I've done this several times to save on components and unnecessary battery drain by putting 100nF adjacent to the ADC input pin. Potential divider charges the 100nF in slow time, charge is transferred to the ADC sampling cap when the channel is selected, 100nF acts as a low impedance source to the ADC. Only works for slow-to-change analogues though (like battery volts). I've got a horrible feeling someone's going to say I shouldn't do it but it works for me :) Nigel Duckworth ----- Original Message ----- From: "Mike Hord" To: Sent: Thursday, May 27, 2004 2:28 PM Subject: [PIC:] ADC source impedence > > How does one calculate the source impedence for input to > an ADC? > > More particularly, given a resistor divider, how do you find > the Rs at the midpoint? I'm looking at using a 1M-2M > divider in a low power circuit to provide a quick and dirty > battery voltage estimate for a 9V battery driving a 3.3V > system. Obviously, this VASTLY exceeds the 2.5k max > the datasheet on the PIC18F2320 I'm using recommends, > but since I'm only using one channel of the ADC, I can > leave that connected all the time, allowing for > tremendous acquisition times. I don't need a lot of > accuracy, just a ballpark figure (say, tenths of a volt, > +/- 50 mV) to let the user know when to change the > battery. > > I REALLY don't want to add another opamp to this > circuit, and most of the discrete component voltage > followers that immediately spring to mind would consume > rather more current than I want to spend. > > Thanks! > Mike H. > > _________________________________________________________________ > FREE pop-up blocking with the new MSN Toolbar get it now! > http://toolbar.msn.click-url.com/go/onm00200415ave/direct/01/ > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics