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At 09:46 AM 4/10/2004 -0700, Chad Russel wrote: >What am I missing here. Can someone help??? > >I figure that measuring 15 volts with a .01 volt or 10 millivolt >accuracy is .067%. The Fluke 87 is .05% accurate and lists for >$349.00. A 12F675 costs how much? Is the reference voltage at least that accurate? Most band-gaps in micros are pretty bad. The pic probably doesn't do multiple functions, and true-RMS AC voltage reading. |
AD with 12F675 fluctuations
Started by ●April 8, 2004
Reply by ●April 10, 20042004-04-10
Reply by ●April 10, 20042004-04-10
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I thought about this a bit. You can find many ap notes at maxim and linear tech. Probably the cheapest way to do what you want is to use a precision 5 volt or 10 volt reference to scale and level shift through a good op amp/s. You should then be able to get by with a 10 bit AtoD and 2 or 4 precision resistors depending if the reference is 5 or 10 volt. Another way is to use a differential 10 bit AtoD with 10 and 15 volt references. There are a few other ways, but probably more expensive. Good luck, Chad --- Chad Russel <> wrote: > What am I missing here. Can someone help??? > > I figure that measuring 15 volts with a .01 volt or 10 millivolt > accuracy is .067%. The Fluke 87 is .05% accurate and lists for > $349.00. A 12F675 costs how much? > > My head hurts. > > Chad > > --- d1camero <> wrote: > > The source voltage under the scope looks fine. I am also using 1k > > and 2k resistor for the voltage divider. > > > > What alternatives are there? > > > > Don > > > > > > > > > > --- In , "Scott Lee" <midl_man@y...> wrote: > > > --- In , "d1camero" <a8723@c...> wrote: > > > > BUT, when I removed the resistor divider network, and simply > > used > > > > the 12F675 to measure 0-5volts, the ADC works great, with an > > > > acceptable 1 bit fluctuation. > > > > > > It seems to me that Igor's explaination, which you seemed to > > > discount, may be at least some of the problem. > > > > > > From what I can tell you are trying to measure 10-15V by using a > > > resistor divider which will scale the 15V down to the necessary > > 5V. > > > It will also scale that 10V down to 3.33V. Thus, although the > A/D > > > is measuring over a 0-5V range, the data of interest is in the > > 3.33- > > > 5V range. > > > > > > You want 0.01V accuracy on the original 10-15V range. The > > resistor > > > divider is also scaling this just like it scaled the original > > > voltage range. Think about it, in the original 10-15V range > there > > > would be 500 steps (501 levels) at the .01V increments you > > desire. > > > 500 steps in the 3.33-5V range works out to .0033V increments. > > > > > > Thus using your resistor divider to measure .01V increments over > a > > > 10-15V range will require that you measure .0033V increments over > > > a > > > 3.33-5V range. Given that the A/D can measure to a resolution > > > of .00488V but at best has an error of +-1 LSB this means that > you > > > are going to be +-0.015V at best. > > > > > > Now I know this doesn't explain all of it but you should see that > > > > there is no way you are going to get the .01V accuracy you desire > > > by > > > taking this approach. > > > > > > One other point is you you say that you have verified the voltage > > > of > > > the 10-15V supply using two DVMs and it seems quite steady -- > have > > > you looked at it with a scope? There is a good chance you will > > find > > > some ripple which the DVMs are filtering out but that the A/D is > > > seeing due to the speed of the sample/conversion. As others have > > > > pointed out, putting a cap close to the input to the PIC should > > help > > > filter this out. As others have also pointed out it is > imperative > > > that you not have too high of resistance as viewed from the PIC > > > A/D. The fact remains that even following these guidelines you > > are > > > not going to acheive the accuracy you desire with a resistor > > divider. > > > > ===== > My software has no bugs. Only undocumented features. > > __________________________________ > ===== My software has no bugs. Only undocumented features. __________________________________ |
