There are two issues here: 1. his input range is 0...-15V or the range to be measured is around -12V (say -10V...-15V ) ? 2. it's unknown if the source is floated or not relative to the microcontroller ground As long those two questions have no answer from the person who asked the initial question, the whole math, physics and electronics together can't help for an appropiate answer. Assuming the source signal is completely floated from the PIC supply/ground, is necessary a simple divider 0...-15V to 0...-5V without any trick. -5V could be connected to PIC ground and 0V to the AD input. And the measurement resolution will be 10 bit. Typical situation when a simple battery (other than PIC supply) is measured. Assuming the signal is not floated (one line of the signal must be connected to the microcontroller ground or VDD) there are at least two choices: a. using a divider on the negative voltage (0...-5V) followed by an inverter (0...+5V), 0V signal connected to PIC GND and PIC supply: GND = 0V, VDD = +5V b. using a divider on the negative voltage (0...-5V), 0V signal connected directly to the AD input, -5V signal connected to the PIC ground, PIC supply as follows: VDD= 0V, GND=-5V, all electronics around the PIC adjusted accordingly. Software adjustment for full scale is required (1023 means 0V, 0000 means -5V) Both situation will gave 10bit resolution. Vasile On 3/5/07, Tobias Gogolin wrote: > Math, Physics, Electronics > lets say he used the voltage div. to Vref at 5V and wanted to measure down > to -10 without offseting with a zener diode... > Then he would have 1/3 range in the positive and 2/3 in the negative... > > > On 3/4/07, Vasile Surducan wrote: > > > > On 3/4/07, Tobias Gogolin wrote: > > > Only one problem... he wouldnt get the full resulution on as neg. > > voltage > > > range... > > > > Why ? > > -- > > http://www.piclist.com PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > -- > Tobias Gogolin > cel. (646) 124 32 82 > skype: moontogo > messenger: usertogo@hotmail.com > > You develop an open source motor controller at > http://groups.yahoo.com/group/GoBox > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist