Chapter-1 Digital Measuring Instruments.pptx

Digital Measuring Instruments.pptx

1. Digital Measuring Instruments Chapter-1

2. Digital Instrumentation Digital Voltmeter (DVM) : The voltmeter is an electrical measuring instrument which is used to measure the Potential Difference between the TWO Points. The measured voltage can be AC or DC Voltage. The DVM are versatile and accurate instruments with small size, less power, and low cost. The DVM are Analog & Digital Voltmeter types. The magnitude of voltage signals can be measured by various analog and digital test & measurement instruments, e.g., the Cathode Ray Oscilloscope (CRO) and the Digital Storage oscilloscope (DSO) . Digital Voltmeter (DVM) Basically, all types of digital meter are a modified forms of the Digital Voltmeters irrespective of the quantity that they are designed to measure. In fact, digital voltmeters contain appropriate electrical circuits to convert current or resistance into voltage signals . The binary nature of the output reading from a digital instrument can be readily (easily) applied to a display that is in the form of discrete numerals. Types of DVM : Ramp Type DVM; Potentiometric Type; Integrating Type; Successive Approximation Type; Continuous Balance Type.

3. Comparison of Digital and Analog Meter Advantages of Digital Measuring Instruments Higher Accuracy and Resolution Greater Speed No Parallax Error Reduced Human Error Compatibility with other digital equipment for further processing and recording, e.g., Connection with Computer, etc. Memory to store data for future analysis and processing Varieties of DVM Digital Voltmeters differ mainly in the technique used to affect the Analog-to-Digital Conversion (ADC) between the measured analog voltage and the output digital reading. Various DVM differ in the following ways, Number of Digits Number of Measurements parameters Accuracy Speed of Reading With/Without Memory Analog Voltmeter Digital Voltmeter Contains a dial with a needle mounting over a calibrated scale Measures voltage directly by giving the discrete numerical output Wrong scale or wrong reading can occur (Ex: 12.1 V) Parallax Error may Occur No doubt in reading (Ex: 12.125 V) No Parallax Error Inferior resolution and accuracy Superior resolution and accuracy Can not measure negative voltage Can measure negative voltage If fall from the table, the analog meter damaged. Digital Voltmeter may still work well if fall from the table, i.e., digital meters may offer stable operations.

4. Block Diagram of DVM Construction : Input Signal : It is the voltage to be measured. Pulse Generator : It is a voltage sources. It uses digital, analog, or both techniques to generate a rectangular pulse. Pulse With and Frequency: It is controlled by the digital circuit. Amplitude, Rise Time and Fall Time: It is controlled by the analog circuit. AND Gate : It gives HIGH Output, only when both the inputs are HIGH. When a train of pulse is fed to it along with a rectangular pulse, it provides the train of pulses as output. The duration of a pulse is same as the duration of rectangular pulses (generated from the Pulse Generator). Decimal Display : It counts the number of impulses and hence the duration thereafter it displays the value of voltage on the LED/LCD Display. Block Diagram of DVM

5. Working of DVM Working of DVM: It uses digital, analog or both techniques to generate a rectangular pulse. The width and frequency of the rectangular pulse is controlled by the digital circuitry inside the generator while amplitude and rise & fall time is controlled by analog circuitry. A digital voltmeter DVM displays the value of a.c. or d.c. voltage being measured directly as discrete numerals in the decimal number. The use of digital voltmeters increases the speed with which readings can be taken. It is simply an Analog to Digital Converter . WORKING OF DVM Input Voltage (Voltage Signal to be measured) Pulse Generator (Pulse Proportional to Input Signal) AND Gate Inputs = Rectangular Pulse + Train of Pulses (Output: Positive Triggered train of pulses of duration same as the width of rectangular pulse) Inverter (Output: Inverted Triggered train) Counter Counts the Number of triggers in the pulses (Number of Triggers = Input Voltage to be measures) Display (Number of Triggers = Input Voltage

