Signal conditioning is the technique of making a signal from a sensor or transducer suitable for processing by data acquisition system.
Types of Signal Conditioning
Amplification: Amplification increases a voltage signal to a level suitable for digitization by the DAQ equipment. Typically a data acquisition device is calibrated for input voltages in the 0 to 10 V range. A small voltage, such as that coming from a thermocouple or strain gauge bridge may need to be amplified 1000 times to make it between 0 and 10 V.
Excitation: Many transducers, like strain gauges and RTDs (resistance temperature devices), need a power supply. The signal from these transducers is either a voltage or a mA current. For many transducers the supply will be low voltage DC, but for transducers based on capacitance measurement an AC supply may be required. Excitation is commonly needed for measuring force, pressure, relative humidity, temperature, level, light level, concentration and vibration.
Linearisation: Linearisation is needed when the signals produced by a sensor don't have a straight-line relationship with the physical measurement, as is the case when using thermocouples to measure temperature. Linearisation is achieved using signal conditioning.
Filtering: Filtering reduces noise errors in the signal. For most applications a low-pass filter is used. This allows through the lower frequency components but attenuates the higher frequencies. The cut-off frequency must be compatible with the frequencies present in the actual signal (as opposed to possible contamination by noise) and the sampling rate used for the A-D conversion. A low-pass filter that's used to prevent higher frequencies, in either the signal or noise, from introducing distortion into the digitised signal is known as an anti-aliasing filter. These generally have a sharper cut-off than the normal low-pass filter used to condition a signal. Anti-aliasing filters are specified according to the sampling rate of the system and there must be one filter per input signal. They are commonly used when measuring, for example, vibration.
Isolation: A high transient voltage at one input may damage not only the input circuit, but an also propagate to other equipment connected to that input. You can prevent this type of damage by providing isolation between inputs.
High Impedance: Certain types of transducer have a very high output impedance and are not able to supply enough current to use a normal voltage input. When connected to a normal amplifier, the currents drawn from the transducer can seriously distort the input signal. Typically glass electrodes used to measure pH, or gas concentration probes, are of this type.