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4-20mA galvanic isolation

William Schepers
Prodigy 20 points
Other Parts Discussed in Thread: ISO1541, INA220, AMC1301, AMC1200, AMC1100, SN6501, ISO7341C

I have two 4-20mA signals that I need to galvanically isolate before connecting to a microcontroller. I have already looked at one magnetically coupled analog isolator, but it was too expensive for my budget. Can somebody help point toward an inexpensive solution? Thanks.

  • 0
    TI__Guru 53330 points
    Hi William,

    Based on whether you want analog equivalent or digital equivalent representation of 4-20mA input current on the isolated side, there could be two possible ways to achieve isolation using few external components.

    Digital Equivalent of 4-20mA on isolated side:
    You could use current shunt monitor like INA220 to convert the analog current into equivalent digital values on I2C bus then use I2C isolator ISO1541 to isolate the digital signals.

    Analog Equivalent of 4-20mA on isolated side:
    Assuming the common-mode voltage on the current loop is less than 5V, you could use AMC1100 / AMC1200 / AMC1301 along with a shunt resistor to generate equivalent analog voltage on the isolated side.

    In either case, I am assuming you have isolated voltage available on both the sides. If not, you could use SN6501 / SN6505 to generated isolated supply voltages.

    I hope the above options answer your question.

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Prodigy 20 points
    To clarify, I am looking for the analog equivalent of the 4-20mA signal on the isolated side, and I already have isolated voltage available. However, the AMC1100, AMC1200, and AMC1301 are too expensive for my design requirements. Do you know of any solutions that are less expensive? Thanks.
  • 0 in reply to William Schepers
    TI__Guru 53330 points
    William,

    I am not sure what kind of resolution & accuracy you are looking at, here is one other approach to address this. Choose a very basic MCU with ADC inbuilt, convert current into digital equivalent value, isolate through SPI using digital isolator (like ISO7341C) and then convert the digital data back to analog using MCU with inbuilt DAC. You could also eliminate MCU by using just ADC, Digital ISO & DAC and make them run continuously and independently. This would mean 3 ICs and cost I believe can be manageable based on the kind of ADC/DAC required.

    Regards,
    Koteshwar Rao