Signal conditioning is useful in making of the circuits related to small signals and setting the signals ranges. Sensors are having different outputs and we can set the desired ranges of the voltages as per the necessity.
• Signal conditioning refers to operations performed on
signals to convert them to a form suitable for interfacing
with other elements in the process-control loop.
• In this chapter, we are concerned only with analog
conversions, where the conditioned output is still an
analog representation of the variable.
• Even in applications involving digital processing, some
type of analog conditioning is usually required before
analog-to-digital conversion is made.
3. PRINCIPLES OF ANALOG SIGNAL
• 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
• 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.
4. PRINCIPLES OF ANALOG SIGNAL
• A sensor measures a variable by converting information
about that variable into a dependent signal of either
electrical or pneumatic nature.
• To develop such transducers, we take advantage of
fortuitous circumstances in nature where a dynamic
variable influences some characteristic of a material.
• We often describe the effect of the signal conditioning by
the term transfer function. By this term we mean the effect
of the signal conditioning on the input signal.
5. PRINCIPLES OF ANALOG SIGNAL
• We perform the required signal conditioning by first
changing the zero to occur when the sensor output is 0.2
• This can be done by simply subtracting 0.2 from the
sensor output, which is called a zero shift, or a bias
• 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.
6. PRINCIPLES OF ANALOG SIGNAL
• We distinguish between amplification and attenuation by
noting whether the gain of the amplifier is greater than or
less than unity.
• In designing bias and amplifier circuits, we must be
concerned with issues such as the frequency response,
output impedance, and input impedance.
• As pointed out at the beginning of this section, the
process-control designer has little choice of the
characteristics of a sensor output versus a process
variable. Often, the dependence that exists between input
and output is nonlinear.
7. PRINCIPLES OF ANALOG SIGNAL
• Historically, specialized analog circuits were devised to
• For example, suppose a sensor output varied nonlinearly
with a process variable, as shown in Figure 1a.
8. PRINCIPLES OF ANALOG SIGNAL
• A linearization circuit, indicated symbolically in Figure 1b,
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 1c.
9. PRINCIPLES OF ANALOG SIGNAL
• 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
• In these cases, it is necessary to provide a circuit to
convert this resistance change either to a voltage or a
• Signal Transmission: An important type of conversion is
associated with the process-control standard of
transmitting signals as 4- to 20-mA current levels in wire.
10. PRINCIPLES OF ANALOG SIGNAL
• This gives rise to the need for converting resistance and
voltage levels to an appropriate current level at the
transmitting end and for converting the current back to
voltage at the receiving end.
• Of course, current transmission is used because such a
signal is independent of load variations other than
accidental shunt conditions that may draw off some
• Thus, voltage-to-current and current-to-voltage converters
are often required.
11. PRINCIPLES OF ANALOG SIGNAL
• Digital Interface: The use of computers in process
control requires conversion of analog data into a digital
format by integrated circuit devices called analog-to-digital
• Analog signal conversion is usually required to adjust the
analog measurement signal to match the input
requirements of the ADC.
• For example, the ADC may need a voltage that varies
between 0 and 5 V, but the sensor provides a signal that
varies from 30 to 80 mV. Signal conversion circuits can be
developed to interface the output to the required ADC
12. PRINCIPLES OF ANALOG SIGNAL
• Filtering and Impedance Matching
• Two other common signal-conditioning requirements are
filtering and matching impedance.
• Often, spurious signals of considerable strength are
present in the industrial environment, such as the 50/60-
Hz line frequency signals.
• In many cases, it is necessary to use high-pass, low-pass,
or notch filters to eliminate unwanted signals from the
13. PRINCIPLES OF ANALOG SIGNAL
• Impedance matching is an important element of signal
conditioning when transducer internal impedance or line
impedance can cause errors in measurement of a
• Both active and passive networks are employed to
provide such matching.
• 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.
14. PRINCIPLES OF ANALOG SIGNAL
• Figure shows such an element modeled as a voltage Vx
and a resistance Rx . Now suppose a load, RL, is
connected across the output of the element as shown in
• This could be the input resistance of an amplifier, for
15. PRINCIPLES OF ANALOG SIGNAL
• A current will flow, and voltage will be dropped across Rx . It is
easy to calculate that the loaded output voltage will thus be
• The voltage that appears across the load is reduced by the
voltage dropped across the internal resistance.
• This equation shows how the effects of loading can be
reduced. Clearly, the objective will be to make RL much larger
than Rx—that is RL>>Rx, . The following example shows how
the effects of loading can compromise our measurements.
