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Saturday, 16 March 2019

Dual Slope Converter Type of DVM


The block diagram of dual slope Digital Voltmeter (DVM) is shown in Figure

Description and Working of the Block Diagram:

The input chopper applies the input signal voltage to the integrator for a fixed time t1 to t2. During this time the integrator's output raises linearly as the capacitor charges. At the end of the charge period the integrator is disconnected from the input voltage and is connected to the discharging circuit.

The integrator capacitor discharges linearly during the period t2 - t3. During the total period of t1 to t3, the output of the zero crossing detector is positive. When the voltage on the integrator capacitor reaches zero the output of the zero crossing detector falls to zero. This is used to control the clock pulses to the counter.
The operation is effected by the control logic block shown in the block diagram. Before each measuring period the counter and control logic are reset by a pulse from the reset oscillator. Negative voltage is also measured in the same way. The difference is that the output of zero crossing detector is negative. This is sensed by the polarity detector and it is used to reverse the polarity of the constant current from the discharge circuit. The drift compensator circuit nulls out the effect of zero drift in the integrator and zero crossing detector. 



The following factors are to be considered in choosing an electronic voltmeter.

1. Input impedance: The input impedance must be as large as possible. This prevents the loading effect.

2. The ranges: The ranges on the meter scale can be in 2-3-10 sequence with 10 dB separation or in the 1.5-5-15 sequence. The readings on the scale must be compatible with the accuracy of the meter. A mirror below the dial of the meter is preferred when the accuracy is 1% or less.

3. The meter must be provided with a decibel scale using the standard reference level. A portion of the dial of the meter must indicate the additions required on the other scales while measuring dB.

4. The sensitivity: The sensitivity and band width are inversely proportional. High sensitivity gives low band width. Low sensitivity gives large bandwidth. Typically a voltmeter with 1 mV sensitivity may have its bandwidth of 10 MHz.
A voltmeter with a sensitivity of 100 mV may have its bandwidth as 5 MHz only. Selection is to be done with a compromise between sensitivity and bandwidth.

5. Power supply requirements: Battery operation is preferred for field work. However when power is available at site, and in laboratories mains operation is economical. Hence provision is to be made to have both mains and battery operations. A quick change over facility from mains to battery and vice versa is essential.

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