Post on 01-Mar-2018
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Understanding Basic
Data Acquisition Specifications
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Key Takeaways
Analog vs Digital Signals
Sampling Rate
Triggering
Resolution
Accuracy vs Precision Signal Conditioning
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What Is Data Acquisition (DAQ)?
Data acquisition (DAQ) is the process of measuring an
electrical or physical phenomenon such as voltage,
current, temperature, pressure, or sound with a computer.
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All Measurements Are Technically Inexact
Input value
Measured values
+ error - error
error
Manufacturers specification of
measurement uncertainty
24 bits resolution
250 Vrms Ch-Ch Isolation
Simultaneous Sampling
Analog Triggering
0.55 Offset Error
157 uV accuracy
320 Vrms Input Noise
0.13 Gain Error
12.5 kS/s
10V Range
24.56 kHz Bandwidth
10 G
Input Impedance
2 MS/s
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Analog
Signals Come in Two Forms: Digital and Analog
Digital
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Digital Signals
Digital signals have two states: high and low
TTL, CMOS, PECL, etc.
Can measure state or rate
+0.8 V
0 V
+5.0 V
+2.2 V
High State
Low State
Indeterminate
State
Rate
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Digital Terminology
Bit
The smallest unit ofdata. Each bit is either a1 or a 0.
Byte
A binary numberconsisting of eight
related bits of data.
Line
One individual signal ina port. Bit refers to thedata transferred. Linerefers to the hardware.
Port
A collection of digitallines (usually four or
eight).
0
1
01101001
10101100
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Analog
Signals Come in Two Forms: Digital and Analog
Digital
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Analog Signals
Analog signals are continuous signals that can be any
value with respect to time.
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Analog Terminology
Level The instantaneous value of the signal at a given point in time.
Shape
The form that the analog signal takes, which often dictates furtheranalysis that can be performed on the signal.
Frequency
The number of occurrences of a repeating event over time.
4.71 V
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Sampling Rate
Sampling rate is the frequency at which you measure the
original input signal.
Resulting signal is a series of discrete samples acquired
at a specified sampling rate.
Actual Signal Sampled Signal
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Simultaneous Sampling
Multiplexed Sampling
Multiplexing Limitations on Sampling Rate
Sample
Clock
Ch 1
Ch 2
Ch 3
Ch 1
Ch 2
Ch 3
Sample
Clock
Ch 1
Ch 2
Ch 3
Ch 1
Ch 2
Ch 3
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Sampling Rate
Original Waveform (10 Hz)
Sampling Rate = 11 HzSampling Rate = 25 HzSampling Rate = 100 Hz
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Sample rate: how often an A/D conversion takes place
Alias: misrepresentation of a signal
Aliasing
Adequately Sampled
Aliased Due to Undersampling
Frequency
Amplitude
6
Frequency
Amp
litude
2
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Following the Nyquist Theorem Prevents Aliasing
To accurately represent the frequencyofyour original signalFrequency
You must sample at greater than 2 times the maximum frequencycomponent of your signal.
To accurately represent the shapeof your original signal
Shape
You must sample between 510 times greater than the maximumfrequency component of your signal .
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The Nyquist Theorem in Action
Aliased Signal
Adequately Sampled
for Frequency Only
Adequately Sampled
for Both Frequency
and Shape
100 Hz Sine Wave
100 Hz Sine Wave
Sampled at 100 Hz
Sampled at 200 Hz
Sampled at 1 kHz100 Hz Sine Wave
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Triggering
Event-driven acquisition or generation
Example: acquire 5 samples on a start trigger:
Start Trigger
Clock
Start of
Acquisition
1 2 3 4 5
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Trigger on Falling Edge
Accepts TTL/CMOS-compatible signals
0 to 0.8 V = logic low
2.2 to 5 V = logic high
Trigger on rising or falling edge of signal
Trigger TypesDigital Edge Triggering
Trigger on Rising Edge
Begin Acquisition Begin Acquisition
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Falling Slope with Level 2.7Rising Slope with Level 2.7
Trigger TypesAnalog Edge Triggering
Trigger off signal level and slope
Slope can be rising or falling
0
2.7
Level and slope
initiate data capture
Captured
data
0
2.7
Level and slope
initiate data capture
Captured
data
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Resolution
6-Bit
Resolution
3-Bit
Resolution
Original
Signal
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Input Range
Original
Signal
3-Bit
Resolution
Range of-10V to 10V
Range of-2V to 2V
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Why Resolution Matters
7.777777 Volts
Static Signal Quantization Levels on ADC
12-bit ADC
7.778
7.773
7.783
7.769
7.764
7.759
14-bit ADC
7.7830
7.7590
7.77707.77827.7782
7.77707.7758
7.7794
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Why Resolution Matters
777 Volts
Signal Quantization Levels on ADC
24-bit ADC
User Reading
7.7830
7.7590
7.77777
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Precision
Precision degree to which repeatedmeasurements under unchanged
conditions show the same results
Does not necessarily mean that themeasurement is accurate
Completely separate from sensor
precision
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Accuracy
Accuracy degree of closeness of ameasurement to its actual (true) value
Provides worst-case level of
uncertainty for a DAQ device at aspecific range
Completely separate from sensor
accuracy
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Absolute Accuracy
Offset
error
Noise
+ Reading (Gain Error)
= Absolute Accuracy
Range (Offset Error )
+ Noise Uncertainty
+ INL
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Are All 16-Bits Created Equal?
