Understanding Basic Daq Specifications En

7/25/2019 Understanding Basic Daq Specifications En

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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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challenges quicklywith unrivaled global

support

Meet your specific

application needs and

achieve more insight

with customizable and

easy-to-use

measurement software

NI Offers a Smarter Measurement Solution

Capture accurate

measurements with

high-quality data

acquisition hardware


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