Computerized Data Acquisition Systems. Chapter 4

Transcription

1 Computerized Data Acquisition Systems Chapter 4

2 Data Acquisition - Objectives State and discuss in terms a bright high school student would understand the following definitions related to data acquisition bits, resolution, range, sampling frequency, ADC, DAC, multiplexing, single-ended input, differential input Convert between equivalent digital and analog values for 8, 10, and 12 bit analog-to-digital converters using offset binary notation

3 Data Acquisition - Objectives Describe the operation of common type of analog-to-digital converters integrating successive approximation Use a LabView data acquisition module to simultaneously collect data from multiple sensors and write data to text file for offline plotting and analysis

4 Computerized Data Acquisition Systems Almost exclusively talking about computerized systems. Non-computerized systems do exist Few and far between Typically, at a minimum, data will be transferred to a computer for analysis and/or storage

5 Number Systems Decimal numbers: base ,356 = Each digit can take one of 10 values: 0-90

6 Binary Numbers Computers are constructed of massive arrays of flip-flops The output of a flip-flop can be _ or _ Collections of flip-flops can be used to store (and manipulate) binary numbers Binary: base-_: each digit can be one of two values:

7 =? Binary Numbers

8 Conversion from Decimal to Binary

9 Computer Interface Terminology Bits: The number of digits in a binary number. A 1 bit binary number is either 0 or 1, a 2 bit binary number is either 00, 01, 10, or 11, etc. An N bit binary number can have 2 N different values. Hex (or hexadecimal): Digital values expressed in base 16. Note that 1 hex digit is exactly 4 binary digits. 1AE 16 =? FF 16 =?

10 Binary Numbers: Who Cares? Sample 0-5 V signals with your computer and convert the voltages to 3-bit binary numbers:

11 Resolution affects Accuracy From

12 Quantization of Signals Digitization of signals limitation Improved resolution achieved with higher bit counts: Suppose you want < 1mV resolution on a 0-5 V signal. How many bits do you need to store all possible numbers?

13 Sampled-Data Systems Computers are event-driven systems Addition, subtraction, multiplication, and division, all performed only at specific time intervals Clock-driven system: rising edges of the clock pulses are the events that drive/synchronize the operations of the computer. So

14 Quantization and Sampling of Signals

15 Quantization and Sampling of Signals

16 Chapter 4 - Data Acquisition From

17 Computer Interface Terminology D/A converter (or DAC): Digital to Analog converters are used to map a finite number of integer values onto a physical output range (usually a voltage) A/D converter (or ADC): Analog to Digital converters are used to convert continuous physical signals (usually voltages) into equivalent digital or binary numbers.

18 Computer Interface Terminology Range: The difference between the upper (V ru ) and lower (V rl ) analog voltages that can be accurately converted to binary. Common ranges are +5 volts (10 V range), 0 to +10 V (10 V range), and +10 volts (20 V range). Resolution: Amount of analog voltage equivalent to a single binary bit = (V ru -V rl ) / 2 N resolution of 12 bit A/D with a + 5 V range

19 D/A Converter R/2R ladder used V out R R closing switches gives output voltages 2R 2R 2R 2R S2 S1 S0 V out /8 V in /8 V in /8 V in V in S2 S1 S

20 Integrating / Dual Slope ADC integrate input signal, V in, for known period then integrate known voltage, -V ref to zero C V in R - V I comparator - -V ref + + Control Logic and clock

21 Integrating / Dual Slope ADC integrate input signal, V in for known period integrate voltage (-V ref ) for unknown period V I large V in small V in known period unknown period - proportional to V in

22 A/D Converter - Successive Approximation V in N+1 bit D/A Converter comparator + - V ref Control Logic

23 A/D Converter - Successive Approximation DAC is used to sequentially guess bits N, N-1, N- 2, 1, 0 as 1 s output of D/A is compared to input voltage keep current bit a 1 if V in > V DAC current approximation is lower than actual set current bit to 0 if V in < V DAC current approximation is higher than actual all previously set bits are maintained as the next bit is guessed as a 1

24 Successive Approximation

25 Offset Binary 3 Bit A/D Converter Digital (Binary) Output V -3.75V -2.5V -1.25V V V +3.75V +5V Analog Voltage Input

26 ADC Formulas (p. 81) Digital output from offset binary ADC: D o = Nominal value of analog input: V in =

27 Example #1 Given a 12 bit, + 5V ADC, what is the nominal voltage for a digital value of 1000? V nom = What range of input voltages would all be converted to the digital value of 1000? resolution=

28 Example #2 Given a 10 bit, 0 to 10 V ADC, what is the nominal voltage for a digital value of 763? V nom = What range of input voltages would all be converted to the digital value of 763? resolution=

29 Example #3 Given a 12 bit, +10V ADC, what digital value D o would V in = V convert to? D o = Given an 8 bit, 0 to 5V ADC, what digital value D o would V in = 1.234V convert to? D o =

30 Computer Interface Terminology Multiplexer : a switching" device to connect different analog signals to the same ADC 8 or 16 different analog inputs common, single-ended or differential inputs used, inputs often amplified near multiplexer

31 Computer Interface Terminology Single-Ended Inputs : All inputs use the same ground, so only 1 switch needed per signal generally all signals must be of the same type Differential Inputs : Each input signal has its own separate ground, so two switches are required for each signal - one for signal and one for common/ground.

32 Single-Ended Multiplexer All 16 inputs must have the same ground (common) From

33 Differential Input Multiplexer From + E E 7- Must be careful with these All 8 inputs can have different grounds (commons)

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35 Typical Connect inputs here DAQ System (inside computer)

36 National Instruments PCI-MIO-16E-4 Board 12 bit resolution (~2.4 mv/bit) accuracy specs on manufacturer s datasheet + 10 volts full scale input (bipolar) offset binary coding 8 channel differential input multiplexer maximum single channel sampling rate of 500 khz (500,000 samples/sec) maximum multi-channel sampling rate of 250 khz (250,000 samples/sec)

37 Sources of Additional Info Analog Input/Output (from Omega Transactions Vol II) Tutorials and Applications Notes from National Instruments ( Go browse the web!