19. PASSIVE CIRCUITS
• Modern active circuits often replace these techniques,
there are still many applications where their particular
advantages make them useful.
• Bridge circuits are used primarily as an accurate means of
measuring changes in impedance. Such circuits are
particularly useful when the fractional changes in
impedance are very small.
• Another common type of passive circuit involved in signal
conditioning is for filtering unwanted frequencies from the
20. PASSIVE CIRCUITS
• It is quite common in the industrial environment to find
signals that prossess high- and/or low-frequency noise as
well as the desired measurement data.
• Divider Circuits
The elementary voltage divider
shown in Figure
often can be used to provide
conversion of resistance variation
into a voltage variation. The voltage
of such a divider is given by the
21. PASSIVE CIRCUITS
• Either R1 or R2 can be the sensor whose resistance
varies with some measured variable. It is important to
consider the following issues when using a divider for
conversion of resistance to voltage variation:
• The variation of VD with either R1 or R2 is nonlinear; that
is, even if the resistance varies linearly with the measured
variable, the divider voltage will not vary linearly.
• The effective output impedance of the divider is the
parallel combination of R1 and R2 . This may not
necessarily be high, so loading effects must be
22. PASSIVE CIRCUITS
• In a divider circuit, current flows through both resistors;
that is, power will be dissipated by both, including the
sensor. The power rating of both the resistor and sensor
must be considered.
24. PASSIVE CIRCUITS
• Bridge Circuits
• Bridge circuits are used to convert impedance variations
into voltage variations. One of the advantages of the
bridge for this task is that it can be designed so the
voltage produced varies around zero.
• This means that amplification can be used to increase the
voltage level for increased sensitivity to variation of
• Another application of bridge circuits is in the precise
static measurement of an impedance.
25. PASSIVE CIRCUITS
• Wheatstone Bridge The simplest and most common
bridge circuit is the dc Wheatstone bridge, as shown in
Figure . This network is used in signal-conditioning
applications where a sensor changes resistance with
process variable changes.
26. PASSIVE CIRCUITS
• The object labeled D is a voltage detector used to
compare the potentials of points a and b of the network. In
most modern applications, the detector is a very high-
input impedance differential amplifier.
• In some cases, a highly sensitive galvanometer with a
relatively low impedance may be used, especially for
calibration purposes and spot measurement instruments.
• For our initial analysis, assume the detector impedance is
infinite—that is, an open circuit. In this case, the potential
difference, delta V , between points a and b is simply
31. PASSIVE CIRCUITS
• Galvanometer Detector The use of a galvanometer as a
null detector in the bridge circuit introduces some
differences in our calculations because the detector
resistance may be low and because we must determine
the bridge offset as current offset.
Fig: When a galvanometer is
used for a null detector, it is
convenient to use the Thévenin
equivalent circuit of the bridge
32. PASSIVE CIRCUITS
• The Thévenin resistance is found by replacing the supply
voltage by its internal resistance and calculating the
resistance between terminals a and b of the network.
• We may assume that the internal resistance of the supply
is negligible compared to the bridge arm resistances. It is
left as an exercise for the reader to show that the
Thévenin resistance seen at points a and b of the bridge
33. PASSIVE CIRCUITS
• The Thévenin equivalent circuit for the bridge enables us
to easily determine the current through any galvanometer
with internal resistance, RG , as shown in Figure . In
particular, the offset current is
36. PASSIVE CIRCUITS
• Lead Compensation In many process-control applications, a
bridge circuit may be located at considerable distance from the
sensor whose resistance changes are to be measured.
• In such cases, the remaining fixed bridge resistors can be
chosen to account for the resistance of leads required to
connect the bridge to the sensor.
• There are many effects that can change the resistance of the
long lead wires on a transient basis, such as frequency,
temperature, stress, and chemical vapors.
• This problem is reduced using lead compensation, where any
changes in lead resistance are introduced equally into two
(both) arms of the bridge circuit, thus causing no effective
change in bridge offset.
37. PASSIVE CIRCUITS
• Current Balance Bridge One disadvantage of the simple
Wheatstone bridge is the need to obtain a null by variation
of resistors in bridge arms.
• In the past, many process-control applications used a
feedback system in which the bridge offset voltage was
amplified and used to drive a motor whose shaft altered a
variable resistor to re-null the bridge.
• This method uses a current to null the bridge. A closed-
loop system can even be constructed that provides the
bridge with a self-nulling ability.
