Explain Dual Slope ADCs

Dual slope ADCs make use of a capacitor connected to a reference voltage. The capacitor voltage begins at zero and is charged for a set time through the output voltage of a sample-and-hold circuit.  The capacitor is then switched to a fixed negative voltage reference, and charged in the opposite direction till it arrives zero volts again.  This second charge is timed along with a digital counter.  The last count is proportional to the input voltage.  

This method is very precise and can generate ADCs with high resolution.  The dual slope method makes sure that the greatest number of component variations is cancelled out.  For instance, drifts or scale errors have no effect as we start and finish at the same voltage.  Though, these converters are very slow and usually more expensive than successive approximation ADCs.