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

Data Acquisition ConceptsData Acquisition Concepts

Data Translation, Inc.Basics of Data Acquisition


Typical data acquisition applications

• Physiological studies

• Environmental studies (temperature, etc.)

• Materials testing

• Medical research

• Quality control and testing


Typical data acquisition applications

• Strain, pressure, temperature and vibration analysis

• Fluid analysis

• Manufacturing automation

Data Acquisition ConceptsData Acquisition ConceptsComponents of a data acquisition systems• Analog to digital conversion (A/D)

• Digital to analog conversion (D/A)

• Digital input/output (DIO)

• Pacer clock


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


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


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


Pacer Clock

• Used to initiate repetitive data conversions

• Used to control the sampling rate of conversions


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


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


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


A/D Interface

• Converts analog data into digital data which can be processed

• Typical components:– Multiplexers– Amplifiers– Sample and hold circuits– A/D converters


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


Full Scale Range (FSR)

• Full scale range refers to the largest voltage range which can be input into the A/D converter


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


Full scale range examples

+10v

0v

+10v +1.25v

-10v -1.25v

FSR = 10v FSR = 20v FSR = 2.5v


Input/Output

Analog Digital(101110010011)


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


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


Output Levels/Resolution Example

1-bit A/D Converter

1

0

+10v

0v



2-bit A/D Converter

11

00

+10v

0v

10

01



4-bit A/D Converter

+10v

0v

1111

111011011100

1011

10101001

1000011101100101

0100

0011001000010000



12-bit A/D Converter

1111 1111 1111

0000 0000 0000

+10v

0v

4096 Output Levels



16-bit A/D Converter

1111 1111 1111

0000 0000 0000

+10v

0v

65,536 Output Levels


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


What affects conversion speed?

• A/D converter only

• A/D converter and related circuitry

• A/D system and host


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


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


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


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


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


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


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


Amplification

• The output of a sensor usually requires amplification

• Apply gain


Gain

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


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


Selecting Gain and Range

5 volts

0 volts

0 volts

10 volts

User Input Selected Range (uni-polar)

Only 50% of the available A/D range is used.



5 volts

0 volts

-10 volts

+10 volts

User Input Selected Range (bipolar)


0 volts



0.25 v

-0.25 v

-10 volts

+10 volts



0 volts

Gain Amp

10



1 volt

0 volts

-10 volts

10 volts


Using this range only utilizes 1/20th of the range. This would allow the input to be divided into 205 increments.

0 volts



1 volt

0 volts

0 volts

1.25 volts

User Input Selected Range (uni-polar)

Using this range utilizes 8/10ths of the range. This would allow the input to be divided into 3277 increments.


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



• 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



• 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


PGL Range/Gain Combinations

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

Uni-Polar and PGL Example




Step Size (LSB)

12-bit 16-bit

1 ± 10 v 4.8828mv 305 µv

10 ± 1 v 488.28 µv 30.5 µv

100 ± 100 mv 48.828 µv 3.05 µv

500 ± 20 mv 9.7656 µv 0.61 µv

Bipolar and PGL Example




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

Uni-Polar and PGH Example




Step Size (LSB)

12-bit 16-bit

1 ± 10 v 4.8828 mv 305 µv

2 ± 5 v 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

Bipolar and PGH Example


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)


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


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


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


DMA: Two Methods

• Single channel for medium throughputs

• Dual channel for high throughputs


Conclusion

Additional data acquisition questions? Contact Data Translation at

(800) 525-8528

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