Data Acquisition Toolbox

Signal Conditioning

Sensor signals are often incompatible with data acquisition hardware. To overcome this incompatibility, the sensor signal must be conditioned. The type of signal conditioning required depends on the sensor you are using. For example, a signal might have a small amplitude and require amplification, or it might contain unwanted frequency components and require filtering. Common ways to condition signals include

• Amplification
• Filtering
• Electrical isolation
• Multiplexing
• Excitation source

Amplification

Low-level signals -- less than around 100 millivolts -- usually need to be amplified. High-level signals might also require amplification depending on the input range of the analog input subsystem.

For example, the output signal from a thermocouple is small and must be amplified before it is digitized. Signal amplification allows you to reduce noise and to make use of the full range of your hardware thereby increasing the resolution of the measurement.

Filtering

Filtering removes unwanted noise from the signal of interest. A noise filter is used on slowly varying signals such as temperature to attenuate higher frequency signals that can reduce the accuracy of your measurement.

Rapidly varying signals such as vibration often require a different type of filter known as an antialiasing filter. An antialiasing filter removes undesirable higher frequencies that might lead to erroneous measurements.

Electrical Isolation

If the signal of interest contains high-voltage transients that could damage the computer, then the sensor signals should be electrically isolated from the computer for safety purposes.

You can also use electrical isolation to make sure that the readings from the data acquisition hardware are not affected by differences in ground potentials. For example, when the hardware device and the sensor signal are each referenced to ground, problems occur if there is a potential difference between the two grounds. This difference can lead to a ground loop, which might lead to erroneous measurements. Using electrically isolated signal conditioning modules eliminates the ground loop and ensures that the signals are accurately represented.

Multiplexing

A common technique for measuring several signals with a single measuring device is multiplexing.

Signal conditioning devices for analog signals often provide multiplexing for use with slowly changing signals such as temperature. This is in addition to any built-in multiplexing on the DAQ board. The A/D converter samples one channel, switches to the next channel and samples it, switches to the next channel, and so on. Because the same A/D converter is sampling many channels, the effective sampling rate of each individual channel is inversely proportional to the number of channels sampled.

You must take care when using multiplexers so that the switched signal has sufficient time to settle. Refer to Noise for more information about settling time.

Excitation Source

Some sensors require an excitation source to operate. For example, strain gauges, and resistive temperature devices (RTDs) require external voltage or current excitation. Signal conditioning modules for these sensors usually provide the necessary excitation. RTD measurements are usually made with a current source that converts the variation in resistance to a measurable voltage.

Sensors

The Computer