Data Acquisiton and Conversion

8/3/2019 Data Acquisiton and Conversion

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DATA ACQUISITION AND

CONVERSIONThe process of taking analogue information, often from a number of

sources, and converting it into a digital form is often termed data

acquisition.

The purpose of a Data Acquisition system is to measure a physical

phenomenon such as light, temperature, pressure, sound, etc. Thebuilding blocks of a Data Acquisition system are as follows:

Transducer Signal Signal Conditioning Data Acquisition (DAQ) device Driver level and application level software

These five building blocks allow you to bring the physical phenomena

you want to measure into your computer for analysis and presentation.

In the following pages, we will discuss each one of these blocks

individually to give you knowledge of each building block, and how they

fit together to make up your Data Acquisition system.


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TRANSDUCERto use for measuring the following physical

phenomena:

Temperature Light Sound Force Pressure


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Position Fluid flow pH levelsThe purpose of a transducer is to convert physical phenomena (light,

temperature, pressure, sound, etc.) into a measurable electrical signal,

such as voltage or current.

With the help of a transducer we have converted physical

phenomena (light, temperature, pressure, sound, etc.) into a signal.

Not all signals are measured in the same manner, so we will need to

learn how to categorize our signal as one of two types:

Digital

Analog

Once we have categorized our signal we need to figure out what

type of information we want out of that signal. The possible types of

information we can obtain from a signal are:

State Rate Level Shape


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Frequency

SIGNAL CONDITIONING

In the signal conditioning stage, electrical signals are conditioned so

they can be used by an analog input board. The signal may be

conditioned by amplification, where the power of the signal is

increased to make it easier to read in more detail. Isolation may also

occur, so that the input and output circuitry do not interfere with

each other. The signal may also be filtered, to remove noise or


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Amplification is a way of increasing a signal from a transducer

that is too small for your DAQ device to accurately measure. A

common example is a thermocouple. Thermocouples output a

voltage in the mill volt range. If you were to send the signal from

your thermocouple straight to your DAQ device, it is feasible that a

change of a degree or two in temperature would not be detected by

your system. However, if we amplify the signal we will be measuring

a signal that is better suited to the range of our DAQ device. Your

signal can either be amplified on the DAQ device or externally. The

problem with amplifying the signal on the DAQ device is that we also

amplify the noise the signal has picked up on its way to the DAQ

device. In order to minimize the amount of noise that is amplified it

is best to place the amplifier as close to the signal source as possible.

Thus it is usually best to use some form of external amplification. As

we will see next, we can show the benefit of external amplification

with an index called the Signal to Noise Ratio

MUTIPLEXING

Although it is quite possible to have a system with a single

analogue input or a single analogue output, it is usual to

have multiple inputs and outputs. Clearly, one solution to this problem


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is to use a separate converter for each input and output signal, but

often a more economical solution is to use some form ofmultiplexing.

A number of analogue input signals can be connected to a single ADC

using an analogue multiplexer. This is a form of electrically controlled

switch based on the use of analogue switches; analogue signal is

connected in turn to the ADC for conversion, the sequence and timing

being determined by control signals from the system. For certain

applications, the arrangement in Figure (a) is unsuitable as each

analogue input signal is sampled at a different time. This may make itimpossible to obtain detailed information as to the relationship between

the signals, such as their phase difference. The problem can be

overcome by


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Sampling all the inputs simultaneously using a number of sample and

hold gates, as shown in Figure (b). Once the input signals have been

sampled, they can be read sequentially without losing the time

relationship between the channels.

SAMPLE AND HOLD SYSTEM

In electronics, a sample and hold circuit is an analog device that

samples (captures, grabs) the voltage of a continuously varying analog

signal and holds (locks, freezes) its value at a constant level for a

specified minimal period of time. Sample and hold circuits and related

peak detectors are the elementary analog memory devices. They are

typically used in analog-to-digital converters to eliminate variations in

input signal that can corrupt the conversion process.

