I have to connect a MIL 2-pin IEPE accelerometer to my Data Acquisition System (DAQ). What cable shall I choose and how shall I ground the cable shield to obtain the best possible signal to noise ratio / immunity to electromagnetic interference (EMI).
To get the best possible signal quality the usage of shielded twisted pair cable is recommended. Twisted pair conductors create wire loops, which provide improved immunity to electromagnetic interference.
The method of connecting the cable shield is of primary importance. The cable shield has to be grounded; otherwise it will work like an antenna and due to its capacitive coupling will transfer EMI to the conductive lines in the cable interior. As the transducer’s signal and common are isolated from the transducer’s body, cable shield can be grounded through the sensor case, DAQ signal common or DAQ ground. The optimal way in which cable’s shield shall be grounded depends on few factors like the transducer case grounding or type of the DAQ inputs. For all MIL connector transducers, Meggitt Sensing Systems offers part numbers 6923M9 and 6923M16 cable assemblies. Both of these cables can be connected to DAQ in a different way. Let’s consider particular cases.
Cable grounded at instrumentation end.
If the transducer is mounted on a surface/object that does not provide the grounding to the transducer case, and/or transducer body is ground isolated, cable shield has to be grounded at instrumentation end.
Meggitt Sensing Systems is providing cables compatible with MIL connector equipped IEPE transducers’ 6923M6 and 6923M16.
In cases where the transducer is isolated from ground and our DAQ provides only IEPE BNC inputs, there is no common and ground separation. In this case, it is necessary to short the cable shield to the common lead.
For this purpose cable 6923M18 featuring MIL connector at the transducer and BNC male at the DAQ end, can be used.
In the case where the 6923M18 cable is used and transducer’s body is grounded, the measurement chain and the unit under test (UUT) shall be grounded in one point to prevent possible ground loops. This is similar to the case of signal to ground non-isolated transducers.
Cable shield grounded at the transducer end.
As mentioned above, in the case of the accelerometers, featuring MIL receptacles, the transducer body is isolated from signal and common lines. Therefore transducer body can be used to ground the cable shield.
This approach shall be considered only when we are sure of proper grounding of transducer body. As the cables 6923M9 and 6923M16 cable shield is connected to the connector shell, it will be necessary to float the cable shield at the instrumentation side.