Basics of signal conditioning

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.


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


Electrical characteristics



Low-voltage output Low sensitivity Nonlinear output

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


Low resistance (100

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


Resistive device High resistance and sensitivity Very nonlinear output

Current excitation or voltage excitation with reference resistor Linearization

Integrated circuit (IC) temperature sensor

High-level voltage or current output Linear output

Power source Moderate gain

Author Information

Travis Ferguson is signal conditioning product manager for National Instruments, Austin, Tex. Comments? Email

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Learn how to create value with re-use; gain productivity with lean automation and connectivity, and optimize panel design and construction.
Go deep: Automation tackles offshore oil challenges; Ethernet advice; Wireless robotics; Product exclusives; Digital edition exclusives
Lost in the gray scale? How to get effective HMIs; Best practices: Integrate old and new wireless systems; Smart software, networks; Service provider certifications
Fixing PID: Part 2: Tweaking controller strategy; Machine safety networks; Salary survey and career advice; Smart I/O architecture; Product exclusives
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Look at the basics of industrial wireless technologies, wireless concepts, wireless standards, and wireless best practices with Daniel E. Capano of Diversified Technical Services Inc.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.