Basics of signal conditioning

10/01/1999


Signal conditioning is a basic component of all measurement devices. It converts incoming measurements into a form acceptable to digitization hardware. Signal conditioning not only defines what types of signals the system can accept, but also defines what additional features the system has to offer. Here are some commonly encountered terms and their definitions:

  • Isolation -Isolation provides the protective barrier between digitization hardware and the real world, preventing common-mode voltage or signal spikes from damaging the measurement system. Additionally, channel-to-channel isolation prevents one input signal from arcing to another input channel and back out of the system. Finally, isolation prevents noise producing ground loops, which decrease signal quality. It is required when incoming signals have common-mode voltages higher than (10 volts, or there is a chance for large spikes in the signal.

  • Transducer excitation -Many common sensors require power to generate a signal. These include strain gauges and RTDs. Transducer excitation provides this power so sensors do not require external power sources.

  • Cold-junction compensation -This specific type of signal conditioning is required by thermocouples. Cold-junction compensation removes small voltage errors caused by connecting a thermocouple using terminal blocks made of different metals than the T/C itself. It does this by reading the ambient temperature at the point where the thermocouple connects to the system.

  • Filtering -The filtering process blocks unwanted signal frequencies arising from external noise sources (generators, motors, power lines, etc.) from incoming signals. Proper filtering also prevents anti-aliasing, where higher frequency components of a signal appear as lower frequency components.

  • Amplification/Attenuation -Amplification increases signal amplitude before digitization occurs. Amplification increases the measurement accuracy of small signals and reduces the effects of surrounding noise sources. Attenuation reduces signal amplitude before digitization occurs, increasing the signal input range capabilities of the system.

  • Linearization -Often sensors do not have a linear relationship between their signal value and the physical quantity they are measuring. A thermocouple's nonlinear temperature-to-voltage relationship is a prime example. Linearization maps the relationship between a sensor's signal value and the physical quantity it is measuring so that an incremental change in the physical quantity corresponds to a similar incremental change in the signal. It can be implemented in either the hardware or software component of a system.

  • Multiplexing -Expansion of a measurement system's I/O channel count can be expanded by passing multiple signals to the same digitization hardware. Use of multiplexing techniques allows acquisition of more signals for less money.

  • Bridge completion -This specific type of signal conditioning is used with strain gauges. If a given strain gauge is either quarter-bridge or half-bridge configuration, then the measurement device's signal conditioning must provide the necessary completion resistors to make a full Wheatstone bridge.

  • Shunt calibration -Also used with strain gauges, shunt calibration provides a comparative signal value for a precisely known strain value (load) that can be used to calibrate the measurement system.

  • Switching relays -Both electromechanical and solid-state relays can be used to control whether external system components or equipment receive power or not. Relays use low voltage (ac or dc) to control devices that can require much larger voltages and currents to operate than available in the measurement system. Relays are typically used to control motors, fans, lights, or even other relays.

Electrical Characteristics and Basic Signal Conditioning Requirements of Common Transducers

Sensor

Electrical characteristics

Requirements

Thermocouple

Low-voltage output
Low sensitivity
Nonlinear output

Reference temperature sensor (cold-junction compensation)
High amplification
Linearization

RTD

Low resistance (1003/4 typ.)
Low sensitivity
Nonlinear output

Current excitation
4-wire/3-wire configuration
Linearization

Strain gauge

Low-resistance device
Low sensitivity
Nonlinear output

Voltage or current excitation
Bridge completion
Linearization

Current-output device

Current loop output (4-20 mA typical)

Precision resistor

Thermistor

Resistive device
High resistance and sensitivity
Very nonlinear output

Current excitation or voltage
Linearization

Integrated circuit (IC) temperature
sensor

High-level voltage or current output
Linear output

Power source
Moderate gain





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