Welcome to the list :) I haven't seen any vref's that low but the resistor divider should work. Maxim makes some highly accurate super easy to use references, the MAX6126 comes to mind, it's powered by 3-12V, so you can run it much lower and comes in a 2.048V and 2.5V output to start nice and low. Then using some precision resistors you could get a decent reference to start with. I'm sure others on the list w/more experience will have a better suggestion from experience. Programatically obtaining the calibrating voltage? You mean the low voltage reference? It's possible, you could use the initial 2.5V (for example) reference to start low and a high-resolution DAC. Two notes: 1) If this is a prototype, sample! No reason to buy the 16F873 on eBay, Microchip has a great sampling program just for this purpose. Also might want to look at the 18F series. 16's are fading and the 18 series is not much more expensive (less expensive in some cases). 2) Avoid HTML, I noticed the tags in your name (silly AOL), some people on this list have nervous breakdowns when they see HTML tags and lash out at any and all HTML users, careful! :) nick@veys.com | www.veys.com > I am a new electronic hobbyist trying my first post to the list. > > I need a design for a "precision" millivolt reference to > calibrate a precision op-amp circuit which will feed into an > ADC. The sources will range from -25 to +25 mV with a > resolution of 0.05mV. The prototype will use a 16F873 (bought > from recent EBay auction). I have seen a circuit using a > 10.0000V standard and resister divider. Can this be done with > a lower (<5V) standard? Is it possible to use a 12-16 bit DAC > to programatically obtain the calibrating voltage? > > The project involves recording bio electrical potentials from > 3-6 channels, reading the digital signal through a home made > 16bit ISA card to collect in a data base for further analysis. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body