GC offset

janneman · 2nd July 2004, 08:49 PM

I, ehh, ahum, err, have these two LM3886's gathering dust, maybe I should built something usefull with them?
There's one aspect bothering me, the offset. If I leave out the series cap in the feedback network, also leave out a servo, what range of offset can I expect? The data sheet difference between 'typical' and 'max' is rather great. I would use a gain of 20.
Anybody has any experience in this respect?

Jan Didden

peranders · 3rd July 2004, 07:40 AM

The datasheet doesn't promise you anything really and reports tell us that you can get +- 200 mV and all values in between. So if you don't want surprises prepare for offset adjustment or DC-servo if you really want to be sure. If you are lucky you may not need any trimming.

Peter Daniel · 3rd July 2004, 07:56 AM

With a gain of 30, I never got more than 80mV and usually it's much lower: 30-40mV being an average. It will also depend on input impedance.
That is for LM3875.

janneman · 3rd July 2004, 08:57 AM

Peter,

The LM3886 datasheet specs 10mV worst case. That's 200mV @ 20 x gain. I could live (or rather, my speakers could) with the 30-40mV you quote. I think I'll put a placeholder for a cap on the PCB and see how it works out.

Jan Didden

peranders · 3rd July 2004, 09:10 AM

Don't forget the input bias currents. They contribute also.

2nd July 2004, 08:49 PM	#1
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	GC offset I, ehh, ahum, err, have these two LM3886's gathering dust, maybe I should built something usefull with them? There's one aspect bothering me, the offset. If I leave out the series cap in the feedback network, also leave out a servo, what range of offset can I expect? The data sheet difference between 'typical' and 'max' is rather great. I would use a gain of 20. Anybody has any experience in this respect? Jan Didden __________________ /New Linear Audio publication: Baxandall & Self on Audio Power!

3rd July 2004, 07:40 AM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	The datasheet doesn't promise you anything really and reports tell us that you can get +- 200 mV and all values in between. So if you don't want surprises prepare for offset adjustment or DC-servo if you really want to be sure. If you are lucky you may not need any trimming. __________________ /Per-Anders (my first name) or P-A as my friends call me

3rd July 2004, 07:56 AM	#3
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	With a gain of 30, I never got more than 80mV and usually it's much lower: 30-40mV being an average. It will also depend on input impedance. That is for LM3875. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

3rd July 2004, 08:57 AM	#4
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	Peter, The LM3886 datasheet specs 10mV worst case. That's 200mV @ 20 x gain. I could live (or rather, my speakers could) with the 30-40mV you quote. I think I'll put a placeholder for a cap on the PCB and see how it works out. Jan Didden __________________ /New Linear Audio publication: Baxandall & Self on Audio Power!

3rd July 2004, 09:10 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Don't forget the input bias currents. They contribute also. __________________ /Per-Anders (my first name) or P-A as my friends call me


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