Lecture 7 -Data Acquisition Basics and Intro to NI-ELVIS.ppt

8/10/2019 Lecture 7 -Data Acquisition Basics and Intro to NI-ELVIS.ppt

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Data Acquisition Basics


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Overview Ground References

Floating versus Ground Referenced Differential and Common Mode Rejection Ratio

(CMRR) Analog multiplexers

Ground Referenced Single Ended (GRSE) and Non

Referenced Single Ended (NRSE) Sampling Rate and Nyquist Criterion

Aliasing

ExampleNyquist.VI , Al iasing.VI , Acq One Sample w

loop and waveform chart and freq.VI Quantization Error

ExampleQuantization.VI , L ineari ty Check.VI

Resolution from Range and Number of Bits

Intro To NI-ELVIS

Circuit Safety


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Floating Versus Ground References

Voltage is a measurement of the difference in electrical potential between twopoints

As such, voltage measurements must always be referenced to a known level

Traditionally voltage measurements are made with respect to earth ground

In self generating voltage systems, like batteries and thermocouples, theground reference is usually the negative terminal of the source

If the negative terminal of a self generating system is connected to anearth ground, then it is grounded

If the negative terminal of the self generating system is not connected toearth ground, then it is floating

Floating means that the local ground reference of a system is nottied to earth ground

Accumulation of static charge, electromagnetic coupling and otherphenomena can cause the local ground to raise to a energy potentialthat is above earth grounds

Other power systems, such as dc-dc converters and transformer coupling, cangenerate local grounds that are isolated from earth ground


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Differential Voltage Measurements Ideally every measurement of voltage would be purely differential

We would measure the potential difference between two points

These points are typically referred to as

A differential amplifier is a device that amplifies the difference between two

voltages

This requires two wires from every measurement and someway to connect both

wires to a differential amplifier to measure the signal

Either a dedicated differential amplifier for each measurement or

A switch (multiplexer or mux) that switches both wires into a differential

amplifier for each measurement A reference to instrumentation system ground is established through the

amplifier

This allows comparison between measurement channels in a system

In large instrumentation systems this is a problem as it doubles the system

)(

VVGainVoutput

VandV


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Differential

From Labview

Data Acquisition

Basics Manual

Analog

Multiplexers


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Analog Multiplexers

Normally there is one analog to digital converter that is shared

in all the analog channels

In order to switch the different analog channels into the analog

to digital converter at the appropriate times, there is an analog

multiplexer

Definition of multiplexer is a set of electromechanical orsemiconductor switches arranged to allow the selection of one

of many inputs to a single output

Digital multiplexers, allow the selection of a digital value or

pulse train to an output Analog multiplexers allow the selection of one of several

analog line voltages signals to an output


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Common Mode Voltage Rejection Ratio (CMRR)

Any voltage measured with respect to the instrumentationamplifier ground that is present at both of the inputs to adifferential amplifier is called Common Mode Voltage

Common Mode Voltage is rejected by an ideal amplifier, i.e.not measured

This is an important noise reduction feature as noise due to

electromagnetic coupling and other sources is usually presenton both inputs

A differential amplifier can improve the signal to noise ratio

Practical devices are imperfect and can be described byparameters such as common mode voltage range and CommonMode Rejection Ratio (CMRR)

CMRR is frequency dependent

Most data acquisition devices will specify the CMRR up to 50hertz, the power line frequency


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CMRR Measurement

refV

GainModeCommon

GainalDif ferentidbCMRR

V

VVdbCMRR

out

log20)(

)log(20)(

Test requires

Periodic signal

Source at frequencies

of interest


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Ground-referenced Single Ended

In GRSE, all measurements are made with respect to a single node, AI GND,

that is directly connected to measurement system ground .

This reduces the number of wires and channels of multiplexing required.

High frequency signals often require the use of coaxial cables

A coaxial cable utilizes a solid center conductor surrounded by an insulatorwhich is surrounded by a grounded shield

Coaxial cables are needed in high frequency because most of the signal

travels along the outside surface of the cable

The shield also reduces the amount of noise coupling in high frequency

signals

These are by necessity single ended measurements


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Ground Referenced Single Ended

(GRSE) and NRSE

In an non-referenced signal ended system, the channel and the sense line (low

reference point at the sensor) are not direct connected to a ground but have a

finite resistance to ground. This may be large or small. Bias resistors may be

installed to control this resistance to a known value to reduce the error in the

signal.


