Signal conditioning Noisy Key Functions of Signal Conditioning

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Signal conditioning Noisy

Signal conditioning Noisy

 • Key Functions of Signal Conditioning: üAmplification üFilter q Attenuation q Isolation q

• Key Functions of Signal Conditioning: üAmplification üFilter q Attenuation q Isolation q Linearization

ATTENUATION • Attenuation is a general term that refers to any reduction in the

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 • It is essential that any attenuator or voltage

• Simplest attenuation circuit • 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 d. B

• A passive attenuator circuit has an insertion loss of -32 d. B and an output voltage of 50 m. V. What will be the value of the input voltage? Gain in d. B = 20 log 10 -32 = 20 log 10 -1. 6 = log 10 Use antilog (log -1)

 • Buffered Voltage Divider However, a simple voltage divider circuit is considered to

• 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 ii. Convert the gain in d. B Note: The total

i. Find the gain ii. Convert the gain in d. B Note: The total gain in d. B can also be calculated as follows: 20 log 10 5 = 13. 98 20 log 10 0. 5 = -6. 02 20 log 10 4 = 12. 04 By taking the sum, the total gain in d. B = 20

SUMMARY • An attenuator is a device that reduces the amplitude or power of

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

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 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

• 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

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

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+1. 4 – 24 = 0 ID = 9. 826 m. A

 • Capacitive Isolation Capacitive isolation is based on an electric field that changes

• 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

• 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

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

• 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

• 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 where a and b are coefficients. REF: http: //www. edn. com/design/analog/4371308/Linearize-optical-distance-sensorswith-a-voltage-to-frequency-converter

OPTICAL DISTANCE SENSORS 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 R 0 = 30 kΩ at T

An NTC thermistor has a resistance of R 0 = 30 kΩ at T 0 = 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) RP = 27 k RG = 16. 4 k Linearization circuit