Convert analog voltages to digital using 5 Volt 4bit ADC
You're thinking of a 4-bit resolution and wishing for a decimal number to represent a voltage value. In that case, consider this equation:

\$\displaystyle \frac{\textrm{Resolution of ADC}}{\textrm{System Voltage}}=\frac{\textrm{ADC Reading}}{\textrm{Measured voltage}}\$

So for an example, you have a 10-bit resolution on a 5v channel and you're reading 2.12v on your multimeter. The decimal value would be:

\$ \displaystyle \frac{2^{10}-1}{5}=\frac{x}{2.12}\$

Your ADC would read 434 once you solve for  \$x\$. These types of numbers are good for microcontrollers like the Arduino that can read the voltage this way and you can make conditions based on the voltage it was reading, i.e. from sensors.

As far as receiving negative numbers from a ADC, you might have to refer to the datasheet of your ADC. I'm not too familiar with negative voltage readings from an ADC unless you do some clever programming on your part using some type of differential mode.