38. PASSIVE CIRCUITS
• Potential Measurements Using Bridges A bridge circuit
is also useful to measure small potentials at a very high
impedance, using either a conventional Wheatstone
bridge or a current balance bridge.
• ac Bridges The bridge concept described in this section
can be applied to the matching of impedances in general,
as well as to resistances.
• The analysis of bridge behavior is basically the same as
in the previous treatment, but impedances replace
39. PASSIVE CIRCUITS
• RC Filters
• To eliminate unwanted noise signals from measurements,
it is often necessary to use circuits that block certain
frequencies or bands of frequencies. These circuits are
called filters. A simple filter can be constructed from a
single resistor and a single capacitor.
• Low-pass RC Filter The simple circuit shown in Figure is
called a low-pass RC filter. It is called low-pass because it
blocks high frequencies and passes low frequencies.
40. PASSIVE CIRCUITS
• In the case of the low-pass RC filter, the variation of
rejection with frequency is shown in Figure . In this graph,
the vertical is the ratio of output voltage to input voltage
without regard to phase.
41. PASSIVE CIRCUITS
• The horizontal is actually the logarithm of the ratio of the
input signal frequency to a critical frequency.
• This critical frequency is that frequency for which the ratio
of the output to the input voltage is approximately 0.707.
In terms of the resistor and capacitor, the critical
frequency is given by
• The output-to-input voltage ratio for any signal frequency
can be determined graphically from Figure or can be
42. PASSIVE CIRCUITS
• Design Methods A typical filter design is accomplished by
finding the critical frequency, Fc , that will satisfy the design
• 1. Select a standard capacitor value in the MicroF to pF range.
• 2. Calculate the required resistance value. If it is below 1KOhm
or above 1MOhm , try a different value of capacitor so that the
required resistance falls within this range, which will avoid
noise and loading problems.
• 3. If design flexibility allows, use the nearest standard value of
resistance to that calculated.
• 4. Always remember that components such as resistors and
capacitors have a tolerance in their indicated values. This must
be considered in your design. Quite often, capacitors have a
tolerance as high as +-20% .
43. PASSIVE CIRCUITS
• 5. If exact values are necessary, it is usually easiest to
select a capacitor, measure its value, and then calculate
the value of the required resistance. Then a trimmer
(variable) resistor can be used to obtain the required
47. PASSIVE CIRCUITS
• High-Pass RC Filter A high-pass filter passes high
frequencies (no rejection) and blocks (rejects) low
• A filter of this type can be constructed using a resistor and
a capacitor, as shown in the schematic of Figure .
• Similar to the low-pass filter, the rejection is not sharp in
frequency but distributed over a range around a critical
51. PASSIVE CIRCUITS
• Band-Pass RC Filter It is possible to construct a filter that
blocks frequencies below a low limit and above a high
limit while passing frequencies between the limits. These
are called band-pass filters.
• The lower critical frequency, fL, defines the frequency
below which the ratio of output voltage to input voltage is
down by at least 3 dB, or 0.707.
• The higher critical frequency,fH , defines the frequency
above which the ratio of output voltage to input voltage is
down by at least 3 dB, or 0.707. The frequency range
between fH and fL is called the passband.
52. PASSIVE CIRCUITS
• Equation gives the ratio of the magnitude of output
voltage to input voltage for this filter as a function of
frequency. This equation includes the effects of loading by
a constant, r, which is the ratio of the high-pass filter
resistance to the low-pass filter resistance, r=RH/RL
55. PASSIVE CIRCUITS
• Band-Reject Filter Another kind of filter of some
importance is one that blocks a specific range of
frequencies. Often such a filter is used to reject a
particular frequency or a small range of frequencies that
are interfering with a data signal.
56. OP AMP CIRCUITS IN
• Voltage Follower : Figure shows an op amp circuit with
unity gain and very high input impedance. The input
impedance is essentially the input impedance of the op
amp itself, which can be greater than 100 MOhm .
The op amp voltage follower.
This circuit has unity gain
but very high input
57. OP AMP CIRCUITS IN
• The unity gain voltage follower is essentially an
impedance transformer in the sense of converting a
voltage at high impedance to the same voltage at low
• Inverting Amplifier: Equation shows that this circuit
inverts the input signal and may have either attenuation or
gain, depending on the ratio of input resistance,R1 , and
feedback resistance,R2 .