A typical sample and hold circuit stores electric charge in a capacitor

and contains at least one fast FET switch and at least one operational

amplifier. To sample the input signal the switch connects the capacitor

to the output of a buffer amplifier. The buffer amplifier charges or

discharges the capacitor so that the voltage across the capacitor is

practically equal, or proportional to, input voltage. In hold mode the

switch disconnects the capacitor from the buffer. The capacitor is

invariably discharged by its own leakage currents and useful load

currents, which makes the circuit inherently volatile, but the loss of


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voltage (voltage drop) within a specified hold time remains within an

acceptable error margin.

One of the factors to consider in converting analog signals to digital is

the sampling rate. The sampling rate determines how often conversions

take place. The higher the sampling rate, the better. This example

shows three different sampling rates for an analog input signal. The 16

samples per cycle digitized signal looks closer to the original analog

input than the 4 samples per cycle signal. The reason one would use a

lower sampling rate is because the amount of total samples that can be

taken is limited, the processing power required to handle that much


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data is limited, or the extra precision obtained by the high sampling rate

is unnecessary.

A problem with using too low of a sampling rate is that aliasing might

occur. Aliasing is when the acquired signal gets distorted by a sampling

rate that is too small. In this example, the original signal is sampled so

slowly that the sampled signal looks like a completely different

frequency than the original signal.

The minimum sampling frequency required to represent the signalshould be at least twice the maximum frequency of the analog

signal under test.(this is called nyquist rate)

If the sampling frequency is equal or less than twice the frequencyof the input signal ,a signal of low frequency is generated from

such a process.(this is called aliasing)


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For accurate frequency representation: Sample at least 2x the highest frequency signal being measured.

For accurate shape representation

Sample 510x the highest frequency signal being measured

Anti-aliasing filter

An analog filter that removes frequencies above Fs/2,where Fs is

the sample frequency.

Analog-to-digital converter circuits (ADC) are usually equipped

with analog low-pass filters to pre-condition the signal prior to

digitization. This prevents signals with frequencies greater than the

sampling rate from being seen by the ADC


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Resolution

an Analog-to-Digital Converter (ADC) takes an analog signal and turns it

into a binary number. Therefore, each binary number from the ADC

represents a certain voltage level. The ADC returns the highest possible

level without going over the actual voltage level of the analog signal.

Resolution refers to the number of binary levels the ADC can use to

represent a signal. To figure out the number of binary levels available

based on the resolution you simply take 2Resolution. Therefore, the higher

the resolution, the more levels you will have to represent your signal.

For instance, an ADC with 3-bit resolution can measure 23 or 8 voltage

levels, while an ADC with 12-bit resolution can measure 212 or 4096

voltage levels. Even though ADCs are not made with only 3-bit

resolution let us further examine our example of a 3-bit ADC. The

lowest voltage level will correspond to 000, the next highest to 001, and


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so on all the way up to 111. As we will see next this is usually not

enough resolution to properly represent a signal.

We will compare a 3-bit ADC and a 16-bit ADC. As we learned earlier a

3-bit ADC can represent 8 discrete voltage levels. A 16-bit ADC can

represent 65,536 discrete voltage levels. As you can see the

representation of our sine wave with 3-bit resolution looks more like a

step function than a sine wave. However, the 16-bit ADC gives us a

clean looking sine wave. One way to think of resolution is by

considering your television screen. The higher the resolution of the

screen, the more pixels you have to show the picture, so you will get a

better picture. Another way to think resolution is by considering the

amount of colors your computer monitor uses to display an image. Ifyou are only using 16 colors the picture is choppy and doesnt look very

good, but if you use 16-bit color the picture is smooth and looks great.

Keep in mind that resolution is a fixed quantity of an ADC, and it


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depends on the DAQ device that you use. Your standard National

Instruments DAQ device has either 12-bit or 16-bit resolution.

Quantization:Each flat region in the sampled signal is rounded-off to the

nearestmember of a set of discrete values (e.g., nearest integer)

Range:

Sample

and hold

ckt

output

3-bit

ADC

3 bit

digital

output


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ADCs also have a parameter called the range. The range refers to the

minimum and maximum analog voltage levels the ADC can digitize.

Unlike the resolution of the ADC, the range of the ADC is selectable.

