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Need assistance for HART COMMUNICATION

Ankush
Intellectual 500 points
Other Parts Discussed in Thread: DAC8760, DAC8740H, DAC8830

Dear Sir/Madam,

 I am getting some issue while using DAC8760IPWPR for 4-20mA generator and DAC8740HRGET for HART communication 

if initialize as 4-20mA option in DAC 8760 without Enabling HART  in config register it is working fine ,

but when I am initializing  HART (Enabling by setting config register 0x57 to 0x0010) it will lift my 4-20mA signal by 6.6mA 

for example 4mA  -> 10.6mA  and 5mA -> 11.6 and  so on upto  20mA -> 26.6mA and in this situation my HART communication 

Working fine.

  • 0
    TI__Mastermind 42243 points

    Hello,

    My colleague Joseph is out of office. He will be able to help you here and will be able to respond in a few days. In the meantime, can you let me know what supplies you are using? What does your HART input signal look like? Also, do you have a schematic you can share? 

    Thank you,

    Katlynne Jones

  • 0
    TI__Guru* 98025 points

    Bhagyesh,

    Katlynne mentioned quite a few things that I think we would want to know to help debug this problem. The most important thing is likely the schematic. I would want to know how you have your HART signal coupled into the device.

    Figures 32 and 33 in the datasheet give two different ways to couple the HART signal into the device. One is for the external bandpass filter coupling into MOD_INF and the second is for the internal filter coupled into MOD_IN, I think it's important to try to match the datasheet for these recommendations for the schematic. It's possible that with any changes, you might change the output current.

    One other thing, you might want to post the register configuration that you've written to the device.

    (And thanks Katlynne!)

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 500 points


    Hi Joseph,
    Thank you for your quick response, please check schematic and please suggest us some solution
    My device is working 3-wire 4-20mA mode and my HART communication working properly, at 1200 baud rate but output
    Current output  is adding offset of 6.6 mA.
    after configuring config register (0x57 = 0x0010) my 4-20mA current getting disturb

    without setting config register (without initializing HART communication)my 4-20mA output is working fine.

    3wire 4-20 OPwith HART.pdf

  • 0 in reply to Ankush
    TI__Mastermind 42243 points

    Hi Bhagyesh,

    Thank you for providing your schematic. I am not sure how often Joseph will be checking the forum while he's away, so his response may be delayed. Thank you for your patience!

    Best,

    Katlynne Jones

  • 0 in reply to Ankush
    TI__Guru* 98025 points

    Bhagyesh,


    In the schematic, I don't think that you want MOD_OUT, MOD_IN of the DAC8740H and the HART-IN of the DAC8760 all capacitively coupled to the same node. Instead, I think that you should have the MOD_IN of the DAC8740H capacitively coupled to the IOUT of the DAC8760. Additionally, the MOD_OUT of the DAC8740H should be capacitively to the HART-IN of the DAC8760.

    This is similar to the example shown in Figure 34 of the DAC8740H datasheet, which is connected to a DAC8830. In that example, the MOD_OUT is capacitively coupled to the buffered voltage output of the DAC8830 before the signal is converted to the current output. Again, you should pay attention to the capacitances used at the DAC8740H MOD_IN and MOD_OUT nodes.

    As an example, there was a DAC8740H question in E2E that I found with a schematic that connected a DAC8740H and a DAC8760. You can see the schematic and comments in the link here:

    https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/694081/dac8740h-hart-communication-design


    Joseph Wu