LM3875 Basic Questions

DreadPirate · 23rd January 2011, 02:12 PM

What is the orientation of optional Ci (specified as 22uf) in the basic configuration recommended by national? Positive to ground?

http://www.national.com/ds/LM/LM3875.pdf

bob91343 · 23rd January 2011, 05:44 PM

Ideally, there is zero dc across the capacitor so it doesn't matter. In practice there will only be millivolts on it so put it in any way you like.

AndrewT · 23rd January 2011, 06:05 PM

the input offset current * the input Rin results in a voltage across the DC blocking cap and across the RF attenuation cap.
If the output offset is exactly zero then this same capacitor voltage appears across the DC blocking cap in the NFB loop.

It varies with the chip but can be anywhere between 20mV and 300mV.

It should be measured after building, to check that orientation is correct.

Kinnja · 23rd January 2011, 07:04 PM

If you are going to use an electrolytic here, then generally speaking, connect the side marked with the minus signs to ground. If you are using a film, or a bipolar electrolytic then it doesn't matter.

23rd January 2011, 02:12 PM	#1
DreadPirate diyAudio Member Join Date: Jan 2006 Location: Northern Virginia, USA	LM3875 Basic Questions What is the orientation of optional Ci (specified as 22uf) in the basic configuration recommended by national? Positive to ground? http://www.national.com/ds/LM/LM3875.pdf

23rd January 2011, 06:05 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	the input offset current * the input Rin results in a voltage across the DC blocking cap and across the RF attenuation cap. If the output offset is exactly zero then this same capacitor voltage appears across the DC blocking cap in the NFB loop. It varies with the chip but can be anywhere between 20mV and 300mV. It should be measured after building, to check that orientation is correct. __________________ regards Andrew T.

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23rd January 2011, 05:44 PM	#2
bob91343 diyAudio Member Join Date: Mar 2010	Ideally, there is zero dc across the capacitor so it doesn't matter. In practice there will only be millivolts on it so put it in any way you like.

23rd January 2011, 07:04 PM	#4
Kinnja diyAudio Member Join Date: Jan 2007	If you are going to use an electrolytic here, then generally speaking, connect the side marked with the minus signs to ground. If you are using a film, or a bipolar electrolytic then it doesn't matter.

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