CHAPTER 3
Measurement Systems with Electrical Signals
• In this chapter, common aspects of electrical-signal measuring
systems are being described.
Electrical output sensing devices have several significant
advantages over mechanical devices:
1. Ease of transmitting the signal from measurement point to the
data collection point
2. Ease of amplifying, filtering, or otherwise modifying the signal
3. Ease of recording the signal
Stages in electrical signal measuring system.
Electrical output transducers are available for almost any
measurement.
A partial list includes transducers to measure
displacement, linear velocity, angular velocity,
acceleration, force, pressure, temperature, heat flux, neutron flux,
humidity, fluid flow rate, light intensity, chemical characteristics, and
chemical composition.
sensor and transducer often used interchangeably,
There are other words used to name transducers for particular
applications-the terms gage,cell,pickup,and transmitter being
common.
SIGNAL CONDITIONERS
There are many possible functions of the signal-conditioning stage.
The following are the most common:
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Amplification
Attenuation
Filtering (highpass, Iowpass, bandpass, or bandstop)
Differentiation
Integration
Linearization
Combining a measured signal with a reference signal
Converting a resistance to a voltage signal
Converting a current signal to a voltage signal
Converting a voltage signal to a current signal
Converting a frequency signal to a voltage signal
More than one signal-conditioning function, such as amplification
and filtering, can be performed on a signal.
General Characteristics of Signal Amplification
• Many transducers produce signals with low voltages
• Signals in the millivolt range are common, and in some
cases, signals are in the microvolt range.
• It is difficult to transmit such signals over wires of great
length, and many processing systems require input
voltages on the order of 1 to 10 V.
• The amplitude of such signals can be increased using a
device called an amplifier, shown as a block diagram in Fig.
The degree of amplification is specified by a
parameter called the gain, G:
Generic voltage amplifier.
Gain is more commonly stated using a logarithmic scale, and the
result is expressed in decibels (dB). For voltage gain, this takes the
form
Using this formula, an amplifier with G of 10 would have a decibel
gain, Gdb, of 20 dB, and an amplifier with a G of 1000 would have a
decibel gain of 60.
The range of frequencies with
close to constant gain is known
as the bandwidth.
The upper and lower
frequencies defining the
bandwidth, called the corner
or cutoff frequencies,
An amplifier with a
narrow bandwidth will
change the shape of
an input time varying
signal by an effect
known as frequency
distortion.
Although the gain of
an amplifier will be
relatively constant
over the bandwidth,
another characteristic
of the output signal,
the phase angle, may
change significantly.
The voltage input signal to the amplifier:
The output signal will be:
The shape of the signal is
changed dramatically and
shows significant phase
distortion'
Another important characteristic of amplifiers is known as commonmode rejection ratio (CMRR).
When the same voltage (relative to ground) is applied to the two input terminals, the input
is known as a common-mode voltage . Instrumentation amplifier will produce an output in
response to differential-mode voltages but will produce no output in response to commonmode voltages.
The measure of the relative response to differential- and common
mode voltages is described by common-mode rejection ratio,
defined by
Gdif is the gain for a differential-mode voltage
Gcm is the gain for a common-mode voltage
High-quality amplifiers often have a CMRR in
excess of 100 dB.
Input-loading and output loading are potential problems that can occur when
Using an amplifier (and when using many other signal-conditioning devices)'
CHAPTER 3 Continued…
Measurement Systems with Electrical Signals
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displacement,
linear velocity,
Angular
velocity,
acceleration,
force,
pressure,
temperature,
heat flux,
humidity,
fluid flow rate,
light intensity,
Chemical
Characteristic
chemical
composition.
• gage,
• cell,
• pickup,
• Transmitter
Converts
physical
changes to
electrical pulses
• Amplification
• Attenuation
• Filtering (highpass,
Iowpass, bandpass, or
bandstop)
• Differentiation
• Integration
• Linearization
• Converting a resistance to
a voltage signal
• Converting a current
signal to a voltage signal
Amplifiers Using Operational Amplifiers
• Practical signal amplifiers can be constructed using a common, low-cost.
Integrated circuit
• component called an operational amplifier, or simply an op-amp. An opamp is represented schematically by a triangular symbol as shown in
figure below.
Figure 3.10
• The input voltages (Vn , Vp) are applied to two input terminals (labeled +
and -), and
• the output voltage (Vo) appears through a single output terminal.
• There are two power supply terminals, labeled V+ and V-.
The op-amp gain is given by small g to distinguish it from G, the gain
of amplifier circuits using the op-amp as a component. The output of
the op-amp in the open-loop configuration shown in Figure 3.10 is
given by
The current flow from point B into the op-amp negative terminal will be
small due to the high op-amp input impedance and will be neglected.
Analyzing the circuit, the current through resistor
Above fc, the gain
starts to decrease,
or roll off, and this
roll-off occurs at a
rate of 6 dB per
octave.
“octave” is a doubling of the frequency…
This roll off in gain at high frequencies is an inherent characteristic of op-amps.
The cutoff frequency, fc, depends on the low-frequency gain of the amplifier-the
higher the gain, the lower is fc . This low-frequency gain-cutoff frequency relationship
is described by a parameter called the gain-bandwidth product (GBP). For most op-ampbased amplifiers, the product of the low-frequency gain and the bandwidth is a
constant. Since the lower frequency limit of the bandwidth is zero, the upper cutoff
frequency can be evaluated from
Although the gain is constant over the bandwidth, the phase angle between the input
and the output, @, shows a strong variation with frequency.
For the non-inverting amplifier in Figure 3.11, the phase-angle variation with
frequency is given by