Other DAQ card
16-bit
500 kS/s 10 V input range
NI PXIe-6341
16-bit
500 kS/s 10 V input range
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Are All 16-Bits Created Equal?
Other DAQ card
NI PXIe-6341
Accuracy = Gain Error + Offset Error + Noise Uncertainty
Gain Error = FSR * .06%
= 10*.0006
= 12 mV
Offset Error = 1 mV
Noise Uncertainty (not specified)
Accuracy = 12 mV + 1 mV =13 mV
Effective Bits of Resolution = 10.5
No math required
Specified as Absolute Accuracy at Full
Scale in data sheet.
Accuracy = 2.19 mV
Effective bits of resolution = 13
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Conditioning Signals for Quality Measurements
Signal conditioning improves a signal that is difficult for
your DAQ device to measure Signal conditioning is not always required
Noisy, Low-Level Signal Filtered, Amplified Signal
Signal Conditioning
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Transducer/Signals Signal Conditioning
ThermocouplesAmplification, Linearization, Cold-Junction
Compensation
RTD (Resistance Temperature
Detector)Current Excitation, Linearization
Strain GageVoltage Excitation, Bridge Configuration,
Linearization
Common Mode or High Voltage Isolation Amplifier
Loads Requiring AC Switching
or Large Current Flow
Electromechanical Relays or Solid-State
Relays
High-Frequency Noise Low-Pass Filters
Common Signal Conditioning Examples
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Examining Common Signal Conditioning for Voltage
Measurements
Amplification
Attenuation
Filtering
Isolation
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Amplification
Used on low-level signals
Maximizes use of analog-to-digital converter (ADC) range
and increases accuracy
Increases signal-to-noise ratio (SNR)
Amplifier
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Example: Amplification and the Signal-to-Noise
(SNR) Ratio
10 mV Signal 1 mV Noise
SNR = 10
1000xAmplifier
1 mV Noise
ADC
ADC
1000x
Amplifier
10 mV Signal
SNR = 10,000
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Attenuation
Decreases the input signal amplitude to fit within the
range of the DAQ device
Necessary when input signal voltages are beyond the
range of the DAQ device
Attenuator
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Filtering
Filters remove unwanted noise from a measured signal
and block unwanted frequencies
Time Domain Lowpass
Filter
Frequency Domain
Time Domain
Frequency Domain
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Isolation
Isolation helps to pass a signal from its source to a
measurement device without a direct physical connection
Blocks high common-mode signals
Breaks ground loops
Protects your instrumentation
Electromagnetic
Capacitive
Optical
Isolation
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Types of Isolation
Isolation Topologies Isolates:
Channel-to-Earth All channels from instrument ground
Bank (Channel-to-Bus) Groups of channels from other groups
All channels from instrument ground
Channel-to-Channel Individual channels from each other
All channels from instrument ground
Channel-to-Earth Isolation Bank Isolation
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Measurement Category Ratings
Classification of live electrical circuits
Accounts for transients devices might experience when
connected in the power distribution system.
CAT I Instruments connected to low voltage sources and
electronicsCAT II Instruments connected directly to standard wall
outlets or plug in loads
CAT III Instruments connected to main installation like a
central distribution board or circuit breaker
CAT IV Instruments connected directly to power source
such as the power grid
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Summary
Analog vs Digital signal Characteristics
Sampling Rate Considerations
Types of Triggering
Resolution (Precision) vs Accuracy
Amplification, Attenuation, Filtering, and Isolation
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