Transcript PowerPoint on Basic Concepts

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
Analog
Signal
Computer
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 2n – e.g. 12-bit
converter = 212 = 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
+10v
+1.25v
0v
-10v
-1.25v
FSR = 10v
FSR = 20v
FSR = 2.5v
Data Acquisition Concepts
Input/Output
Analog
Digital
(101110010011)
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
+10v
1
0
0v
Data Acquisition Concepts
Output Levels/Resolution Example
2-bit A/D Converter
+10v
11
10
01
0v
00
Data Acquisition Concepts
Output Levels/Resolution Example
4-bit A/D Converter
+10v
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0v
0100
0011
0010
0001
0000
Data Acquisition Concepts
Output Levels/Resolution Example
12-bit A/D Converter
+10v
1111
1111
1111
4096
Output
Levels
0v
0000
0000
0000
Data Acquisition Concepts
Output Levels/Resolution Example
16-bit A/D Converter
+10v
1111
1111
1111
65,536
Output
Levels
0v
0000
0000
0000
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) ÷ 2n
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
User Input
Selected Range (uni-polar)
10 volts
5 volts
0 volts
0 volts
Only 50% of the available A/D range is used.
Data Acquisition Concepts
Selecting Gain and Range
User Input
Selected Range (bipolar)
+10 volts
5 volts
0 volts
0 volts
-10 volts
Only 25% of the available A/D range is used.
Data Acquisition Concepts
Selecting Gain and Range
User Input
Selected Range (bipolar)
+10 volts
Gain Amp
0.25 v
10
-0.25 v
0 volts
-10 volts
Only 25% of the available A/D range is used.
Data Acquisition Concepts
Selecting Gain and Range
User Input
Selected Range (bipolar)
10 volts
1 volt
0 volts
0 volts
-10 volts
Using this range only utilizes 1/20th of the range. This would allow
the input to be divided into 205 increments.
Data Acquisition Concepts
Selecting Gain and Range
User Input
Selected Range (uni-polar)
1.25 volts
1 volt
0 volts
0 volts
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
Uni-Polar and PGL Example
Gain
Effective
Range
Step Size (LSB)
12-bit
16-bit
1
0-10 v
2.4414 mv
152.6 µv
10
0-1 v
2.4414 µv
15.26 µv
100
0-100 mv
24.414 µv
1.526 µv
500
0-20 mv
4.8828 µv
0.305 µv
Data Acquisition Concepts
PGL Range/Gain Combinations
Bipolar and PGL Example
Gain
Effective
Range
Step Size (LSB)
12-bit
16-bit
1
± 10 v
4.8828mv
305 µv
10
±1v
488.28 µv
30.5 µv
100
± 100 mv
48.828 µv
3.05 µv
500
± 20 mv
9.7656 µv
0.61 µv
Data Acquisition Concepts
PGL Range/Gain Combinations
Uni-Polar and PGH Example
Gain
Effective
Range
Step Size (LSB)
12-bit
16-bit
1
0-10 v
2.4414 mv
152.6 µv
2
0-5 v
1.2207 mv
76.29 µv
4
0-2.5 mv
610.35 µv
38.147 µv
8
0-1.25 mv
305.17 µv
19.073 µv
Data Acquisition Concepts
PGL Range/Gain Combinations
Bipolar and PGH Example
Gain
Effective
Range
Step Size (LSB)
12-bit
16-bit
1
± 10 v
4.8828 mv
305 µv
2
±5v
2.4414 mv
152.6 µv
4
± 2.5 v
1.2207 mv
76.27 µv
8
± 1.25 v
610.35 µv
38.147 µv
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) 525-8528