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Lecture 8

EE 213 is a 2-credit course on electrical instrumentation and measurement that covers analog signal conditioning. Some key topics include: - Signal conditioning involves operations performed on signals to convert them to a form suitable for interfacing, such as adjusting the level (magnitude) and bias (zero value) of a voltage representing a process variable. - Transducers measure variables by converting information about the variable into a signal, taking advantage of how certain materials' characteristics vary with changes in variables like light intensity. - Linearization circuits are used to condition sensor outputs with nonlinear dependencies between input and output variables so a linear voltage is produced. Modern computers can perform linearization using software. - Filtering and conversions between

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22 views20 pages

Lecture 8

EE 213 is a 2-credit course on electrical instrumentation and measurement that covers analog signal conditioning. Some key topics include: - Signal conditioning involves operations performed on signals to convert them to a form suitable for interfacing, such as adjusting the level (magnitude) and bias (zero value) of a voltage representing a process variable. - Transducers measure variables by converting information about the variable into a signal, taking advantage of how certain materials' characteristics vary with changes in variables like light intensity. - Linearization circuits are used to condition sensor outputs with nonlinear dependencies between input and output variables so a linear voltage is produced. Modern computers can perform linearization using software. - Filtering and conversions between

Uploaded by

Muhammad Usman
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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EE 213: Electrical Instrumentation

& measurement
(2 Credit Hours)
Analog Signal Conditioning
Signal conditioning refers to operations performed on signals
to convert them to a form suitable for interfacing with other
elements.
A transducer measures a variable by converting information
about the variable into a signal.

To develop such transducers, we take advantage of nature


where a variable changes some characteristic of a material.

once we have researched nature and found that cadmium sulfide


resistance varies inversely with light intensity, we must then learn to
employ this device for light measurement within the confines of that
dependence.
Signal-Level and Bias Changes

One of the most common types of signal conditioning involves


adjusting the level (magnitude) and bias (zero value) of some
voltage representing a process variable.

For example,

some sensor output voltage may vary from 0.2 to 0.6 V as a process
variable changes over a measurement range.

However, equipment to which this sensor output must be connected


perhaps requires a voltage that varies from 0 to 5 V for the same
variation of the process variable.
We perform the required signal conditioning by first changing the
zero to occur when the sensor output is 0.2 V.

This can be done by simply subtracting 0.2 from the sensor output,
which is called a zero shift, or a bias adjustment.

Now we have a voltage that varies from 0 to 0.4 V, so we need to make


the voltage larger.

If we multiply the voltage by 12.5, the new output will vary from 0 to 5 V as
required.

This is called amplification, and 12.5 is called the gain


In some cases, we need to make a sensor output smaller,
which is called attenuation.

You should note that the circuit that does either is called an
amplifier.

We distinguish between amplification and attenuation


by noting whether the gain of the amplifier is greater than or
less than unity.
Linearization

Often, the dependence that exists between input and output of a sensor
is nonlinear
Historically, specialized circuits were devised to linearize signals.

For example, suppose a sensor output varied nonlinearly with a


process variable, as shown in Figure.

A linearization circuit, indicated symbolically in Figure, would ideally


be one that conditioned the sensor output so that a voltage was
produced which was linear with the process variable, as shown in
Figure.

Such circuits are difficult to design and usually operate only within
narrow limits.
The modern approach to this problem is to provide the nonlinear
signal as input to a computer and perform the linearization using
software.

any nonlinearity can be handled in this manner and, with the


speed of modern computers, in nearly real time.
Conversions

Often, signal conditioning is used to convert one type of electrical


variation into another.

Thus, a large class of sensors exhibit changes of resistance with


changes in a dynamic variable.

In these cases, it is necessary to provide a circuit to convert this


resistance change either to a voltage or a current signal.
Filtering

Often, spurious signals of considerable strength are present in the


industrial environment, such as the 60-Hz line frequency signals.

Motor start transients may also cause pulses and other unwanted
signals in the process-control loop.

In many cases, it is necessary to use high-pass, low-pass filters to


eliminate unwanted signals from the loop.
Concept of Loading

One of the most important concerns in analog signal conditioning


is the loading of one circuit by another.

This introduces uncertainty in the amplitude of a voltage as it is


passed through the measurement process.

If this voltage represents some process variable, then we


have uncertainty in the value of the variable.
Figure shows an element modeled as a voltage Vx and a resistance Rx.

Now suppose a load, , is connected across the output of the element as


shown in Fig.

This could be the input resistance of an amplifier,


For example. A current will flow, and voltage will be dropped
across .

It is easy to calculate that the loaded output voltage will thus


be given by

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