Note-13 1
Signal Conditioning Systems 2
Generalized Measurement System: The output signal from a sensor has generally to be processed or conditioned to make it suitable for the next stage Signal conditioning refers to operations performed on signals to convert to a suitable for interfacing with other elements in the instrumentation process The raw signal may: be too small have to be amplified contain noise have to be removed be non-linear may need linearization be analog may be converted to digital be digital may be converted to analog be hazardous require isolation SIGNAL CONDITIONING 3
Elements of Signal Conditioning System 1. Conversion getting the signal into the right type of signal Conversion can mean taking the signal into a DC/AC voltage or current For example, the resistance change of a strain gauge has to be converted into a voltage change: - Resistance to voltage conversion. This can be done by using a Wheatstone bridge and using the out-of-balance voltage. Conversion can also mean taking the signal to Digital or Analog - This would require ADC (A/D Converter) or DAC (D/A Converter) 4
2. Change of signal level getting the level of the signal right It means adjusting the level (magnitude) and bias (zero value) of signal - For example, 0.1 V 0.8 V 0 V 5 V Signal conditioning circuit 0.1 V 0.8 V 0 V 0.7 V 0 V 5 V Zero shift Amplification For example, the signal from a thermocouple might be just a few milivolts. To feed the signal into an A/D converter for inputting to a microprocessor, it needs to be made much larger. Operational amplifier circuits are widely used for amplification. 5
3. Eliminating or reducing noise increasing the signal-to-noise ratio (SNR) Noise may come from variety of sources Filter or compensation circuits may be used to eliminate or reduce noise from a signal Example filters: (a) (b) (c) (d) low-pass high-pass Band-pass Band-stop (notch filters) 6
4. Linearization making the output linear of the measurand Often, the characteristic of a sensor is nonlinear Special circuit are available to linearize the signals Modern approach is to use computer software to linearize 7
5. Impedance or load matching for maximum power coupling Connecting a sensor with different impedances causes signal reflection Need a circuit to match impedance; thus to reduce signal reflection When a sensor output is connected to the next stage directly, it will introduce uncertainty in the measurement. Sensor V x R x L Vy Vx R L R x V y R L V 1 R x R x RL Rx Output voltage is reduced by the voltage drop So, a module between sensor and load is needed for proper coupling. This module is generally a buffer circuit with very high input impedance and very low output impedance. 8
6. Protection to prevent damage to the next element Normally protection is provided against high current and high voltage which may damage important components How to protect? A series resistor to limit the current to an acceptable level A fuse to break if current does exceed a safe level The use of a Zener diode circuit Zener diode can also protect against wrong polarity Opto-isolator can isolate two circuits optically 9
Amplifiers for Instrumentation 10
Introduction An amplifier is a device that amplifies a signal almost always a voltage The low voltage output of a sensor, say of a thermocouple, may be amplified to a level required by a controller or a display. Amplifiers can also be used for impedance matching purposes even when no amplification is needed. Power amplifiers, which usually connect to actuators, provides the power necessary to drive them. 11
Desirable Characteristics of an Amplifier An amplifier should have following characteristics for instrumentation applications The amplification factor should be constant (for linear scaling) The frequency response curve should be flat over the operating band The power extracted from the measurement system should be as small as possible Should not add noise to the sensor signal It should operate on the least possible auxiliary power It should have a long operating life and a high degree of reliability The amplifier should have a very high input impedance and very low output impedance Why? 12
Operational Amplifier Almost always, operational amplifiers (Op-amp) are used for amplification, as they have almost all the desirable characteristics mentioned earlier. Vary high gain, more than 10 6, ideally infinite Adjustable gain can be achieved by external circuit element High input impedance (ideally infinite) and low output impedance (ideally zero, practical values 20-100 Ω) Internal structure of an op-amp is very complex (made of a number of basic devices), but it may be thought of as a basic device and can be viewed as a basic building block of a circuit. 13
Instrumentation Amplifier It is available in single IC and is designed to have: Very high input impedance (300 MΩ), very low output impedance High common-mode rejection gain (more than 100 db) High voltage gain and can be controlled by external circuit elements 2R Total differential gain: 2 R1 R4 G d R1 R3 14
Some other Example Circuit Configurations Using Op-Amps (1) 15
Op-Amp as Comparator The ckt is designed to control temperature with a certain range. When temperature is below a certain value the thermistor resistance R 1 is more than R 2. It gives an output at the lower saturation limit of the op-amp which keeps the transistor OFF. When temperature rises and R 1 falls, the op-amp switch to +ve saturation value and switch the transistor ON. 16
Some other Example Circuit Configurations Using Op-Amps (2) 17
Differential Amplifier Example Example: The difference in the emfs of the two junctions of the thermocouple is being amplified. Assume a temperature difference of 10 C produces an emf difference of 530 µv. then the values of R 1 and R 2 can be chosen to give a circuit with an output of 10 mv. So if we select, R 1 as 10 k Ohm Then, R 2 =189 k Ohm 18