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AFE881H1: AFE881H1 use for non loop powered application

LS
Prodigy 140 points
Part Number: AFE881H1


Tool/software:

Dear Sir,

We are preparing a schematic for 4-20mA transmitter with HART Compatibility, that needs to have 8 channels for (HART+4-20mA functionality)

We selected AFE881 for this purpose,

Query is:

i) How to convert below schematic to non-loop powered application, our microcontroller section already remains powered up whether (4-20mA or HART) is connected or not,

So instead of making 3.3V using transistors and as shown in below picture form datasheet, what other way we should follow

ii) Can we use an Isolator for generating 3.3V that powered AFE881H, for that can you indicate us which circuitry needs to be removed from below schematic 

iii) Thirdly can we use only SPI interface between AFE881H and microcontroller instead of using both UART and SPI.

iv) what will be the limitation if we do not connect CD signal to microcontroller

  • 0
    TI__Mastermind 42273 points

    Hi LS, 

    Joe will review and respond to your questions soon.

    Best,

    Katlynne Jones

  • 0
    TI__Guru* 98195 points

    LS,


    I'm not entirely sure of what type of circuit you're looking for. However, let me just start with some basic answers.

    1. If you have power to the microprocessor, then you could just remove the DC-DC converter at the top side of the circuit that you clipped out. Normally this would be power brought across the isolation to power the microprocessor. However, the AFE881H1 would still be attached to the 4-20mA loop and powered through the loop supply.

    2. You probably could bring power across from the microprocessor side, but I'm not sure how you would set up the voltage to current section to control the load current from the loop. Can you draw your application a bit more? I need more descriptions about what you're trying to do and how you want to set up the device.

    3. There is an SPI only mode for communication, UART is not necessary for communication.

    4. I'm not aware of any limitations by omitting the CD signal. You would likely need to constantly poll the MODEM_STATUS register just to see that data has come in to be read.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 140 points

    Joseph,

    For point 2:

    2. I wanted to power up AFE881 with DC-DC isolated converter(3.3V output) powered from microcontroller side

    so wanted to minimize circuit ,

    wanted to confirm can i remove circuit indicated(red marked) in below picture

    - will the 4-20mA(V to I) and other circuit will operate as expected that is the loop current transmitted on loop will be as per Code transferred to AFE881H

    LS

  • 0 in reply to LS
    TI__Guru* 98195 points

    LS,

    I think you could probably do something like that, but I've never tried it that way. Note that Q4 now takes the bulk of the loop current and that at the high end, it pulls the 20mA from the loop. That means the emitter resistance must be significantly reduced, or the OPA wouldn't be able to drive the base high enough to pull current.

    Joseph Wu