This voltmeter is an integrating A/D converter, where the output represents the integral of an input voltage over a fixed period of time. the advantage of this method of measurement is in the reliable result in the presence of high frequency noise and the extremely small nonlinearity errors since it uses the time to quantize the answer. Moreover the auto-zero feature allows the manufacturer to use operational amplifiers with up to 10mV offset.
In the dual-slope technique each measurement is divided into three phases. The first phase is the auto-zero phase, where the errors in the analog components, as the offset voltage, are automatically nulled out by grounding the input and closing a feedback loop such that the error information that makes dV/dt of the integrator equal to zero is stored on the auto-zero capacitor and the reference capacitor is charged to the reference voltage Vref. In the second phase the input signal to be measured Vin, is integrated for a fixed number of clock pulses, in this case the frequency of clock is 120khz, and the integration period is 1000 clock pulses. The integrator capacitor will ramp up at a rate proportional to Vin. In the third and final phase the integrator input is switched from Vin to Vref.The integrator ramps down at a rate that is proportional to Vref. The number of clock pulses beetwen the beginning of this cycle and the time when the integrator back to its auto-zero value is a digital measure of Vin. Vin will be = (number of clock pulses)/1000 * Vref.