Different Types of Signal Conditioners

Signal conditioners are used in various applications to interface between the input signal and digital acquisition (DAQ) system. Therefore, the design of signal conditioning circuitry should match measuring sensor characteristics. Otherwise, the signal conditioning circuitry will attenuate the sensor input signal. As a result, the subsequent stages of DAQ will not function as per its design features.

The signal conditioner depends on the input sensor and measurement system. Due to the diverse nature of physical quantities, one sensor cannot measure all physical quantities. Instead, there is a separate sensor for different physical quantities. Therefore, the DAQ systems should match the sensor characteristics to represent the natural value accurately.

A few common signal conditioners include a thermocouple, resistance-dependent temperature (RTD), strain gauge, and linear variable differential transformer (LVDT).

 

Thermocouple Signal Conditioning

Thermocouples are common and inexpensive sensors for measuring temperature because they can withstand many environments. Signal conditioners are designed for thermocouple values with small values, typically in millivolts (mV), with non-linear outputs.

Their small range makes them easily affected by noise. The signal conditioner should be able to cancel out noise by acting as a filter. The signal conditioner should also linearize the input to either 0–10 V or 4–20 mA signals, depending upon the application requirement.

A thermocouple measures temperature when two dissimilar metals are joined together. The junction of two different metals generates voltage directly proportional to the temperature. This is the natural junction for the thermocouple to execute its measurement. However, an additional link is formed at the terminal point of the measuring device and generates a different voltage. This junction is called cold junction, and the final output includes both the cold junction’s and thermocouple’s voltages. 

To cancel out the cold junction effect, the signal conditioner should compensate for cold junction temperature; otherwise, the result does not represent the actual temperature. An external temperature sensor such as thermistor measures the cold junction temperature. 

 

Resistance-dependent Temperature (RTD) Signal Conditioning

Resistance-dependent temperature (RTD) is also a temperature measuring sensor using resistance to represent the temperature. Its resistance increases and decreases with the growth and decline in temperature. The signal conditioner must provide a current to excite the RTD to produce a voltage representing the actual temperature. 

 

Figure 1. A pt100 RTD. Image used courtesy of Amazon

 

One of the most common RTDs is the pt100 temperature sensor, which has a resistance of 100 ohms at 0 ℃. Thus, its resistance changes 0.4 ohm /℃. The resistance per degree change in temperature is small for a pt100; the signal conditioner should provide high enough gain to processes the sensor signal by the DAQ. 

RTDs come in two-wire, three-wire, and four-wire configurations. Therefore, the conditioning circuitry should provide an interface for all of these.

The two-wire RTD is often faced with voltage drop problems, where the RTD is installed at a considerable distance from the measuring circuit. Because the connecting wire’s resistance also adds to the RTD resistance, the output combines RTD and connected wire resistance. To offset resistance due to the connecting wire, four-wire RTDs are often used. 

 

Strain Gauge Signal Conditioning

Strain gauges are used to measure force. Its resistance changes as the power is applied to the sensor. Using additional components allows measured force to calculate other variables such as weight and pressure. The strain gauge is used in combination with a circuit called Wheatstone bridge. It consists of four resistive elements having known resistance. The strain gauge can be placed in any of four positions.

 

Figure 2. A quarter-bridge strain gauge circuit.

 

The signal conditioning circuitry should provide the power source to the bridge, called excitation voltage. According to specification, the excitation voltage should be stable, as the bridge output depends on the excitation voltage. Therefore, if the excitation voltage is not stable, the result will also fluctuate and produce non-stable readings.

The signal conditioning circuitry should amplify the strain gauge signal because strain gauge sensors output is small, typically in millivolts (mV) per excitement voltage. The signal conditioning circuitry amplifies the signal level to prevent noise and increase resolution. 

The signal conditioning circuity should provide provision for balancing the bridge and reference resistive elements. It should also offset the output values in case of no strain signal; there is often no strain on the sensor, but the circuitry generates a small voltage. This small voltage produces an output indicating a false value.

 

Linear Variable Differential Transformer (LVDT) Signal Conditioning

A linear variable differential transformer (LVDT) measure linear displacement. It works on the principle of the transformer, having coil and core. The coils are stationary, while the core is moveable. The core is attached to the object for which position is measured. 

 

Figure 3. Cutaway view of an LVDT. Image used courtesy of Honeywell [PDF]

 

Since LVDT works on the principle of the transformer, it involves a sinusoidal signal. Therefore, signal conditioning circuitry should calculate the magnitude and phase of the alternating current (AC) signal. The magnitude calculates the displacement value and phase to calculate the direction of movement.

The excitation source for LVDTs is the AC signal for the primary coil. The secondary signal is demodulated with the primary signal. The demodulated or direct current (DC) signal represents the actual displacement that occurred. The DC polarity indicates the direction of displacement. 

Signal conditioning is a necessary step in any DAQ system, which provides an interface for physical variable sensors. Unfortunately, the physical variable is often non-linear with changing electric characteristics. This makes their detection and application challenging to detect and process.

Signal conditioners take care of these physical sensors by providing suitable electrical characteristics to take the maximum reading from these sensors. As with all electrical and electronic components, their correct selection is crucial.