Thanks Neil, I think I get the point. /Ake -----Ursprungligt meddelande----- Fr=E5n: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]F=F6r PicDude Skickat: den 18 april 2004 21:11 Till: PICLIST@MITVMA.MIT.EDU =C4mne: Re: SV: [EE:]Light detector On Sunday 18 April 2004 01:45 pm, Ake Hedman scribbled: > Doing mostly software work for the last decade... Why does the ADC input > see the two resistances in the divider in parallel as its input > impedance? Why isn't it just the LDR that set this? Sorry if this is a > very dumb question... Same here (software for the last 12+ years) but from the remnants of knowledge from an old EE degree, an ideal voltage source has 0 resistance. So from an impedance standpoint, the positive voltage rail is 0 ohms away from the negative voltage rail (ground). In reality it's quite low, and can be considered to be zero for these purposes. This is good for you, since the parallel combination brings down the impedance seen by the PIC. > I plan to use the 12F675 and you are right there is a 10K recommended > impedance of the analog voltage source. I', glad I asked the question. > Nothing is ever as simple as it first look is it! No kidding! Cheers, -Neil. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.