SCXI-1520: Should Pressure Sensors Cases Be Grounded to Earth?

[ScottWenzel]

We are using full bridge strain guage type pressure transducers. All 5 transducers are mounted to a large metal system that is not connected to earth ground. Sometimes we get very erratic, noisy signals when nothing should be happening and also during the "pressure event". When all channels are tested with a steady pressure all looks fine. All 5 channels will show trouble at the same time but each channel shows different and unique symptoms. Othertimes every thing works great.

[Bill B]

There is not an earth ground signal associated with a full-bridge configuration. The analog voltage that is measured from the bridge is connected to a differential amplifier that does not use an earth ground. However, if the metal enclosures has an earth ground connection, you can connect it to the earth ground (solder lug) inside the SCXI-1314 terminal block. The solder lug is located just above the strain relief opening to the left of the CH1 terminal strip. This will ensure that the SCXI-Chassis and the metal enclosures for your pressure transducers are all at the same potential.

[ScottWenzel]

We connected the ungrounded large metal system to its own earth ground and for now it seems to have solved the erratic and noisy signals. The pressure transducer cases are isolated from the chassis ground of the 1520 but are connected to the large metal system through their pipe fittings.

It seems that since the input impedance of the 1520 is spec'ed at >1 GOhm and the pressure transducers have a capacitance between the bridge elements and the case of ~200 - 500 pF we can get significant AC coupling, even at very low frequencies, into the 1520 if the case of the pressure transducer (and the large metal system) is floating and susceptible to EMI or static charging.