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AD5161: Error in equation 4 / effect of wiper resistance, VW

I am wondering how the wiper resistance influences the output voltage of a
digital potentiometer in voltage divider. In the AD5161 datasheet, there is
equation (4) on page 16 where Vw depends on Rwiper. Is this equation correct
(except the Raw replaced by 256)?

 

I’ll say that the correct equation should looks like:
Vw(D)=  Va∙Rwb(D)/Rab + Vb∙Rwa(D)/Rab
Where Rwb(D) and Rwa(D) is the resistance between w and b/a terminals at any
specific code.

There is an article that explain how is the internal digiPOT architecture,
http://www.analog.com/library/analogDialogue/archives/45-08/digipot.html

The wiper resistance in a digiPOT is the resistance due to the internal
switches.
As there are always connected switches in the main signal path, the wiper
resistance effect should be add.
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