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LMP90080: LMP90080

Tomas Heitel
Prodigy 90 points
Part Number: LMP90080
Other Parts Discussed in Thread: ADS124S08, ADS114S08

Dear TI Engineer and Community,

i'm developing a RTD board with LMP90080.
I would like to ask You, how to proper connect a 4-wire PT100 sensor with a shielded cable to the LMP90080? At the RTD end there is no connection between shield and metal cover of the sensor.
Should I connect the cable shield to PE (metal chassis) or LPM90080's GND at the LMP90080 end?

For detailed info please refer to the attached picture

Thank You in advance, Ing. Tomas Heitel

  • 0
    TI__Guru* 92655 points
    Tomas,


    I think it is more common to connect the RTD shielding to an earth ground, which would be the same earth ground that is connected to the chassis of the system.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 90 points
    Hi Joseph,

    thank You for the comment. Could You please explain me the background of Your decision? Is it because of EMC, functionality,..? I forgot to mention that it is an industrial environment. Are there available any TI application notes regarding this issue?

    Tomas
  • 0 in reply to Tomas Heitel
    TI__Guru* 92655 points
    Tomas,


    I based the comment on recommendations that I have seen from RTD manufacturers. However, using earth ground makes sense to me. First, earth ground should be enough to protect the circuitry from RFI and EMI. For the purpose of ground loops, the return path will go through the shielding to the earth ground, bypassing any circuitry on sensitive boards. If the shielding had returned through the circuit ground, there is less certainty about how the ground would be able to sink current and possibly disrupt the circuit. If you are in an industrial environment, I think the sensors would end up in a location where the potential is at earth ground.

    There are generic TI application notes on shielding for EMI/RFI, but I don't know of any specific application notes for RTD shielding.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points
    Tomas,


    Incidentally, if you are in the process of choosing an ADC for making RTD measurements, you should also consider the ADS124S08, and the ADS114S08 (the difference being 24-bit and 16-bit ADCs). Both devices have many channels and a PGA. These devices may work a bit differently than the LMP90080, but they often are used in the same applications.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Hi Joseph,

    thank You for the reply. The PT100 Ex-proof sensor itself comes with an metal cover (please see attached picture). During installation, the metal cover is electrically connected to PE, but as I mentioned previously the shield of the cable is not internally connected to the metal cover of PT100.

    What do You mean by "I think the sensors would end up in a location where the potential is at earth ground."?
    The MCU's and LMP90080's GND is floating in view of PE (earth ground) - picture above. Do You suggest to connect the GND and PE to be on the same level?

    One more question. Is there an End Of Life date for the LMP90080 chip?

    Tomas

  • 0 in reply to Tomas Heitel
    TI__Guru* 92655 points
    Tomas.


    I would have connected the shielding to earth ground but not connected that to the MCU ground. I was merely suggesting that it would be better to connect the shielding to earth ground, so that any return current bypasses the MCU ground. I was concerned that the MCU ground wouldn't be able to easily sink and source any ground loop current.

    In the end, there may be several valid ways to deal with the earth ground and the MCU ground. In some cases, I've seen the earth/chassis ground and MCU ground connected together with a low impedance path, but close to the power connection of the board. In other cases, I've seen the earth and MCU ground separated, but connected with some high impedance connections to prevent the grounds from being too far apart in potential. Regardless of what you do, I think the least preferable would be to tie the earth ground shielding to the board, and let any return ground loop current go through the board. I would note that I'm not really an expert in the system ground aspect of circuits. I generally answer questions on the ADC.

    As for EOL of the device, I don't know of any plans to EOL this or any other device. In my product line, it is generally not done, and I don't know of any devices we've ever EOL.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 90 points
    Hi Joseph,

    Thank You for support and advice.

    Tomas Heitel