Test: Digital Voltmeter (DVM) - Electrical Engineering (EE) MCQ

Dual slope integrating type is the popular method of Analog to Digital conversion. In dual slope method, the noise is averaged out by the positive and negative ramps using the process of integration.
The basic principle of this method is that the input signal is integrated for a fixed interval of time and then same integrator is used to integrate the reference voltage with reverse slope.
Hence the name is given to the technique is dual slope integration technique.
Dual slope integrating type ADC is the most accurate type of ADC because of  non-dependency on variation in components values caused by noise.
These reduce design complexity and improves measurement quality.
The accuracy of the measured voltage is inde­pendent of the integrator time constant and the accuracy of the method is also independent of the oscillator frequency.

Concept:

Resolution: It is the smallest increment an instrument can detect and display.

• The resolution for the 'N' bit digital voltmeter is given by:
  
• The range of an instrument is the sum of the smallest and the largest number it can display.

For 4­-digit, the smallest and the largest number that can be displayed is:

Range = 1
Resolution = 1/10⁴
Resolution = 10^-4 = 0.0001
In percentage, the resolution is 0.01%

Digital multimeters have the following advantages over analog multimeters:

• More accurate
• Reduced reading and interpolation errors
• Parallax error is eliminated
• Faster and do not require zero adjustment
• Smaller size therefore portable

However, analog meters have some advantages over digital multimeters, cheaper and less electric noise.

In a digital voltmeter, during start of conversion, zero indication is displayed. This is known as auto zeroing.
The circuit is used for this purpose is shown below.

It consists of an integrator. To achieve auto zeroing, the switch S is connected from Vin to the ground potential and the capacitor C of the dual-slope integrator discharges to the ground.

The resolution in a N bit DVM is given by

Where N is the number of full digits.
In a DVM, a full digit counts 0 to 9 and half digit counts from 0 to 1.

Calculation:
The given DVM has 3 ½ digit display, i.e. it has 3 full digits and 1 half digit.
Resolution for the given DVM is

Minimum count for the given DVM = 0.001

Maximum count = 1.999 V

Digital Multimeter:

• A digital multimeter is an electronic instrument that can measure very precisely the dc and ac voltage, current (dc and ac), and resistance.
• All quantities other than dc voltage are first converted into an equivalent dc voltage by some device and then measured with the help of a digital voltmeter.
• Multimeters measure resistance by injecting a small current into the circuit and then measuring the voltage drop across those points in the circuit.
• The known current and the resulting voltage drop are then used to calculate the resistance using Ohm's Law.

Digital multimeter:

• Digital multimeter present measurement data in a direct format that does not require calculating the exact value, and they are free from the parallax error of analog meters. 
• Unlike meter movements in analog meters, digital multimeter displays have no moving parts, and they are free from wear and shock failures.
• Digital multimeters automatically detect polarity, show positive and negative values, have much better overload protection, and offer automatic as well as manual range selection options.

Calculation:

Here, MV = Measured value
TV =  True value
Given, Lowest range of Measured Value(M_L) = 97
Highest range of Measured Value(M_H) = 103
True Value = 100
therefore,

Concept:

The voltmeter sensitivity (Sv) is determined by dividing the sum of the resistance of the meter (Rm)

Mathematically, sensitivity is expressed as:

E = Rated Voltage or Full scale Reading
Voltmeter sensitivity is expressed in Ohm/Volt
R_m = S_v × E

Calculation:
Sensitivity of voltmeter = 500 Ω/V
Internal resistance of voltmeter = 500 ×
20 = 10 kΩ
Effective resistance across A – B is
= 10 || 15 = 6 kΩ

The voltmeter reading is 10 V
⇒ V_AB = 10 V
By voltage division,

To enable the ohmmeter to indicate any value being measured, with the least error, scale multiplication features are used in most ohmmeters.

• A typical meter will have four test lead jacks: COMMON, R X 1, R X 10, and R X 100
• The jack marked COMMON is connected internally through the battery to one side of the moving coil of the ohmmeter
• The jacks marked R X 1, R X 10, and R X 100 are connected to three different size resistors located within the ohmmeter

Concept:
The resolution (R) in an N bit DVM is given by: 

Where N is the number of full digits.
In a DVM, a full digit counts 0 to 9 and a half digit counts from 0 to 1.
For N and half DVM resolution (R) is given by: 

Calculation:
Given that,
N = 3(1/2)

%R = (1/2000) × 100 × of full range
%R = 0.05 % of full range