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ADP7182 Noise reduction question in datasheet

Question asked by mex on Jun 26, 2018
Latest reply on Jul 4, 2018 by AIlustrisimo

Hello all,

 

first time poster here, so I am not sure if this ends in the right forum:

 

I am working with some poer regulators: LDO type, positive and negative:

 

ADP7118 and ADP7182 in SOT23-5 Package, just for some information. Adjustable versions

 

Now I am having trouble to follow the datasheets in regard of the noise reduction techniques.

page 16 ( on Rev D datasheet)

noise = 11uV * (Rparallel + R2)/R2;   this one is kind of understandable ( its for the positive LDO)

 

now regarding the negative LDO, suddenly there appear to be 2 formulas, one is just describes in textform? what is that about, the other one has the same reading but suddenly is slightly different and in the text it says:

 

The noise of the LDO is approximately the noise of the fixed output
LDO (typically 18 µV rms) times RFB2, divided by the parallel
combination of RNR and RFB1

 

The noise of the LDO is approximately the noise of the fixed output
LDO (typically 18 µV rms) times the high frequency ac gain

 

 

now is that supposed to be the same, those two sentences ( simply someone forgot to dlete one after reviweing it

 

those two statement dont make any sense in combination, either the first one is adifferent noise, for example fixed output vs adjustable output, but two noises for one and the same thing, cannot exist.

 

So I would appreciate any further explanation, since i cannto explain this any more

 

Thanks a lot for reading,

cheers

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