Signal conditioning is a set of operations performed on a signal that makes it suitable for interfacing with other devices or systems. Signal conditioners are the actual devices that perform this operation. These devices have an input and an output. Normally the input is a sensor that measures a variable, not necessarily and electrical signal.

The signal conditioning process is also known as a transfer function because the final effect is to convert an input signal (or measurement) into a suitable output signal. For instance, when a temperature sensor measures the temperature of a system or environment, the output of the sensor (temperature) is not suitable to be an input signal to an electrical system. Therefore, the temperature measurement must be converted into an electrical signal.

The following diagram shows the evolution of a signal from the sensor through the central processing unit and to the output or load:

Signal Conditioner Functions

Signal conditioners provide filtering, amplification, converting, and/or other processes required to make sensor outputs suitable for reading by computer boards. They are used primarily for data acquisition, in which sensor signals must be normalized and filtered to levels suitable for analog-to-digital conversion. The digital signal is then available to be analyzed or interpreted by a computerized device.

Filtering

Filters can be constructed from either active or passive components. A passive filter uses only resistors, capacitors, and inductors with a maximum gain of one. An active filter uses passive components and active components like operational amplifiers and transistors. They have a higher gain with sharper frequency response curves.

The filter architecture can be analog or digital.

Analog (RC) - Analog filters are designed with resistors and capacitors. They are used for analog signals only, and are often used in low-noise requirement applications.

Digital (FIR, IIR) - Digital filters are designed with solid-state components and used for digital signals and quantized signals from a sample-and-hold amplifier. This category includes finite impulse response (FIR) and infinite impulse response (IIR) filters. Digital filtering can approach ideal bandpass characteristics.

Filter Type

The function of the filter is to separate the signal's frequency spectrum into valid data while blocking noise. The standard types of filter responses are low-pass, high-pass, band-pass, and band-reject (or notch filter). Filters are selected based on the frequency of the signal to be analyzed.

Low-pass filters block high frequency components; or allow the passage of low frequency signals. A simple passive low-pass filter can be constructed with only a resistor and a capacitor.

Band-pass filters allow the passage of signals within a range of frequencies and blocks signals with frequencies below the smallest frequency in the range and above the highest frequency in the range. If the range (band) of frequencies is between f₁ and f₂ then the filter allows the passage of signals with frequencies between f₁ and f₂ only.

Band-notch filters, also known as a band-reject filters, allow the passage of all frequencies with the exception of signals within a range of frequencies.

Amplification

Amplification is a process which increases (amplifies) the signal for possessing or digitization. Signal amplifiers often include electronic components that amplify signals without producing significant amounts of thermal noise. In some applications a signal must be amplified or attenuated in order to drive a circuit or a system. There are many types of amplifiers used in signal conditioning including the following:

• Voltage followers have a unity gain, so the output signal is a reproduction of the input signal. This type of amplifier is mainly used as an impedance matching device.
• Isolation amplifiers are designed specifically to isolate high DC levels from the data acquisition device while passing the relatively small AC or differential signal. The inputs and outputs are electrically isolated.
• Instrumentation amplifiers are differential amplifiers that have been optimized for use with DC signals. They are characterized by high gain, high common mode rejection ratio (CMRR), and high input impedance.
• Sample-and-hold amplifiers freeze analog voltage instantly. During this process the HOLD command is issued and analog voltage is available for an extended period.

Signal Converting

In many instances it is required to convert a signal from one type to another, in order to accommodate the driving input of circuits. Some important signal converters are:

• Current-to-voltage converters scale and convert current signal input to the desired output voltage range.
• Voltage-to-frequency converters accept a voltage signal and convert its analog level to a signal with a corresponding frequency.
• Frequency-to-voltage converters accept a signal and convert its frequency to a corresponding analog voltage level.
• Current loop converters convert an analog or digital signal to a current loop output such as 4-20 mA or 0-20 mA.
• Charge converters convert the charge output from a piezoelectric, capacitive or other charge-producing sensor to a signal such as analog voltage or current.

Digital Conversion

Data acquisition is the digitizing and processing of multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes. Analog sensors and signals are first normalized by the use of filters, amplifiers and signal converters. The next wave of the signal chain is the exchange of this signal to a digitized format. The two most important conversion functions in this phase of the process are analog-to-digital conversion and digital-to-analog conversion.

Analog-to-Digital ConverterAn analog-to-digital (ADC) converter is a device that accepts, as input, an analog signal and at the output, produces an equivalent digital signal. Most of our sensors and transducers produce analog signals that have to be converted to digital signal in order to be processed by computers or other digital device. There are several types of analog-to-digital (ADC) converters including: direct conversion, successive-approximation, integrating and sigma-delta ADCs.

Digital-to-Analog Converter  A digital-to-analog (DAC) converter produces the reverse operation of an ADC. These devices accept digital signals and convert them to analog signals (normally voltages).

Selection Criteria

After defining the function of the signal conditioner, the form factor, device specifications, signal inputs, sensor inputs, excitation, outputs and user interface are important parameters to consider when searching for signal conditioners.

Form Factor

Common form factors for signal conditioners include circuit board, panel or chassis mount, modular bay or slot system, rack mount, DIN rail, and stand-alone.

• Printed circuit boards(PCBs) attach to enclosures or plug into computer backplanes.
• Panel or chassis mounts are used to install the device in cabinets, enclosures, or panels with bolts.
• Modular style units include stackable units that dock in bays, slots, or boxes.
• Rack units that fit inside a standard 19” telecommunications rack.
• Devices can be designed for mount on a Standard Deutsches Institute for Normung (DIN) rail, which is a German standard.
• Benchtop or freestanding devices often feature full casings or cabinets and integral interfaces.

Device Specifications

Device specifications that are important to consider when searching for signal conditioning products include analog input channels, digital I/O channels, and accuracy.

Analog signals are a wave signal which means that the value changes steadily over time and can have any value in a range. Signal converters with analog inputs can have multiple channels. Channels are either single-end or differential.

Single-end inputs have only one low wire shared by all inputs. For example a board could have 2 single-end inputs; there will be two input lines and one ground line. Single-end inputs are less expensive and allow for twice the number of inputs in the same size wiring connector since they only require one analog input and one ground input which is shared by all the inputs. They save space and are easier to install. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs.

Differential channels have two inputs. The voltage is the signal processed between the two inputs. The board will have one signal and one ground pin for each input to allow for measurement voltage difference between two signals tied to the same ground. Differential channels provide excellent common-mode noise rejection. This type of input should be used when EMI, RIF or noise is present.

Digital signals do not have 'in between' values. They are an on or off signal producing a square wave. Digital signals break down the information into binary code, which is a series of 1sand 0s. The data receiver reassembles the code into useful information. Digital signal allows users to send more information in a smaller space.

Accuracy is defined as the difference (error) between the true value and the indication expressed as percent of the span. Accuracy, which is represented as a percentage of a full-scale measurement range, depends on signal conditioning linearity, hysteresis, and temperature considerations. It includes the combined effects of method, observer, apparatus and environment.

Static accuracy is the combined effects of Linearity, Hysteresis, and Repeatability. It is expressed as +/- percentage of full scale output. The static error band is a good measure of the accuracy that can be expected at constant temperature.

• Linearity is the deviation of a calibration curve from a specified straight line. One way to measure linearity is to use the least squares method, which gives a best fit straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.
• Repeatability is the ability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.
• Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure. Temperatures hysteresis is the sensor's ability to give the same output at a given temperature before and after a temperature cycle.

Signal Inputs

The input signal can have a variety of specifications as it enters the converter. The type of converter selected depends largely on the type of input signal from the system and the desired output signal. The input signal could have properties such as:

• DC type voltage and/or current
• AC type voltage and/or current
• Frequency waveforms for varying frequency, pulse or specialized waveforms.
• Chargewhich comes from a piezoelectric device and usually requires conditioning.

Sensor Inputs

Sensor inputs can be accelerometer, thermocouple, thermistor, RTD, strain gauge or bridge, and LVDT or RVDT. Specialized inputs include encoder, counter or tachometer, timer or clock, relay or switch, and other specialized inputs.

Excitation

Sensors can be classified as either active or passive devices. Passive devices like thermocouples can generate a signal without a power supply. Active sensors need a power supply in order to control the flow of electrons and make a measurement. In some cases active sensors are powered by the signal conditioner. The output from the signal conditioner that powers the device is referred to as an excitation source. Signal excitation can either be a voltage or current output. The following schematic showcases how an excitation voltage powers a Wheatstone bridge:

Outputs

Outputs for signal conditioning products can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer, and other specialized outputs.

User Interface

Signal converters have several user interfaces available that allow the user to make adjustments to the system.

• A front panel is a local interface with integral controls, a keypad, and/or display on the panel of the unit
• Computer programmable converters are interfaced with a separate supervisory or host computer.
• Touch screens have a visual display which interacts with the user through touch. The user can directly put in information through the contact-sensitive screen.
• Remote and handheld devices can be mobile while the user enters program parameters.

Devices with no user interface for input or programming are used for storage. The downloading and processing is done in another location.

Resources

Dataforth - Signal Conditioning Tutorial

Engineers-Excel.com - Analysis and Simulation of Filters

University of St. Andrews - High Pass Filter

Interfacebus.com - Electronic Engineering Dictionary

Electronics-Tutorials.WS - Basic Electronics Tutorials