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Ground Loop

A ground loop is when there is difference in potential between two ground points

resulting in current flow between the two ground points. This can introduce error

into measurements through direct effects (raising the ground) and indirect effects

(electromagnetic coupling)


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Aliasing

A false lower frequency component that appears in sampled data acquired at too

low a sampling rate compared to the Nyquist Frequency.

This is the phenomena that explains why wagon wheels seem to turn backwards in

movies. The sampling rate of the pictures (60 hz) is not fast enough to capture more

than one cycle of the wheel so in subsequent frames, the wheel appears to turn

backwards


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Nyquist Sampling Criteria


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Sampling Rules

If data signal maximum frequency content is f, then

Must sample at greater than 2f to get frequency correct Must sample at 10-20f to get shape correct

f is the highest frequency present, not necessarily the highest frequencydesired

This is why it is important to lowpass filter prior to sampling

Once it is sampled, the aliasing has occurred and further filtering canthelp

Anti-aliasing filtering cant be done in software unless initial samplerate is high enough to be greater than the Nyquist criterion for allfrequencies present

Sampling at a much higher rate so that you can digitally filter iscalled oversampling

In generalfilter before you sample or be sure you are oversamplingand then digitally filter

Example Nyquist.VI


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Frequency Domain Representation of

Aliasing

Signal Spectrum Before Sampling at less than Nyquist frequency

Signal Spectrum after Sampling at less than the Nyquist frequency


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Quantization error

Quantization - The process of converting an analog signal to a digital

representation.

Normally performed by an analog-to-digital converter (A/D converter orADC).

Quantization Error - The inherent uncertainty in digitizing an analog value.

The quantization error depends on the number of bits in the converter, along

with its errors, noise, and non-linearities
http://zone.ni.com/devzone/nidzgloss.nsf/10ce50b61f969f5a86256879000d3a01/f2f8a166d032e01e8625686a007941b3?OpenDocumenthttp://zone.ni.com/devzone/nidzgloss.nsf/10ce50b61f969f5a86256879000d3a01/f2f8a166d032e01e8625686a007941b3?OpenDocument


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Quantization Error

Quantization error varies within a range


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Quantization

Increasing the number of bits decreases the quantization

error for the same input range


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The greater the input range, the greater the

effect of quantization error (for a fixed number

of bits of conversion)

Accuracy

Example Quantization.VI


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Setting Limits Allows DAQ Device to

Make Better Use of Number of Bits in

ADC


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Relative Accuracy

Examplesee L inearity Check.VI


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LabVIEW Data Acquisition

O i


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Overview Data Acquisition Structures Functions & locations

Timing, triggering and buffering - Software versus

board Data Acquisition Libraries

Express

DAQ MX - the latest

Single Shot Acquisition of a single channel usingDAQMX

Software timing functions and time functions

Multi-channel acquisition using DAQMX

Multiple Samples of a single or multiple channelsusing DAQMX

Timing

Triggeringevent and analog using DAQMX

Traditional NI-DAQ

NI-MAX

L f D t A i iti


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Layers of Data Acquisition

DAQ Board Functions/ Onboard Registers

DAQ Board Specific Interface

Driver and its memory settings based on library calls (DLL)

Driver (DLL ) Interface (Code Interface Node)

DAQMX VIs - memories

DAQMX VI definition

VI Using DAQMX VIs

This allows applications to move between different computers and different dataacquisition systems with similar capabilities without reprogramming


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Timing, Triggering and Buffering

There are three critical support functions performed indata acquisition

Timingdetermining and controlling the separationbetween samples

Triggeringdetermining when to start and stop dataacquisition

Bufferingstoring information as it is acquired before itis processed or transferred between elements of the dataacquisition system

These functions can be performed in

The hardware in the data acquisition board The software in the computer

Both the data acquisition board and the software in thecomputer


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Buffers

Abufferis a contiguous set of memory (in a computer) or registers (on a board) where data

can be written for future processing or read from as an input to processing

Incoming or outgoing information is temporarily stored in a buffer which acts as a

synchronizing element and allows devices on either side of an interface to act at their design

rate

A buffer is an intermediate device

Pointers serve as indexes into the buffer. Buffers can have read pointers (next location to be

read) and write pointers (next location to write)

Buffers allow data to be communicated more efficiently as the overhead in data transfer is

usually fixed and larger transfers are more efficient


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Circular Buffers

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Lecture 7 -Data Acquisition Basics and Intro to NI-ELVIS.ppt

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