Digital Voltmeter - Part I

Digital Voltmeter ( DVM ) is an Analog to digital    converter (A/D) with a

digital display

TYPES: (based on the type of ADC used)

          1. Dual Slope Type

          2. Ramp Type

          3. Integrating Type

          4. Successive Approximation Type

DUAL SLOPE INTEGRATING TYPE:

          Principle: Dual slope integrating type digital voltemeter has both +ve and -ve slope during integration there by it averages the errors (or) noise that are present in the environment.

          Operation: The start pulse resets the counter to zero, the flip flop gives zero output and keeps switch Si closed and Sr open by switch driver circuit .The integrator integrates in +v direction when zero crossing just exceeds zero. The gate is open allowing the counter to count the oscillator clock. When counter resets to maximum count for the next clock, the counter resets to zero forcing the flip flop output to '1'. In this case, switch Si is open and Sr is closed. Vr (reference voltage) is negative reference,now integrated by the integrator with negative slope and the voltage crosses zero,the zero crossing detector closes the gate, stopping the counter.

           The counts in the counter is proportional to 't2' ( time for integrating Vr (negative slope) towards zero ) and is in-turn proportional to input voltage.

An Expression for Input Voltage:

          Charging Equation:      Vout = -(1/RC) ∫ [ Vi * dt ]    ( interval 0 to t1)

                                                 Vout = -(1/RC)*Vi*t1

         Discharge Equation:     Vout = (1/RC) ∫ [ -Vr * dt ]   ( interval 0 to t2)

                                                 Vout = (-1/RC)*Vr*t2

         From above equations, we get

                                                Vi*t1 = Vr*t2

                                                    Vi = (t2/t1)*Vr

                                        since,  t1 = n1*T

                                            and t2 = n2*T

                                                    Vi = (n2/n1)*Vr

        Hence, Vi is directly proportional to n2.

               where, 

Vout	Output Voltage
R, C	Resistor and Capacitor in the integrator
Vr	Reference Voltage
T	Total time of integration
t1	Time of integration of input voltage
t2	Time of integration of reference voltage
n1	Maximum number of counts
n2	Final count value

Coming up next:

* Ramp Type DVM

* Integrating Type DVM

* Successive Approximation type DVM

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