Most DAQ devices offer a range from 0 - +10 or -10 to +10. The range

is chosen when you configure your device in NI-DAQ. We will learn

how to configure our DAQ device in software later in this chapter. Keep

in mind that the resolution of the ADC will be spread over whatever

range you choose. The larger the range, the more spread out your

resolution will be, and you will get a worse representation of your signal.

Thus it is important to pick your range to properly fit your input signal.

As an example let us reconsider the colors we use to represent an

image on our computer monitor. As we said earlier a picture looks

better when more colors are used to represent it. Now let us examine

the effect that changing the range would have on our picture. Let us

compare a picture with 16 color resolution in black and white to a

picture with 16 color resolution in color. Our black and white picture

will be clearer because our resolution is only spread across two colors

instead of all colors. Next we will see this affect with our analog signal.


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Choosing the proper range for a signal is very important to help

maximize the resolution of our ADC. To illustrate this, let us revisit our

sine wave and our 3-bit ADC. Due to poor resolution we are still not

going to be able to represent our sine wave very well. However, an

improper choice of range can make our representation of the sine wave

even worse. Our sine wave has a minimum value of 0 Volts and a

maximum value of +10 Volts. If we choose our range as 0 - +10 Volts

we will have 8 different voltage levels we can represent. If we were to

improperly choose a range of -10 to +10 Volts we would now only have

4 voltage levels to represent our signal, because the other 4 levels

would be used by the 0 to -10 Volt range. Our smallest detectable

voltage would change from 1.25 to 2.50 and we would get a worse

representation of our sine wave. As you can see improperly choosing

the range will negatively impact the representation of your signal.

However, we do not always have a choice as to what range to pick. For


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instance, if our sine wave actually went from -2 to +8 Volts, we could

not choose 0 to +10 Volts as our range, because the signal does not fit

within that range. We would be forced to choose a range of -10 to +

10, even though it spreads out our resolution.

GainGain refers to any amplification or attenuation of a signal. The gain is

not applied by your ADC. Instead the gain is applied by the

instrumentation amplifier that proceeds the ADC on your DAQ device.

The gain setting is a scaling factor. For example, possible gain settings

for an E-Series device are 0.5, 1, 2, 5, 10, 20, 50, or 100. Each voltage

level on your incoming signal is multiplied by the gain setting to achieve

the amplified or attenuated signal. Unlike resolution that is a fixed

setting of the ADC, and range that is chosen when the DAQ device is

configured, the gain is specified indirectly. in Lab VIEW will you find a

place to set the gain. The gain is chosen indirectly through a setting

called input limits. Input limits refers to the minimum and maximum

values of your actual analog input signal. The input limits are specified

in Lab VIEW. Based on the input limits you set, the largest possible gain


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is applied to your signal that will keep the signal within the chosen

range of the ADC. So instead of needing to calculate the best gain

based on your signal and the chosen range, all you need to know is the

minimum and maximum values of your signal. If you dont set the input

limits of your signal a gain of 1 (no change) will be applied.

Applying a gain to an analog input signal is very similar to amplifying a

your voice with a microphone. If you tried speaking in a stadium for

100, 000 people without a microphone, very few of the 100,000 people

will be able to hear your voice. However, if you amplify your voice with

a microphone you can maximize the number of people that can hear

you. In the same way a small signal will not be able to use the entire

resolution of the ADC, unless a gain is applied to amplify the signal. Let

us take a look at how the gain setting affects an analog input signal.

Assume we have a sine wave with a range of 0 to +5 Volts and an ADC


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range of 0 to 10 Volts. As you can see above if we applied a gain of 1

(no change) to our signal we would only be taking up half of the range,

and thus using only half of our resolution. However, if we apply a gain

of 2 to our signal we now have a sine wave with a range of 0 to +10

Volts. Now our signal fits exactly in our range and we will be

maximizing the use of our resolution. Now let us consider a sine wave

with a range of 0 to +6 Volts with the same ADC range of 0 to +10

Volts. We can no longer apply a gain of 2, because our sine wave

would have a range of 0 to +12 Volts which exceeds our ADC range.

The only gain we can apply is a gain of 1. It is also important to note

that if we put a 0 to +5 Volt signal into our device, our graph in

LabVIEW will show a 0 to +5 Volt signal regardless of the gain that is

applied. The gain setting is only used to maximize the use of the ADC

resolution. It will not affect your measurement.

