igital Voltmeter ( DVM ) is an Analog to digital converter (A/D) with a
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.
Resistor and Capacitor in the integrator
Total time of integration
Time of integration of input voltage
Time of integration of reference voltage
Maximum number of counts
Final count value
Coming up next:
* Integrating Type DVM
* Successive Approximation type DVM