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When using very long cables with IEPE sensors, the added capacitance in the cable can affect the frequency response of the sensor by filtering some of the high frequency content. The capacitance of the cable, output level of the sensor, and amount of IEPE current supplied to the sensor all are variables which determine the frequency response. The frequency response can be modeled and theoretically calculated. PCB’s website has good information on this theory. Here is a link: PCB - Driving Long Cables
Table of Contents
Test Setup:
To obtain a frequency response of the cable under test, a swept sine analysis was performed from 1kHz to 80kHz at different voltage amplitudes. The PXI-4461 and the Swept Sine Example for LabVIEW in the Sound and Vibration Measurement Suite provided the hardware and software measurement implementation. A GRAS preamplifier input adapter was used so that a voltage signal could be applied to the preamplifier. The cable length under test connected the GRAS preamplifier and the Dytran current source power unit. The Dytran 4115B can vary the output current between 2mA and 20mA. This test setup allowed us to vary the type of cable, amount of IEPE current, and the sensor signal level.
Results:
Tests were performed on three different cables:
|
Cable |
Capacitance/ft |
Total Capacitance |
|
100 ft RG-62 |
13.5 pF/ft |
1.35 nF |
|
500 ft RG-58 |
28.8 pF/ft |
14.4 nF |
|
1000 ft RG-174 |
30.5 pF/ft |
30.5 nF |
The frequency response was measured from 1 kHz to 80 kHz with three different signal amplitudes (250mV, 1V, 5V) and IEPE currents of 2mA, 4mA, 10mA, and 20mA. The following graphs show the relationship between these variables. The frequency values represent a passband frequency of 0.1dB of variation (frequency at which the response channel has varied by more than 0.1dB in reference to the stimulus channel).
5V peak signal amplitude:
1V peak signal amplitude:
250mV peak signal amplitude:
Note that the areas represented in these graphs are defined by the three cable test points. If you are using a cable length which was not tested and your frequency of interest falls close to an IEPE current boundary, the safest approach is to use a shorter cable or use the higher IEPE current value.
Conclusions:
You should only be concerned about using long cable lengths with IEPE sensors if you are interested in a frequency range of more than 10 kHz and using a cable longer than 300 ft. If your application does require this, these results can give you a better idea of the cable length that you can use. If the distant between the measurement device and sensor is a requirement which cannot be changed, you can choose a measurement device which can provide enough IEPE current to drive the long cable. NI products support several different IEPE current values:
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