Getting Started With Analog Input (Data Acquisition Toolbox)

Data Acquisition Toolbox

Data Range and Engineering Units Properties

This section describes properties that control the range of data expected from your sensor, the range of the A/D converter, and the engineering units of the data returned to MATLAB.

When data is acquired through a sensor and digitized by an A/D converter, you must be aware of these three issues:

The expected range of the data produced by your sensor. This range depends on the physical phenomena you are measuring and the output range of the sensor.
The range of your A/D hardware. For many devices, the A/D hardware range is specified by the gain and polarity.
By default, most A/D hardware outputs data in units of volts. However, after the data is digitized, you may want to define a linear scaling that represents specific engineering units when data is returned to MATLAB.

The sensor and A/D ranges should be selected such that the maximum precision is obtained and the full dynamic range of the source signal is covered. The default A/D range is supplied by the hardware driver.

The sensor range is set with the SensorRange property, the A/D range is set with the InputRange property, and the engineering units are set with the UnitsRange property. These are all channel properties and can be set on a per-channel basis.

For example, suppose you are using a microphone that can measure sound levels up to 120 dB and can produce an output voltage ranging from -1 to 1 volts. If you are measuring street noise in your application, then you might expect that the sound level never exceeds 60 dB. Assuming a linear relationship between the sound level and sensor output voltage, then you are using only 50% of the dynamic range of your sensor (-0.5 to 0.5 volts). In addition, your A/D converter may have several valid voltage ranges that it can be set to. The best A/D hardware range is the one that encompasses the expected sensor range most closely. For this example, let's assume the hardware only supports a range of -1 to 1 volts and that engineering units conversion is not required. The code shown below reflects this example.

ai = analoginput('winsound')
addchannel(ai, 1)
ch = ai.Channel(1)
set(ch, 'SensorRange', [-0.5 0.5])
set(ch, 'InputRange', [-1 1])

Setting the data range and engineering units is described in detail in "Engineering Unit Conversion".

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