Computerized Data Acquisition Systems Chapter 4
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
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
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
Number Systems Decimal numbers: base-10 941,356 = Each digit can take one of 10 values: 0-90
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:
10111010 2 =? Binary Numbers
Conversion from Decimal to Binary
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 =?
Binary Numbers: Who Cares? Sample 0-5 V signals with your computer and convert the voltages to 3-bit binary numbers:
Resolution affects Accuracy From http://www.cyberresearch.com/cyb/cybtechtut.htm
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?
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
Quantization and Sampling of Signals
Quantization and Sampling of Signals
Chapter 4 - Data Acquisition From http://www.cyberresearch.com/cyb/cybtechtut.htm
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.
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
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 1 0 0 4/8 V in 0 1 0 2/8 V in 0 0 1 1/8 V in 0 0 0 0 V in S2 S1 S0 1 0 1 0 1 0
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
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
A/D Converter - Successive Approximation V in N+1 bit D/A Converter comparator + - V ref Control Logic
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
Successive Approximation
Offset Binary 3 Bit A/D Converter Digital (Binary) Output 111 2 110 2-5V -3.75V -2.5V -1.25V 101 2 100 2 +1.25V 011 2 +2.5V +3.75V +5V Analog Voltage Input 010 2 001 2 000 2
ADC Formulas (p. 81) Digital output from offset binary ADC: D o = Nominal value of analog input: V in =
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=
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=
Example #3 Given a 12 bit, +10V ADC, what digital value D o would V in = +5.623V 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 =
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
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.
Single-Ended Multiplexer All 16 inputs must have the same ground (common) From http://www.cyberresearch.com/cyb/cybtechtut.htm
Differential Input Multiplexer From http://www.cyberresearch.com/cyb/cybtechtut.htm + E 0 - + E 7- Must be careful with these All 8 inputs can have different grounds (commons)
Typical Connect inputs here DAQ System (inside computer)
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)
Sources of Additional Info Analog Input/Output (from Omega Transactions Vol II) Tutorials and Applications Notes from National Instruments (www.ni.com) Go browse the web!