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Thread: USM grounds

  1. #1

    Default USM grounds

    Can anyone tell me, are the ground screws in the 4 inputs in the USM box all common-connected or are they isolated? And are they connected to the Vnet ground? I want to measure a very small voltage and ground noise could be an issue. I also have another case where I don't want to accidentally cross connect my sensor ground with my system ground.

    If the USM grounds are not isolated, is there another way to measure an isolated voltage where neither side connects to a ground?

    Thanks,

    keith

  2. #2

    Default Re: USM grounds

    How small is the voltage?

    You may have to consider using shielded wire, the USM can use this.

    I suggest you give Racepak Technical a call and ask they will know the definitive answer.

  3. #3

    Default Re: USM grounds

    After closer inspection of both a USM box and the USM manual I can answer my own question.

    The screws marked "shield" are all hooked together. This would go to chassis/battery ground. These grounds are used only for the shielding braid/foil (assuming shielded wire) and are not used to carry any kind of signal.

    The screws marked "ground" are all hooked together but are not connected to the shield ground. This is a good thing, keeps the chassis/earth ground noise out of the sensor ground if you wire things correctly. These screws correspond to "sensor ground" and should be used only as a sensor signal return. They should not be connected to the shields.

    It appears that I can at least attempt to do what I want regarding certain voltage logging.

    I think racepak also has a separate module that does true differential voltage measurement. I may have to look into that also.

    Keith

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