Data Acquisition Concepts Data Translation, Inc. Basics of Data Acquisition

Data Acquisition Concepts Typical data acquisition applications Physiological studies Environmental studies (temperature, etc.) Materials testing Medical research Quality control and testing

Data Acquisition Concepts Typical data acquisition applications Strain, pressure, temperature and vibration analysis Fluid analysis Manufacturing automation

Data Acquisition Concepts Components of a data acquisition systems Analog to digital conversion (A/D) Digital to analog conversion (D/A) Digital input/output (DIO) Pacer clock

Data Acquisition Concepts Analog to Digital Conversion (A/D) Once data has been converted from analog to digital, the digital information can then be processed by the computer, or transferred to memory

Data Acquisition Concepts Digital to Analog Conversion (D/A) D/A converters convert stored data back to a continuous signal (analog voltage) for display or control purposes Output from the D/A converter can be used to drive external devices which require an analog input

Data Acquisition Concepts Digital Input/Output (DIO) Used primarily for control purposes Used to transmit data between the host processor and an external digital device that expects to receive 1’s and 0’s

Data Acquisition Concepts Pacer Clock Used to initiate repetitive data conversions Used to control the sampling rate of conversions

Data Acquisition Concepts What is analog? Analog signals are “continuous” signals Represented by continuously changing physical quantities Level of signal can be increased or decreased indefinitely Ex. temperature, pressure, strain, voltage

Data Acquisition Concepts What is digital? Digital signals are “discrete” signals Represented by separate, individual units Units are represented by “bits” (binary digit) Bits are represented by one of two possible states: on/off, true/false, 1/0

Data Acquisition Concepts What is data acquisition? A method of acquiring an analog signal and converting (digitizing) it into a binary code which can be manipulated by a computer Computer Analog Signal Analysis

Data Acquisition Concepts A/D Interface Converts analog data into digital data which can be processed Typical components: –Multiplexers –Amplifiers –Sample and hold circuits –A/D converters

Data Acquisition Concepts A/D Converter Functions Input and output (I/O) –Analog input is converted into a digital number, by comparing the voltage with its position within the Full Scale Range –With an n-bit A/D converter the number of output levels equals 2 n – e.g. 12-bit converter = 2 12 = 4096

Data Acquisition Concepts Full Scale Range (FSR) Full scale range refers to the largest voltage range which can be input into the A/D converter

Data Acquisition Concepts Range Range is an input span for an A/D and D/A system Typical ranges are based on available sensors –Uni-polar (positive) 0 to 5 volts 0 to 10 volts Bipolar -5 to +5 volts -10 to +10 volts

Data Acquisition Concepts Full scale range examples +10v 0v +10v+1.25v -10v-1.25v FSR = 10vFSR = 20vFSR = 2.5v

Data Acquisition Concepts Input/Output Analog Digital ( )

Data Acquisition Concepts Resolution Resolution determines the smallest change that can be detected Specified in bits. Determines number of output levels, or steps –8 bits = 256 steps –10 bits = 1024 steps –12 bits = 4096 steps –16 bits = 65,536 steps

Data Acquisition Concepts Typical Resolutions 8-bit – common for image capture 10-bit – general analog acquisition 12-bit – general analog acquisition 16-bit – precision analog acquisition 24-bit – high-accuracy analog acquisition

Data Acquisition Concepts Output Levels/Resolution Example 1-bit A/D Converter v 0v

Data Acquisition Concepts Output Levels/Resolution Example 2-bit A/D Converter v 0v 10 01

Data Acquisition Concepts Output Levels/Resolution Example 4-bit A/D Converter +10v 0v

Data Acquisition Concepts Output Levels/Resolution Example 12-bit A/D Converter v 0v 4096 Output Levels

Data Acquisition Concepts Output Levels/Resolution Example 16-bit A/D Converter v 0v 65,536 Output Levels

Data Acquisition Concepts LSB LSB stands for “least significant bit” An LSB represents the smallest change that can be resolved by the A/D converter An LSB carries the smallest value or weight An LSB is the rightmost bit LSB = Full Scale Range (FSR) ÷ 2 n

Data Acquisition Concepts What affects conversion speed? A/D converter only A/D converter and related circuitry A/D system and host

Data Acquisition Concepts Acquisition time The time required to perform a complete conversion from the analog signal to digital The time required after receipt of “start digitizing” command until the A/D converter has finished digitizing

Data Acquisition Concepts Settling time Time it takes to switch to a new channel Each time a user switches between channels, there is a delay, referred to as settling time

Data Acquisition Concepts Throughput rate The inverse of A/D conversion time + acquisition time Measured in Hertz (Hz), which means the number of conversions per second The maximum rate at which the data conversion system can operate, while maintaining a specific accuracy

Data Acquisition Concepts Throughput Acquisition System –The acquisition system determines the maximum throughput possible –The entire system (host, disk, A/D board, and program) determines the practical throughput

Data Acquisition Concepts Throughput Throughput is specified as an aggregate of all channels –A/D runs at a constant rate –Number of channels determines throughput per channel The host computer and software must be able to service the A/D board before the next conversion is complete

Data Acquisition Concepts Speeding up throughput rate Overlap mode - while one sample is in Sample and Hold circuitry, the next is read into the multiplexer Throughput equals the greater of conversion time OR acquisition time, plus any time needed for Sample and Hold switching

Data Acquisition Concepts Getting signals into the computer Transducers convert physical variables into electrical outputs An input transducer (sensor) then supplies its output to signal conditioning circuitry (on a Screw Terminal Panel) Signal conditioning circuitry prepares for interfacing with the PC

Data Acquisition Concepts Amplification The output of a sensor usually requires amplification Apply gain

Data Acquisition Concepts Gain A scale (multiplying) factor which increases an input signal to better utilize the range of the A/D converter

Data Acquisition Concepts Gain Gain is the amplification applied to a signal to bring it to the range of the A/D system –High Level Gains (PGH) for high level signals – 1, 2, 4, 8 –Low Level Gains (PGL) for low level signals – 1, 10, 100, 500

Data Acquisition Concepts Selecting Gain and Range 5 volts 0 volts 10 volts User InputSelected Range (uni-polar) Only 50% of the available A/D range is used.

Data Acquisition Concepts Selecting Gain and Range 5 volts 0 volts -10 volts +10 volts User InputSelected Range (bipolar) Only 25% of the available A/D range is used. 0 volts

Data Acquisition Concepts Selecting Gain and Range 0.25 v v -10 volts +10 volts User InputSelected Range (bipolar) Only 25% of the available A/D range is used. 0 volts Gain Amp 10

Data Acquisition Concepts Selecting Gain and Range 1 volt 0 volts -10 volts 10 volts User InputSelected Range (bipolar) Using this range only utilizes 1/20 th of the range. This would allow the input to be divided into 205 increments. 0 volts

Data Acquisition Concepts Selecting Gain and Range 1 volt 0 volts 1.25 volts User InputSelected Range (uni-polar) Using this range utilizes 8/10ths of the range. This would allow the input to be divided into 3277 increments.

Data Acquisition Concepts Selecting the Best Gain & Range Analog input and output boards are generally designed to interface to the majority of sensors that are available The ranges used on I/O boards may not always be appropriate for every application

Data Acquisition Concepts Selecting the Best Gain & Range To get the highest degree of accuracy possible out of an I/O board – try to utilize as much of the available range as possible Use internal or external gain selection

Data Acquisition Concepts Selecting the Best Gain & Range Determine the maximum range that the input signal will use Determine if the signal is uni-polar (above zero) or bipolar (above and below zero) Evaluate the available gain and range combinations to select the most appropriate product

Data Acquisition Concepts PGL Range/Gain Combinations GainEffective Range Step Size (LSB) 12-bit 16-bit v mv152.6 µv v µv15.26 µv mv µv1.526 µv mv µv0.305 µv Uni-Polar and PGL Example

Data Acquisition Concepts PGL Range/Gain Combinations GainEffective Range Step Size (LSB) 12-bit 16-bit 1± 10 v4.8828mv305 µv 10± 1 v µv30.5 µv 100± 100 mv µv3.05 µv 500± 20 mv µv0.61 µv Bipolar and PGL Example

Data Acquisition Concepts PGL Range/Gain Combinations GainEffective Range Step Size (LSB) 12-bit 16-bit v mv152.6 µv 20-5 v mv76.29 µv mv µv µv mv µv µv Uni-Polar and PGH Example

Data Acquisition Concepts PGL Range/Gain Combinations GainEffective Range Step Size (LSB) 12-bit 16-bit 1± 10 v mv305 µv 2± 5 v mv152.6 µv 4± 2.5 v mv76.27 µv 8± 1.25 v µv µv Bipolar and PGH Example

Data Acquisition Concepts Methods of Transferring Data to Memory Programmed input/output (PIO) – this is older technology and is slow –Polled I/O –Interrupts (shared and not shared) Direct memory access (DMA)

Data Acquisition Concepts Polled I/O Data transfer is controlled by the CPU CPU monitors the Data Ready bit of the A/D converter When data is ready, the CPU reads it, then transfers it to memory Very slow

Data Acquisition Concepts Direct Memory Access (DMA) Data is transferred directly from the data acquisition board to system memory Frees CPU and speeds throughput Board activates request line to use DMA controllers

Data Acquisition Concepts Direct Memory Access (DMA) DMA controller receives request, acts upon it and sends back an acknowledge to the board Acknowledge is received; board puts data on the bus to be stored in memory

Data Acquisition Concepts DMA: Two Methods Single channel for medium throughputs Dual channel for high throughputs

Data Acquisition Concepts Conclusion Additional data acquisition questions? Contact Data Translation at (800)