Dual Slope type DVM:

As shown in figure 1, the input voltage ‘Vi’ is integrated with the slope of the integrator output proportional to the test input voltage. After a fixed time equal to t1, the input voltage is disconnected and the integrator input is connected to a negative voltage ‘VR’. The integrator output will have a negative slope which is constant and proportional to the magnitude of the input voltage.

Block diagram and working: