EEE312: Electrical measurement & instrumentation

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University of Turkish Aeronautical Association Faculty of Engineering EEE department EEE312: Electrical measurement &

(abbreviated DVM, for Digital Voltmeter, or DMM, for Digital Multimeter) indicates the quantity being measured by a

The heart of the DVM is the circuitry that converts the measured analog signals into digital form.

1 University of Turkish Aeronautical Association Faculty of Engineering EEE department EEE312: Electrical measurement & instrumentation Digital Electronic meters BY Ankara March Introduction The digital electronic meter (abbreviated DVM, for Digital Voltmeter, or DMM, for Digital Multimeter) indicates the quantity being measured by a numerical display rather than by the pointer and scale used in analog meters. The heart of the DVM is the circuitry that converts the measured analog signals into digital form. These conversion circuits are known Aanalog-to Digital (A/D) Converters or ADC. The circuitry that converts digital signals into analog form is called Digital-to-Analog Converters (or DACs). 2

2 Digital-to-Analog Converters DACs 3 Digital-to-Analog Converters DACs 4

Digital-to-Analog Converters DACs Ladder Resistive Summing Networks: The more popular type is R-2R ladder DAC. we analyze it by using the Super-position Theorem.

3 Digital-to-Analog Converters DACs Ladder Resistive Summing Networks: The more popular type is R-2R ladder DAC. we analyze it by using the Super-position Theorem. Assume that an input signal (V REF ) is applied to only one input at a time and all other inputs are at zero. Then determine the output of all these applied individual inputs. Finally, all of these outputs are added to get the total output. 5 The most widely methods used in commercially available analog-to-digital converter circuits are: 1. Tracking or Staircase ramp. 2. Successive approximation. 3. Voltage to frequency. 4. Dual slope. 5. Parallel (or flash). Each sample should be converted into binary code The A/D conversion requires an amount of time to convert the analog voltage value into digital form. Sample and Hold (S/H) 6

Sample and Hold: The Sample and Hold circuit is required at the input of ADC so that the voltage level can be held steady, long enough, during the conversion process.

4 Sample and Hold: The Sample and Hold circuit is required at the input of ADC so that the voltage level can be held steady, long enough, during the conversion process. Each held level is converted into binary code by an ADC Sample and Hold (S/H) 7 1- Staircase ramp ADC (The simple ADC): The start pulse resets the counter to the zero. As long as the analog input V A is greater than V D, the comparator has an output which is high and the AND gate is open for transmission of the clock pulse to the counter. When V D exceeds V A the comparator output changes to low value and the AND gate is disabled. This stops the counting and the counter can be read out as the digital word representing the analog input voltage. V A V D Start 8

5 1- Staircase ramp ADC (The simple ADC): V d The output climbs from zero to the desired value. V in 1LSB V A -V d Quantization error Max = 1 LSB Assume VCC = 8V & Vin= 3.1 V The resolution of the ADC= 1LSB = V CC /2 n ; where n is the bits number. Increasing n? This type have very simple design and cheap. However the conversion time is variable and slow (depend on the amplitude of V in ). Also, it require reset for each conversion Tracking (servo) ADC. UP - DOWN counter. Start command is not used, and the counter continue counting from the previous value. The digital output bounces back and forth by ±1 LSB around the correct value. The conversion time is small for small changes in the sampled analog. UP DOWN 10

6 Example: Using the simple ADC, design a digital voltmeter whose range volts. Display the digital output on 7-segs with a resolution of 0.1 V and maximum measuring time 0.1 sec

7 13 14

8 2- Successive approximation A/D converters: The DAC inputs are all initially set to 0. The control logic first turns the MSB of the DAC to "l" and the comparator tests Vthe in resulting DAC N=4bits 1LSB=0.5 v Time=7ck output against the analog input. A decision is N=5bits 1 LSB = 0.25 v Time = 14 ck N = 10 bits 1 LSB = 8/10 2 v Time =? made by the control logic as to whether to store 1LSB the "1" bit in the storage register or to store a 0. V d Assume VCC = 8V & Vin= 3.1 V The simple ADC Then the second bit of the DAC is turned to 1 and a second decision is made. After n bits, the storage register will contain all those bits stored as "l" or 0" and the total contents will be a digital approximation of the analog input voltage signal Successive approximation A/D converters: Ex: 3bits V in >V d V in <V d 16

9 2- Successive approximation A/D converters: V ref V d <V in V d >V in V d <V in V d >V in V d >V in V d >V in ¾V ref V in Timing diagrams of a successive-approximation search in a 6-bit converter. ½V ref ¼V ref Clock Start Time Successive approximation A/D converters: The conversion time of the successive approximation converters is constant. T convert = n / f, where n is the number of bits in the converter and f is the clock frequency. One very important requirement of these converters is that the input voltage remain constant during the conversion process. This method is very widely applied because of their combination of high resolution and speed (i.e., they can perform conversions within 1 to 50 s rather than the milliseconds required by the staircase ramp, dual-slope, and voltage-to-frequency types). However, they are more expensive than these slower types. 18

10 3- Voltage-to-frequency converter. In these types of A/D converters, the input dc voltage is converted into a set of pulses whose frequency is proportional to the magnitude of the input voltage. The pulses are counted by a frequency counter, and the count is proportional to the magnitude of input voltage. The heart of this A/D converter is the circuit that transforms the input dc voltage to a set of pulses. An integrator is used to carry out this task, as in the following example: Single Slope Integrating ADC. 19 Ex: Single Slope Integrating ADC: Constant & negative Start pulse Reset the counter. Switch S 1 on then off (To discharge C) V in > V 1 Counter counts up. Once V in < V 1 Counter stop counting. Digital output N is proportional to V in. Ncounts during time t Error? How to increase the Accuracy? 20

11 Example: Using the single Slope Integrating ADC, design a digital voltmeter whose range V. Display the digital output on 7-segs with a resolution of 1 V and maximum measuring time 1 msec. Solution: Start Dual-slope A/D converters: - 22

12 4- Dual-slope A/D converters: N 2 & t r are independent of the value of R and C Dual-slope A/D converters: The dc voltage to be converted by the dual-slope converter, Vin, is fed to an integrator, which produces a ramp waveform output. The slope of the ramp is proportional to the magnitude of Vin. At the end of the interval, T1, the carry-out (CO) bit of the ripple counter causes the switch to move to the -VREF position. In this position, a constant current source ( -VREF/R) begins to discharge capacitor C. The ripple counter is reset to zero when there is a CO. The count continues until the zero crossing detector switches state as a result of capacitor C being discharged. The counter is stopped by the zero crossing detector, and the resultant count is proportional to the input voltage. 24

13 5- Parallel (or flash) converters. 3-bits flash ADC Parallel converters perform the fastest A/D conversions. In this technique (by way of example, a 3-bit parallel converter) the input voltage V in is fed simultaneously to one input of each of comparators. The other input of each comparator receives a different reference voltage value. The input voltage V in is thus simultaneously compared to 7 equally spaced voltage values (from V ref /8 to V ref ) Parallel (or flash) converters. The speed of parallel A/D converters is limited by the comparator and encoder delay times. Additional time will also be required due to delays in system components that follow the flash converter, either a microprocessor or DSP chip... 3-bits flash ADC Conversion Speed vs bits number? Develop the truth table of the used encoder. 26

14 References: For further reading students are referred to the following book: Electronic Instrumentation Laboratories Stanley Wolf & Richard F.M. Smith. 2 nd Edition, Prentice Hall, ISBN-10: ISBN-13: END 28

ANALOG TO DIGITAL (ADC) and DIGITAL TO ANALOG CONVERTERS (DAC)

COURSE / CODE DIGITAL SYSTEM FUNDAMENTALS (ECE421) DIGITAL ELECTRONICS FUNDAMENTAL (ECE422) ANALOG TO DIGITAL (ADC) and DIGITAL TO ANALOG CONVERTERS (DAC) Connecting digital circuitry to sensor devices