Wow.. it's amazing what the correct magic incantation will do at google. I think I Was trying everything *but* that one. -forrest On 12/2/2010 9:17 PM, Charles Craft wrote: > Google "low temperature voltage reference" > > The dreaded Maxim but they do exist. Digikey shows stock. > http://www.maxim-ic.com/datasheet/index.mvp/id/4212 > > MAX6143 > High-Precision Voltage Reference with Temperature Sensor > > > On 12/2/2010 11:03 PM, Forrest W Christian wrote: >> I'm thinking there's a trick I'm missing somewhere. >> >> I'm trying to design a circuit which uses an ADC to measure 0-60V at >> least to the nearest 0.1V. >> >> The obvious solution is to take a precision reference, and a voltage >> divider with high stability resistors, and a 12bit ADC (although a 10bit >> ADC may be close enough). >> >> But this brute-force method seems to get expensive very quickly, >> especially since I really need this to work from -30 to +50C. >> >> However, in looking at this closer, it appears I might not need that >> high of stability (or that high of precision resistors). In a voltage >> divider situation, if both resistors exibit the same 'temperature >> coefficient error' (or whatever the correct term really is) at each >> temperature in the range, then the divider won't change over temperature= .. >> >> Is it reasonable to expect say metal film resistors from the same >> manufacturer (and type) to be pretty consistent with how many ppm the >> resistance drifts at any given temperature? For instance if they were >> sitting right next to each other on the board, perhaps with a gob of >> heatsink adhesive encapsulating both of the units? >> >> The other thing that's bothering me is the lack of a precison reference >> with extremely low ppm/C rating. With the 80*C application range I'm >> talking about, and typical voltage references in the 50ppm/C range, I >> end up with +-0.4% overall temperature drift. Or 0.24V at full scale >> error - which isn't going to work. >> >> Short of building an oven, is there something else I can do to get a >> decent reference voltage for the ADC? I haven't seen a >> voltage-reference equivalent of a TXCO, and even if I did I suspect that >> it would be horribly priced. Plus, I'm trying to keep low power here >> (this product will be used at times in a solar powered application), >> and burning power to heat up a voltage reference doesn't sound like >> low-power to me. >> >> And what else am I likely to be missing? >> >> -forrest >> >> >> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .