Signal Conditioners

Signal conditioners are used as part of process and control applications to manipulate the output signal of a transmitter or transducer in such a way as to ensure it meets the requirements of the next stage of the process. Signal conditioning typically involves steps that isolate, filter, amplify, or convert a sensor input signal to a proportional output signal that is transmitted to another control device or system.

Signal conditioners are a common, though not always required, component of process systems. In typical installations, signal conditioners, when needed, are installed between the sensor and the controller or other data acquisition device. Whether or not signal conditioning is required depends on the sensors included in the system as well as the requirements of equipment down the line.

Sensors measure various physical properties such as temperature, force, pressure, position, light intensity, sound, flow, etc. The output of the sensor, conditioned to provide the corresponding measurement of the physical property, is picked up by a display, recorder, process controller or another piece of equipment to initiate a process.

The output of most sensors is a relatively small voltage, current or change to resistance. Sometimes those signals need to be converted from analog to digital, voltage to frequency, or some other conversion. Sometimes the signal has to travel too long of a distance causing it to degrade. Sometimes proximity to sources of high voltage or radio signals inserts noise into the signal. Sometimes the sensor needs to be isolated from the data acquisition device to avoid earth loops or other common mode voltage problems. These are all instances when signal conditioning is needed.

Types of devices that use signal conditioning include signal filters, instrument amplifiers, sample-and-hold amplifiers, isolation amplifiers, signal isolators, multiplexers, bridge conditioners, analog-to-digital converters, digital-to-analog converters, frequency converters or translators, voltage converters or inverters, frequency-to-voltage converters, voltage-to-frequency converters, current-to-voltage converters, current loop converters, and charge converters.

Signal Conditioning Functions

Signal conditioners are equipped with a range of functions that prepare signals from sensors and transmitters for the next stage of the process. The functions you should look for are determined by the needs of your application and equipment.

Adjustability

Though sensor outputs are nearly always shown with convenient endpoint signals (e.g., 0.00 mV through 50.0mV), in the real world sensors provide approximate values such as 4.73mV through 48.9mV. Adjustable signal conditioners can scale the output signal to more convenient endpoints as well as optimize performance by calibrating out errors due to the sensors and/or the system such as voltage drops, other devices, etc.

As sensors age they are also prone to drifting. The ability to adjust zero and span through the signal conditioner increases the useful life of sensors without affecting other devices, burdening host processors, or affecting system accuracy.

Amplification

Output signals often need amplification especially when used with long cable runs. Signal conditioners amplify output signals by increasing the voltage, thus increasing the measurement resolution and sensitivity. When amplification of the signal is paramount, placing the signal conditioner closer to the signal source, or transducer, improves the measurement signal-to-noise ratio by magnifying the voltage level before it is affected by environmental noise.

Attenuation

Attenuation is the opposite of amplification. It is commonly needed when converting a signal from analog to digital and the signal strength is beyond the range of the Analog-to-Digital Converter (ADC). This is typically necessary when measuring voltages that are more than 10 V. Signal conditioners with attenuation decrease the input signal amplitude so that the conditioned signal is within ADC range

Isolation

Signal conditioners isolate output signals by using transformers or optical couplers--rather than a direct physical connection-- between circuits. This replaces a low-impedance path with a high-impedance path from input to output. Isolation breaks up potential ground loops and protects equipment from high voltage surges and spikes.

Filtering

Filtering in signal conditioning involves removing unwanted noise within a certain frequency range, such as using low-pass filters to block out high-frequency noise in electrical measurements. Filtering can also prevent aliasing from high-frequency signals.

RFI/EMI

As commercial and industrial settings become increasingly automated, radio frequency interference (RFI) and electromagnetic interference (EMI) becomes a bigger problem. Whether it is the effects of inductive load switching relays or simply someone keying up a radio transmitter, RFI/EMI protection should be considered when selecting a signal conditioner.

Excitation

Many sensors and transmitters require external voltage or current excitation. For example, strain gauges, accelerometers, thermistors, and resistance temperature detectors use some variation of an introduced electrical signal upon which to base measurement. RTD and thermistor measurements measure and convert the variation in resistance of a current source to a measurable voltage. Accelerometers often have an integrated amplifier, which requires a current excitation provided by the measurement device. Strain gauges typically are used in a Wheatstone bridge configuration with a voltage excitation source.

Linearization

Some sensors, notably thermocouples, produce output signals that are not linearly related to the physical measurement. Linearization is the process of interpreting the signal from the sensor such that it aligns with the physical measurement.

Cold-Junction Compensation

In order get absolute temperature readings from thermocouples, there needs to be a stable reference temperature. This is accomplished through cold-junction compensation. As thermocouples measure temperature based upon the difference in voltage between two dissimilar metals, cold-junction compensation uses another voltage generated at the connection between the thermocouple and terminal of your data acquisition device. Cold-junction compensation improves measurement accuracy by providing the temperature at this junction and applying the appropriate correction.

Bridge Completion

Bridge completion is required for quarter- and half-bridge sensors to comprise a four resistor Wheatstone bridge. Strain gauge signal conditioners typically provide half-bridge completion networks consisting of high-precision reference resistors. The completion resistors provide a fixed reference for detecting small voltage changes across the active resistor(s).

Some signal conditioners can also perform computation functions such as totalization, integration, pulse-width modulation, and other math operations on a signal.

If you have any questions regarding signal conditioners please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.