Hi, I had seen that recommendation in datasheet, but did not check what an input impedance of a ADC pin would be. Probably a buffer would help. On Fri, Feb 6, 2015 at 12:02 AM, Byron Jeff wr= ote: > I've done some testing with a 16F1938 which has the same module. The > datasheet points out that the impedance of the DAC output pin is very hig= h > and suggests adding and opamp voltage follower buffer before using it. > The ADC doesn't work well unless the impeadance is under 10k ohms IIRC. > > So let me throw out a couple of suggestions: > > 1. There is an internal routing of the DAC buffer to the ADC. I would > suggest at least testing ADC channel 30, which connects to the DAC > internally before moving further. > > 2. If you use an op-amp make sure that it's rail to rail for both the inp= ut > and output. Since it's powered by Vdd and you're shooting for a voltage > close to that, having rail to rail op amp is critical for proper operatio= n. > So no LM324, LM358, or LM741's for this. > > BAJ > > On Thu, Feb 05, 2015 at 11:45:06PM +0200, KPL wrote: >> Hi, >> >> I'm trying out an internal DAC of "enhanced core" 16f1824, but output >> never gets close enough to VDD, which is configured as a reference. >> >> in JAL my config looks this way: >> >> DACCON0_DACEN=3D1 --enable DAC >> DACCON0_DACLPS=3D1 -- positive ref >> DACCON0_DACOE=3D1 --enable DAC output on RA0 >> DACCON0_DACPSS=3D0b00 -- reference is VDD >> DACCON0_DACNSS=3D0 -- negative source=3DVSS >> DACCON1_DACR=3D0b11111 -- output level >> >> Section about DAC is very small in the datasheet, probably something >> important is missing. Any ideas? >> I know, some of definitions are for "low power" mode, but I defined them= anyway. >> >> >> Measurement is done using another pin and ADC; which works fine by >> itself, shows 1023 when VDD is fed there. >> When connected to DAC output, I get about 890 max. >> >> -- >> KPL >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > > -- > Byron A. Jeff > Chair: Department of Computer Science and Information Technology > College of Information and Mathematical Sciences > Clayton State University > http://faculty.clayton.edu/bjeff > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 KPL --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .