Design Con 2015

Read sensors with an oscilloscope

-January 02, 2014

Oscilloscopes principally measure voltage and time. Measuring another physical property such as temperature, pressure, flow, velocity, or displacement on an oscilloscope requires use of a transducer or sensor to convert the measured quantity into voltage. Using the oscilloscope's rescale function you can scale the input voltages into units that match the transducer's input.

Some oscilloscopes handle the rescale operation by changing the probe gain in the channel input menus. This provides numerically correct values but does not generally change the units of measure. The math function, rescale, lets you multiply the acquired waveform samples by a constant, add a constant (mx+b), and also modify the readout units.

Let's take an example of a third-party current probe. The current probe's manufacturer specifies the probe's sensitivity of 0.33mV/mA when operating into 50Ω. To read the oscilloscope measurements in milliamperes, you need to multiply the measured values by 3mA/mV, the reciprocal of the current probe's sensitivity. Figure 1 shows an example of using an oscilloscope’s rescale function to convert the vertical scale units from mV to mA.

Figure 1. An oscilloscope's rescale math function produces a display of the output of a current probe directly in milliamps. The voltage values are multiplied by a factor of 3mA/mV (the reciprocal of the probe's 0.33mV/mA published sensitivity, which changes units from Volts (top trace) to Amperes (bottom trace). Click on the image above for a larger view in a new window.

The current probe output is applied to channel 1, where we see the rms amplitude, shown in parameter P1, is 10.6 mV. The rescale math function is setup in math trace F1. Rescale offers the ability to multiply a signal by a constant and add a second constant as shown in the rescale dialog box in the lower right. In Figure 1, the readings from channel 1 are multiplied by 3. Rescale also has the capability to override the units and select other common units of measure. In this case the units are changed to Amperes (A). Parameter P2 reads the rms current applied to channel 1 as 31.8mA. Cursors or other parameters applied to math trace F1 will also read correctly in Amperes.

The second example takes the output of an instrument microphone and reads the sound pressure level (SPL) in units of pressure (Pascals or Pa). In this example we will employ a SPL calibrator to determine the sensitivity of the microphone. Figure 2 shows the use of the rescale function to read the microphone outputs directly in Pascals.

Figure 2: Rescaling the oscilloscope vertical axis to read a microphone output directly in units of sound pressure level. Channel 1 is the microphone input and math trace F1 is the same signals rescaled to read in Pascals. Click on the image above for a larger view in a new window.

The microphone is inserted into a SPL calibrator, which supplies an acoustic signal of 110dB rms relative to 20µPa at 1kHz. The output of the microphone is connected to channel 1. The rms voltage, read by parameter P1, is 265.8mV. The first step in the calibration is to convert the calibration level into Pascals. The calibrated 110dB rms relative to 20µPa level works out to 6.32 Pa. The sensitivity of the microphone is computed as 6.32/0.2658 or 23.78Pa/V. This is entered into the multiplier field in the rescale math function. The Override units box is checked and the output units are specified as PAL for Pascals. The rms value of math trace F1 now reads the expected value of 6.32Pa.


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