6. Characteristics Features & Classification of DVMs Characteristics Features of DVMs Input Range: From ±1.000 to ±1000V with automatic range selection and overload indication Absolute accuracy: As high as ±0.005% of the reading Resolution: 1 part in 10 6 Stability: Short term – 0.002% of the reading for a 24-hr period Long term – 0.008% of the reading for a 6-month period Input resistance: Typically, 10 MΩ Input capacitance: 40 pF Output signals: BCD form (BCD: Binary Coded Decimal a 4-bit Binary number) Classification of DVMs Voltage to Time Ramp type DVM Voltage to Frequency Integrating type DVM Dual slope integrating type DVM Direct Successive approximation DVM Staircase ramp type DVM https://www.manualslib.com/manual/1093785/Velleman-Dvm851.html A 3 1/2 digits meter can  display 1999 (3 full & 1 Half) . A 4 1/2 digit meter can display 19999 (4 full & 1 Half) . The half means that the most significant digit can only go up to 1.

7. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1. Ramp type DVM (Voltage to Time) Ramp type DVM: The operating principle of the ramp-type DVM is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage , or to decrease from the level of the input voltage to zero . Working : 1. Reset the Counter by sending a pulse from Sample Rate Multi-Vibrator (MV) 2. Apply Input Voltage to be measured at the Ranging and Attenuator 3. Input voltage with a decaying ramp signal applied to the Comparator Circuit 4. At First Coincidence Point , the Input Comparator sends a Start Pulse to the Gate to OpenGate. The Oscillator Start Sending Clock Pulses to the Counter. 5. At Second Coincidence Point , the Zero Comparator sends a Stop Pulse to Gate to CloseGate. 6. The Counter counts the pulses and display. Voltage to Time Conversion or Zero Comparator Display Decaying First Coincidence Second Coincidence

8. 2. Integrating type DVM (Voltage to Frequency) Integrating Type DVM : The voltmeter measures the True Average Value of the input voltage over a fixed measuring period. In contrast the ramp type DVM, it samples the voltage at the end of the measuring period . This voltmeter employs an integration technique which uses a voltage to frequency conversion . Working Principle : Integrating type DVM works on the principle of converting Voltage into proportional frequency signal . The frequency is then measured and displayed digitally. Working : 1. Input Voltage (e i ) to be measured at the input of the Integrator. The capacitor in the integrator charges which generates a ramp signal. 2. The output of the integrator (e ) is compared with a Reference signal (e r ). 3. When the e = e r , the comparator will change its output state (from 1 to 0 or from 0 to 1) and generates a pulse in the regular intervals. 4. The output of the Pulse Generator is feed as input to the Integrator which discharges the capacitor. Now, again the input voltage (e i ) is applied at the integrator which will again charge the capacitor and generates a ramp signal. 5. This regular interval of pulse (frequency) is measured by the Digital Frequency Meter. NOTE : If the applied input voltage is large, the capacitor will charge early at points e 01 or e 02 . This shows that the voltage is proportional to the frequency. e 01 e 02 Digital Frequency Counter Part Voltage to Frequency Converter Part

9. 3. Successive Approximation type DVM (Direct) Successive Approximation type DVM : The successive approximation type DVM is a special type of Potentiometric DVM in which a digital divider is used in the place of linear divider . The servomotor replaced by electromagnetic logic. The comparator compares the output of digital to analog converter with unknown voltage.

10. 3. Successive Approximation type DVM (Direct) … Contd Calculations: B7: 128/255 = 0.501x5 = 2.50 NO Large B6: 64/255 = 0.25x5 = 1.25 NO Large B5 : 32/255 = 0.125x5 = 0.627 YES Small B4: 48/255 = 0.188x5 = 0.941 NO Large B3 : 40/255 = 0.156x5 = 0.78 YES Small B2: 44/255 = 0.172x5 = 0.86 NO Large B1: 42/255 = 0.164x5 = 0.823 NO Large B0: 41/255 = 0.160x5 = 0.803 NO Marginally Large

11. Solve the Given Problem • An 8-bit successive Approximation DVM of 5V range is used to measure 1.2V.The contents of the SAR after 5 th clock is • • Ans:00111100

12. DIGITAL MULTIMETER A digital multimeter is an electronic instrument which can measure very precisely the dc and ac voltage, current (dc and ac), and resistance. All quantities other than dc voltage is first converted into an equivalent dc voltage by some device and then measured with the help of digital voltmeter. The block diagram of a digital multimeter is shown in the Figure. The procedures of measurement of different quantities are described below.

13. For measurement of ac voltage, the input voltage, is fed through a calibrated, compensated attenuator, to a precision full-wave rectifier circuit followed by a ripple reduction filter. The resulting dc is fed to an Analog Digital Converter (ADC) and the subsequent display system. Many manufacturers provide the same attenuator for both ac and dc measurements. For current measurement, the drop across an internal calibrated shunt is measured directly by the ADC in the ‘dc current mode’, and after ac to dc conversion in the ‘ac current mode’. This drop is often in the range of 200 mV (corresponding to full scale). Due to the lack of precision in the ac–dc conversions, the accuracy in the ac range is generally of the order of 0.2 to 0.5%. In addition, the measurement range is often limited to about 50 Hz at the lower frequency end due to the ripple in the rectified signal becoming a non-negligible percentage of the display and hence results in fluctuation of the displayed number. At the higher frequency end, deterioration of the performance of the ADC converter limits the accuracy. In ac measurement the reading is often average or rms values of the unknown current. For resistance measurement the digital multimeter operates by measuring the voltage across the externally connected resistance, resulting from a current forced through it from a calibrated internal current source. The accuracy of the resistance measurement is of the order of 0.1 to 0.5% depending on the accuracy and stability of the internal current sources. The accuracy may be proper in the highest range which is often about 10 to 20 MΩ. In the lowest range, the full scale may be nearly equal to>200 Ω with a resolution of about 0.01 Ω for a 4½ digit digital multimeter. In this range of resistance measurement, the effect of the load resistance will have to be carefully considered.

14. DIGITAL FREQUENCY METER A frequency counter is a digital instrument that can measure and display the frequency of any periodic waveform. It operates on the principle of gating the unknown input signal into the counter for a predetermined time. For example, if the unknown input signal were gated into the counter for exactly 1 second, the number of counts allowed into the counter would be precisely the frequency of the input signal. The term gated comes from the fact that an AND or an OR gate is employed for allowing the unknown input signal into the counter to be accumulated. One of the most straightforward methods of constructing a frequency counter is shown in Figure in simplified form. It consist of a counter with its associated display/decoder circuitry, clock oscillator, a divider and an AND gate. The counter is usually made up of cascaded Binary Coded Decimal (BCD) counters and the display/decoder unit converts the BCD outputs into a decimal display for easy monitoring. A GATE ENABLE signal of known time period is generated with a clock oscillator and a divider circuit and is applied to one leg of an AND gate. The unknown signal is applied to the other leg of the AND gate and acts as the clock for the counter. The counter advances one count for each transition of the unknown signal, and at the end of the known time interval, the contents of the counter will be equal to the number of periods of the unknown input signal that have occurred during time interval, t. In other words, the counter contents will be proportional to the frequency of the unknown input signal. For instance if the gate signal is of a time of exactly 1 second and the unknown input signal is a 600-Hz square wave, at the end of 1 second the counter will counts up to 600, which is exactly the frequency of the unknown input signal.

15. The waveform in Figure shows that a clear pulse is applied to the counter at t0 to set the counter at zero. Prior to t1 , the GATE ENABLE signal is LOW, and so the output of the AND gate will be LOW and the counter will not be counting. The GATE ENABLE goes HIGH from t1 tot2 and during this time interval t (= t2 – t1 ), the unknown input signal pulses will pass through the AND gate and will be counted by the counter. After t2 , the AND gate output will be again LOW and the counter will stop counting. Thus, the counter will have counted the number of pulses that occurred during the time interval, t of the GATE ENABLE SIGNAL, and the resulting contents of the counter are a direct measure of the frequency of the input signal.