58. OP AMP CIRCUITS IN
• Summing Amplifier A common modification of the
inverting amplifier is an amplifier that sums or adds two or
more applied voltages.
• This circuit is shown in Figure for the case of summing
two input voltages. The transfer function of this amplifier is
59. OP AMP CIRCUITS IN
• Example : Develop an op amp circuit that can provide an
output voltage related to the input voltage by
60. OP AMP CIRCUITS IN
• Non inverting Amplifier :A non inverting amplifier may
be constructed from an op amp, as shown in Figure . The
gain of this circuit is found by summing the currents at the
summing point, S, and using the fact that the summing
point voltage is Vin so that no voltage difference appears
across the input terminals.
63. OP AMP CIRCUITS IN
• Differential Instrumentation Amplifier: An ideal
differential amplifier provides an output voltage with
respect to ground that is some gain times the difference
between two input voltages
• where A is the differential gain and both Va and Vb are
voltages with respect to ground. Such an amplifier plays
an important role in instrumentation and measurement.
64. OP AMP CIRCUITS IN
• Common Mode Rejection: To define the degree to which
a differential amplifier approaches the ideal, we use the
following definitions. The common-mode input voltage is
the average of voltage applied to the two input terminals,
• An ideal differential amplifier will not have any output that
depends on the value of the common-mode voltage; that
is, the circuit gain for common-mode voltage, Acm , will be
65. OP AMP CIRCUITS IN
• The common-mode rejection ratio (CMRR) of a differential
amplifier is defined as the ratio of the differential gain to
the common-mode gain. The common-mode rejection
(CMR) is the CMRR expressed in dB,
66. OP AMP CIRCUITS IN
• Differential Amplifier There are a number of op amp
circuits for differential amplifiers. The most common circuit
for this amplifier is shown in Figure
67. OP AMP CIRCUITS IN
• If the resistors are not well matched, the CMR will be
poor. The circuit of the last Figure has a disadvantage in
that its input impedance is not very high and, further, is
not the same for the two inputs. For this reason, voltage
followers are often used on the input to provide high input
impedance. The result is called an instrumentation
68. OP AMP CIRCUITS IN
• Instrumentation Amplifier Differential amplifiers with
high input impedance and low output impedance are
given the special name of instrumentation amplifier. They
find a host of applications in process-measurement
systems, principally as the initial stage of amplification for
• Example: A sensor outputs a range of 20.0 to 250 mV as
a variable varies over its range. Develop signal
conditioning so that this becomes 0 to 5 V. The circuit
must have very high input impedance.
69. OP AMP CIRCUITS IN
• Solution: A logical way to approach problems of this sort is
to develop an equation for the output in terms of the input
• The equation is that of a straight line; we can then write
• where m is the slope of the line and represents the
gain(m>1) or attenuation(m<1) required, and Vo is the
intercept; that is, the value Vout would be Vo if Vin=0.
72. OP AMP CIRCUITS IN
• Voltage-to-Current Converter: Because signals in
process control are most often transmitted as a current,
specifically 4 to 20 mA, it is often necessary to employ a
linear voltage-to-current converter. Such a circuit must be
capable of sinking a current into a number of different
loads without changing the voltage-to-current transfer
74. OP AMP CIRCUITS IN
• Current-to-Voltage Converter: At the receiving end of
the process-control signal transmission system, we often
need to convert the current back into a voltage. This can
be done most easily with the circuit shown in Figure . This
circuit provides an output voltage given by
75. OP AMP CIRCUITS IN
• Another op amp circuit to be considered is the integrator.
This configuration, shown in Figure , consists of an input
resistor and a feedback capacitor. Using the ideal
analysis, we can sum the currents at the summing point
77. OP AMP CIRCUITS IN
• It is also possible to construct an op amp circuit with an
output proportional to the derivative of the input voltage.
This circuit, which is shown in Figure , is realized with only
a single capacitor and a single resistor, as in the case of
the integrator. Using ideal analysis to sum currents at the
summing point gives the equation
79. DESIGN GUIDELINES
• For Design Guidelines: See the topic no 6 in the chapter
of Analog signal Conditioning
• Reference Book:
• Curtis D. Johnson. “Process Control Instrumentation
Technology”, Prentice Hall, 8/E, 2005 ISBN-10:
0131194577 • ISBN-13: 9780131194571
• Curtis D. Johnson. “Process Control Instrumentation
Technology”, Prentice Hall, 8/E, 2005 ISBN-10:
0131194577 • ISBN-13: 9780131194571