Swap it out and see what happens.
But Al - I knew somebody was going to say that and all my lovely work will have to be redone!

Oh well, may as well get on with it then! Solder sucker where are you........
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Old 13th April 2005, 10:50 AM   #4
shusha is offline shusha  Croatia
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What gain do you have without resistor from NI input to the ground?

Why not try some of those calculators: http://www.bcae1.com/opamp.htm

I find them very usefull. Can't memorise all of the equations (not using them on a daily basis).
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Old 13th April 2005, 11:05 AM   #5
Nuuk is offline Nuuk  United Kingdom
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Swap out completed but I am leaning ever so slightly toward PCB instead of P2P! (Is there an inverted 3886 PCB out there yet?)

So with the new chip, I get 37 mV. A pity that I can't remember where I bought that other 3886! On with the testing.......
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Old 13th April 2005, 11:09 AM   #6
shusha is offline shusha  Croatia
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Sorry, just saw it is inverted circuit.
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Old 13th April 2005, 11:10 AM   #7
Nuuk is offline Nuuk  United Kingdom
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Quote:
What gain do you have without resistor from NI input to the ground?
The gain is set by the feedback resistor and input resitor as I understand it. This is an inverted circuit!
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Old 13th April 2005, 11:12 AM   #8
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Sorry - just saw your second post!
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Old 13th April 2005, 12:02 PM   #9
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The offset can be caluulated rather easily and if you have rathe low-ohmish resistor values only the voltage offset will have inlfuence. In your case it's 1-10 mV times the DC gain => max 300 mV!

... and if the resistor values _not_ are low-ohmish _and_ you don't have matched resistor values for each input, then you will get additional offset. Why? Current flows into the inputs and they are not small in the context _and_ they are not equal, see the datasheet for input bias offset current.

Guess why I have only 18 kohms as feedback as compared to 270 kohms?
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Old 13th April 2005, 12:13 PM   #10
sek is offline sek  Germany
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Quote:
Originally posted by Nuuk
Swap out completed but I am leaning ever so slightly toward PCB instead of P2P! (Is there an inverted 3886 PCB out there yet?)
Yep.

But only DIY, I didn't want to spend the money for a 'professional' order.
It's a single-sided design on about 80mm x 32mm.

I guess you were talking about a board you can just buy and start soldering.
But anyway, just tell me if you are interested in pictures and details.

Sebastian.

PS: Forgot a little detail: it uses SMT resistors and caps for the low power components...
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