According to the FAE, the spec did change (because of a new =20 manufacturing process), so I'm guessing microchip hasn't gotten around =20 to correcting the datasheets after using an older part datasheet as a =20 starting point. Seems like 3k is the correct value. However, assuming that your voltage source is close to ideal (very low =20 impedance), then the impedance seen by the A/D should be the parallel =20 combination of the voltage-divider resistors. The cap helps, and =20 increasing the sampling-cap-charge time also helps. And of course, =20 the 3K is to achieve a specific (really quick) acquisition time, which =20 I'd guess most apps won't need. At least at this point, I'm not =20 worried about it for my app. I'm not clear on why the sampling caps are discharged after each =20 measurement though. There's an explanation in section 17.2.3 in the =20 datasheet, but I'm not clear on its meaning. I'd think starting from =20 the last value would allow for faster sampling, especially for sources =20 that change slowly. Cheers, -Neil. Quoting Forrest Christian : > ICK. > > I have an 18f26K22 onboard on a new circuit. Fortunately most of the > sources are low-impedance, but I have one which is a voltage divider. > Final rev of the board is going out in the next day or so I'm glad you > mentioned this.. > > What is darn frustrating is if you look at page 302 in the datasheet ( > http://ww1.microchip.com/downloads/en/DeviceDoc/41412D.pdf ), it says > 'The maximum recommended impedance for analog > sources is 10K'. And I know very well I went through this page during > engineering. The 10K figure is even repeated in the footnote on this > page. > > Since this basically measuring the voltage source to the board, and > shouldn't be changing (that fast), I'm just going to add a .1uF ceramic > cap at the input pin. Assuming the values are close in figure 17-5, a > .1uF cap should have plenty of 'storage' to be able to provide an > effective low-impedance source to the PIC. I would be a bit more picky > if this was a critical measurement instead of a nicety... But > considering it needs the low impedance source only a relatively small > percentage of the time, it should be ok... > > -forrest > > On 9/2/2011 9:44 PM, PICdude wrote: >> Heard back from FAE. Seems like there is a process change on the >> 18F's, so the spec really has changed. Sucks. But that allows for >> quite fast acquisition times, which I don't need, so hopefully the >> longer charge-time I'll use will still be fast enough to get the >> sample rate I need. I need to read the specs in detail and run some >> tests. >> >> >> >> Quoting PICdude: >> >>> Some time ago I started porting to an enhanced PIC 16F1936 to replace >>> 16F883's I've been using, so I could get a faster internal osc and >>> touch-switch capability. But there are some annoyances (bugs which >>> have unpleasant workarounds in the errata). But now there's a >>> 18F23K22 that I'm thinking I'll use instead. Same package& pinout >>> (so boards don't change), no banking, faster (in case I need it), and >>> no Vcap, so I'll get that pin back. >>> >>> However, I notice that the Zain (min recommended analog source >>> impedance) on the 18F is 3k, vs. 10k on the 16F. And the 18F does not >>> state that higher is okay with a 0.01uf cap as with the 16F. Why is >>> this? Did something change, or did they re-think the specs on the >>> same module, or...? I've been using sources with over 20k impedance >>> with no problems, but with a much longer sample-cap charge time. Is >>> there some reason I won't be able to do this with the 18F part? >>> >>> Cheers, >>> -Neil >>> >>> -- >>> http://www.piclist.com PIC/SX FAQ& list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist >>> >> >> > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .