Signal conditioning
Noisy
• Key Functions of Signal Conditioning:
Amplification
Filter
 Attenuation
 Isolation
 Linearization
ATTENUATION
• Attenuation is a general term that refers to any reduction in the
strength of a signal.
• occurs with any type of signal, whether digital or analog.
• Most data acquisition system inputs can measure voltages only
within a range of 5 to 10 V.
• Voltages higher than this must be attenuated.
• Simplest attenuation circuit
𝑉𝑜𝑢𝑡 =
𝑅2
𝑉𝑖𝑛 (
)
𝑅1 +𝑅2
• It is essential that any attenuator or voltage divider is driven from a low
impedance source
• the load (the impedance connected to the output) must be high compared to the
divider output impedance.
• A passive attenuator circuit has an insertion loss of -32 dB
and an output voltage of 50 mV. What will be the value of
the input voltage?
Gain in dB = 20 log 10
-32 = 20 log 10
-1.6 = log 10
Vout
Vin
Vout
Vin
Use antilog (log -1)
Vout
Vin
Vout
= 0.025
Vin
0.050 V
Vin =
=𝟐𝐕
0.025
• Buffered Voltage Divider
However, a simple voltage divider
circuit is considered to have a high
output impedance.
But this can be overcome using unitygain buffer amplifiers at the divider
outputs.
A dedicated unity-gain buffer has highinput impedance in the MΩ range and
the buffers’ output impedance is
extremely low
An op amp or a transistor serves as an
impedance matching buffer to prevent
the load from affecting the divider’s
output voltage.
i. Find the gain
Vout
Vin
ii. Convert the gain in dB
Note: The total gain in dB can also be calculated as follows:
20 log10 5 = 13.98
20 log10 0.5 = -6.02
20 log10 4 = 12.04
By taking the sum, the total gain in dB = 20
SUMMARY
• An attenuator is a device that reduces the amplitude or
power of a signal without distorting the signal waveform
• An attenuator is effectively the opposite of an amplifier. An
amplifier provides gain while an attenuator provides loss,
or gain less than 1 (unity).
ISOLATION
Isolated signal conditioning products protect and preserve valuable
measurements and control signals, as well as equipment, from the dangerous
and degrading effects of noise, transient power surges, internal ground loops,
and other hazards present in industrial environments.
Methods of Implementing Isolation
Isolation requires signals to be transmitted across an isolation barrier without
any direct electrical contact.
Light-emitting diodes (LEDs), capacitors, and inductors are three commonly
available components that allow electrical signal transmission without any
direct contact.
The principles on which these devices are based form the core of the three
most common technologies for isolation –
i. optical,
ii. capacitive
iii. inductive coupling.
• Optical Isolation
Optical isolation uses an LED along with a photodetector device to
transmit signals across an isolation barrier using light as the method of
data translation.
LEDs produce
light when a
voltage is
applied across
them
A photodetector
receives the light
transmitted by the LED
and converts it back to
the original signal.
ADVANTAGE:
• immunity to electrical and magnetic noise
DISADVANTAGE:
• transmission speed, which is restricted by the LED switching speed, high-power
dissipation, and LED wear.
B
ID
A circuit with optical isolation is shown in the figure. The photodetector has a
voltage of V = 0.4 V when it is turned on.
a. Explain how the circuit works to produce the output of the inverter to be HIGH
or LOW
b. If the voltage drop of the Light Emitting Diode is 1.4 V, calculate the value of
the current ID.
Part (a)
When the switch is closed, node B will be connected to ground (0 V=low),
hence, the LED is off, and the photodiode will not be on and there is no
current flow. So, node A will be equivalent to 5 V and the output of the
inverter will be 0
When the switch is opened, node B will be equivalent to 2.4 V (high) as
the 24 V power supply is now connected to the circuit. This time, the LED
is on, and the photodiode will be on as well. So, node A will take the value
of V = 0.4 V and the output of the inverter will be 1
Part (b)
KVL: 2200 ID+100 ID+1.4 – 24 = 0
ID = 9.826 mA
• Capacitive Isolation
Capacitive isolation is based on an electric field that changes with the
level of charge on a capacitor plate. This charge is detected across an
isolation barrier and is proportional to the level of the measured signal.
ADVANTAGE:
• immunity to magnetic noise.
• support faster data transmission rates
DISADVANTAGE:
• capacitive coupling involves the use of electric fields for data transmission, it can
be susceptible to interference from external electric fields.
• Inductive Coupling Isolation
− current through a coil of wire produces a magnetic field.
− current can be induced in a second coil by placing it in close vicinity of the
changing magnetic field from the first coil.
− The voltage and current induced in the second coil depend on the rate of
current change through the first.
− This principle is called mutual induction and forms the basis of inductive
isolation.
Inductive isolation uses a pair of coils
separated by a layer of insulation.
Insulation prevents any physical signal
transmission
ADVANTAGE:
• support faster data transmission rates
DISADVANTAGE:
• susceptible to interference from external
magnetic fields.
Signals can be transmitted by
varying current flowing through one
of the coils, which causes a similar
current to be induced in the second
coil across the insulation barrier.
LINEARIZATION
Most sensor outputs are non-linear with respect to the applied stimulus. As a
result, their outputs must often be linearized in order to yield the correct
measurements.
For example:
Thermocouples, for example, have a nonlinear relationship from input
temperature to output voltage
Two methods of linearization (can be analog or digital)
i. Software linearization
ii. Hardware linearization
• Basically, it is a process of mapping/linearizing the output from the
sensors with the stimulus in order to achieve the correct
measurements
By taking
the slope
of these
data
Plot of voltage versus temperature
for three types of thermocouple
Plot of nominal Seebeck coefficient
versus temperature for three types of
thermocouple
• An ideal linear thermocouple would have a constant
Seebeck coefficient
• selecting a thermocouple for a particular temperature
range, we should choose one whose Seebeck coefficient
varies as little as possible over that range
For range between
250C to 500 C
For range between
400C to 750 C
Type S has wider
range of useful
temperature
OPTICAL DISTANCE SENSORS
1
= 𝑎𝐷 + 𝑏
𝑉𝑠
where a and b are coefficients.
REF: http://www.edn.com/design/analog/4371308/Linearize-optical-distance-sensorswith-a-voltage-to-frequency-converter
An NTC thermistor has a resistance of R0 = 30 kΩ at T0 = 20C and
B = 4000 K for the temperature range of interest. The value of RT can be
calculated using equation shown below. If at temperature of 15C and 35C,
the gain should be (in terms of magnitude), 0.9 and 1.1 respectively ,
calculate the values of the resistors RP and RG if RS = 17.8 k
(Note: the temperature must be converted to Kelvin)
𝑅𝑇 = 𝑅0
𝐵(1 𝑇− 1 𝑇 )
0
𝑒
RP = 27 k
RG = 16.4 k
Linearization
circuit