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Cabling Considerations for Variable Reluctance Pressure Sensors

Variable Reluctance pressure sensors operate on an inductive sensing principle that requires and AC excitation and returns an AC signal. Long cabling runs from the sensor to the carrier demodulator are possible without compromise in performance if the proper cable configuration is observed. This application note describes recommended cable geometries for variable reluctance sensors such as models DP15, DP45 or DP103.

The purpose of using the correct cabling configuration is to minimize cross-talk between the carrier wires and the signal wires. If the distance between the sensor and carrier demodulator is just a few feet, no precautions are necessary: even loose pigtail leads will be sufficient. If the distance between the demodulator and sensor is over ten feet, it is best to select one of the cable geometries shown in the diagram below.

The optimal cable type consists of two pairs of separately shielded wires. This is the standard used for all Validyne cable assemblies. Belden #8434 cable has an inner-shielded pair (red and black) and an outer pair (green and white). The two inner-shielded wires are connected together to form the signal line, and this is wired to pin 2 or pin B of the cable connector. The green wire is the positive phase of the carrier, wired to pin A or pin 1, and the white wire is the negative carrier wired to pin 4 or pin D. The signal line is shielded from the excitation wires so that inductive cross-talk to the pressure signal is minimized.

The shields of the cable are generally connected to the connector shell, which, in turn, is mechanically connected to the demodulator and transducer housings. In cases where a difference in ground potential is expected between the plumbing and electronics, the shield should be disconnected at the transducer.

The following diagram shows various possible cabling configurations:

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