Hi Kiste, 2010/10/28 Oliver Seitz <[email protected]>
> >When powered @5V, I loose a lot of resolution, thus information. Even more > when such a voltage divider would be designed for power supply up to 24V... > > That's right, you're loosing resolution. 5V max. on a divider designed for > 24V, you've got a resolution of only about 24 milivolts. So, when measuring > only 5V, accuracy can not be better than 0.5%. > OK, right. I'm starting to accept the idea of just not doing this :) > > Have you put this number into relation to the rest of your circuit? For the > divider, are you using expensive precision resistors with 0.1% accuracy and > do you compensate for temperature errors? Or do you use cheap resistors with > 5%? Something in between. I'm using 1% accurary resistors: http://sirbot.org/sirbot-modules/dc-motor-controller-board/dcmc_rctank_schematic.png > Is there a dedicated 5V reference, or do you simply use one supply to power > different devices? Then your reference voltage could introduce errors of > several percents. > Vref is the one used by the PIC itself. So yes, it may be shared with other devices (but not the motors') > > The magic is quite simple: You're powering the Op-amp with 5V. As there is > no magic inside the Op-amp, it cannot output more than 5V. You should use > so-called rail-to-rail (R2R) op-amps, simple Op-amps will not even output > 4V. > OK, I thought R2R was actually R/2R (ladder I think). I was quite lost :) > >Have I absolutely misunderstood this whole thing ?? > > In general, you understood very well :) > Well, that's a great emotional victory ! I was thinking that... maybe... Could I use internal vref output, as Vasile mentioned, connected to Vref+ input, and adjust Vref output so I could match a better resolution ? Say I have 5V as input, on a 24V divider. More or less I would measure between 0-1V. If I lower Vref+ to 1V instead of 5V, with internal vref output, I would now a better resolution, since 0-1V range would now expand to full resolution. If needed... Cheers, Seb -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