Code width

Code width is the smallest change in your signal that your system can

detect. The formula for the code width is shown above. As you can see

the code width is a property of the resolution, range, and gain. Thesmaller our code width is the better we can represent our signal. The

formula confirms what we have already learned in our discussion of

resolution, range, and gain:


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Larger resolution = smaller code width = better representation ofthe signal

Larger gain = smaller code width = better representation of thesignal

Larger range = larger code width = worse representation of thesignal

Types of ADC

Successive approximation

The DAC is driven by a digital word produced by the successive

approximation logic. Initially, all the bits of this word are set to 0 and

then the most significant bit (MSB) is set to 1. This input word is

converted by the DAC into an analogue signal corresponding to half of

the full range of the DAC. This value is compared with the analogue

input signal using a comparator and the result is fed back to the control

logic. If the comparison shows that the DAC output is less than the

analogue input, the MSB will be left at 1; if not, it will be reset to 0. In

any event, the logic then sets the next MSB and, again, compares the

output of the DAC with the input signal. In this way, each bit of the

input to the DAC is set in turn and its correct state determined. The

conversion is completed when all the bits of the DAC input have been

set correctly. Therefore, for an n-bit conversion, this will take


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approximately ntimes the settling time of the DAC and the comparator.

This compares favorably with the counter type, which requires up to 2n

times the settling time of the DAC and comparator. Typical successive

approximation converters might have settling times of110 s for an 8-

bit conversion, increasing to perhaps 10100 s for a 12-bit device.

High-speed variants are available with considerably improved conversion

times. The complexity of this form of converter is somewhat greater

than that of the counter type. However, its superior speed of operation

makes it one of the most commonly used arrangements for integrated

circuit converters.

DAC

R-2R ladder

The R2Rmethod also makes use of the current-to-voltage converter

but does not require a broad spread of resistorvalues. all the resistors

connected to the switches have the same value. Theother end of the

resistor in each case is joined to a chain of resistors, whichgoes from


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the inverting input of the operational amplifier to earth. The circuit is

arranged such that currents flowing through each of the resistors

connected to the switches see a resistance of2Rlooking in either

direction along the resistor chain. Therefore, half the current will go in

each direction.

Similarly, currents flowing up the chain see equal resistances in either

direction at each node and will again be split. Therefore, each switch

contributes half as much current as the switch above, as its current is

repeatedly halved at each node on its journey to the op-amp. Therefore,

the currents generated by the switches are binary weighted, as in theprevious method, but without the use of a wide range of resistor values.

Here, only resistors ofRand 2Rare required and, if appropriate, these

can be formed using only resistors of one value (R) by connecting two

in series to form the other (2R). This allows temperature-matched

resistors to be used to provide greatly improved temperature stability.

Flash ADC


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The parallel or flash converter is the fastest of the various forms of

ADC. It operates by having a separate comparator to compare the input

voltage with every discernible voltage step within the converters

range.The various voltage steps are produced using a precision resistor

chain from a reference voltage source. Each voltage increment is

connected to a separate comparator that compares it with the input

voltage. The result is that all of the comparators connected to points

along the resistor chain that have voltages greater than the input

voltage will produce an output of one polarity, whereas those connected

to voltages below the input voltage will produce voltages in theopposite sense. Combinational logic is then used to determine the value

of the input voltage from this pattern. The great advantage of this

method is its high speed of conversion, as all the comparisons are

performed simultaneously. This allows sample rates in excess of 150

million conversions per second, with conversion times of only a few

nanoseconds. However, as an n-bit converter requires 2ncomparators,

the hardware is significantly more complicated and therefore more

expensive than it is for other techniques.


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Internal PC bus DAS\External Bus

Remote DAS

Data acquisition software


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Transforms the PC and DAQ hardware into a complete DAQ,analysis, and display systems

Enables the developers to design the custom instrument bestsuited to their application.

Examples: Test point , Lab view etc


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Hard copy

DATA ACQIUSITION and

CONVERSION system


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Documents

Data Acquisiton and Conversion