• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 745 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ground reference for lmp8601

Bernard Pottner
Prodigy 140 points
Other Parts Discussed in Thread: LMP8601-Q1, LMP8601, RCV420

Hi,

I'm looking at datasheet for LMP8601, LMP8601-Q1, document SNOSAR2F –SEPTEMBER 2008–REVISED JANUARY 2014

On page 23, it shows an application of using LMP8601 as a 4-20mA Current Loop Receiver Application.

We are looking to do a similar application where LMP8601 is used to monitor current that a remote device is putting on the dry contacts of a relay output.

Will this work?? seeing as the remote devices could be 1000 feet away - running off a different power supply - with ground potential differences.

Thanks

  • 0
    TI__Guru 51386 points

    Hi Bernard,

    As long as the input pins stay within -2V to +76V of the LMP8601 GND pin (pin 2), the circuit should work.

    The simple circuit assumes the devices share a similar ground (within the same device), and that there are not large differences between the grounds.

    However, if you are thinking of completely separate grounds (between buildings) - there can be differences of several volts - or even hundreds during transients - between grounds across a large geographic area or "outside world". That would easily exceed the -2V limit (or even the +76V limit).

    The LMP8601 is NOT specifically designed to be a 4-20ma receiver. It does not provide the needed large common mode isolation barrier for traversing grounds. I would not be comfortable with the application - even if you add clamping diodes, as these can get blown-out by transients. You really do not want a conduction path between building/plant electrical grounds.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 140 points

    Thank you Paul, that helps.

    Paul wrote; However, if you are thinking of completely separate grounds (between buildings) - there can be differences of several volts - or even hundreds during transients - between grounds across a large geographic area or "outside world". That would easily exceed the -2V limit (or even the +76V limit).

    Yes, that is along the lines of what we are trying to achieve.

    Would a possible workaround be to have the LMP8601 powered locally from an isolated floating DC supply circuit?

  • 0 in reply to Bernard Pottner
    TI__Guru 51386 points

    Hi Bernard,

    The receiver is usually the source for loop power, so the rest of the loop has to "float" above ground. The "transmitter" is usually the floating node. The receiver is usually the grounded side. The loop should NOT be grounded at both ends (also creates "leakage" currents that can distort the reading and is generally considered a big 'no-no').

    Measuring the "ground" (or "low") side is the most accurate way as you do not have to worry about the CMRR, added noise, and common mode range of the differential amplifier. Measuring from the "high" (positive) side is more complicated, adds noise and is error-prone.

    Is it easier to measure a 1mm crack kneeling on the floor, or, on the ceiling on top of a tall, wobbly ladder?? It is always easier to measure at ground level..

    Usually the receiver has a sense resistor on the "ground" end of the loop (usually 250 ohms or less), and the power is applied to the positive side of the loop. That way, you only have to measure the voltage across the sense resistor to ground, which only requires a simple single-ended non-inverting amplifier.

    In actual industrial environments - the receiver ground (common) is also floating from "green" (earth) ground for safety (and to allow passing regulatory requirements - usually requiring >2kV isolation between grounds). The final output is usually converted to digital and passed optically/inductively/capacitively across an isolation barrier to the grounded controller/computer.

    If you have to "float" the receiver ground (common) to make the 8601 to work, then I would go with the low-side sense instead and avoid the noise/drift added by the 8601 input stage since you no longer need the -2 to 76V input range. Then all you need is a good sense resistor and a simple op-amp gain stage to accomplish the same (and even better) results.

    However, if, for some reason, your circuit does not have access to the negative side of the loop and you only can measure from the positive side, then the 8601 should work. But you still have to make sure the inputs are not brought -2V below "common" using (heavy duty) clamping diodes.

    We also make chips to do this...the RCV420. Google "4-20 receive schematic" for more examples.

    Regards,