Signal conditioning Noisy Key Functions of Signal Conditioning

  • Slides: 17
Download presentation
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. Voltage Divider • 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 = Vin ( R 2 / R

• Simplest attenuation circuit • Vout = Vin ( R 2 / R 1+R 2) • 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. • It is generally considered that the signal source should have an impedance of at most 1/10 that of the attenuator, and the load should have an impedance (at least) 10 times the attenuator's output impedance.

There is a need to measure or monitor electrical signals from multiple sensors. So

There is a need to measure or monitor electrical signals from multiple sensors. So normally they are connected to a multiplexer

 • However, for multiplexer inputs, the output impedance of a simple voltage divider

• However, for multiplexer inputs, the output impedance of a simple voltage divider circuit is much too high • For example, consider a 10: 1 divider reading 50 V. If a 900 kΩ and a 100 kΩ resistor are chosen to provide a 1 MΩ load to the source, the impedance seen by the analog multiplexer input is about 90 kΩ, still too high for the multiplexed reading to be accurate. • When the values are both downsized by a factor of 100 so the output impedance is less than 1 kΩ, but the voltage measurement will be affected as well. • Hence, simple attenuation circuit is not practical with multiplexed inputs.

 • Buffered Voltage Divider • The low impedance loading effect of simple voltage

• Buffered Voltage Divider • The low impedance loading effect of simple voltage dividers can be overcome using unity-gain buffer amplifiers on divider outputs. • A dedicated unity-gain buffer has high-input impedance in the MΩ range and does not load down the source, as does the network in the previous example. • Moreover, the buffers’ output impedance is extremely low, which is necessary for the multiplexed analog input. An op amp or a transistor serves as an impedance matching buffer to prevent the load from affecting the divider’s output voltage.

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.

 • 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

MID TERM EXAM • 30 th March 2016 • 10 am, venue will be

MID TERM EXAM • 30 th March 2016 • 10 am, venue will be announced later • Up until Lecture 5 including the handout notes