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DAC8742H: DAC8742

J.C. Geluk
Prodigy 20 points
Part Number: DAC8742H
Other Parts Discussed in Thread: DAC8771, , DAC8771EVM, DAC8760, DAC8740H, PROFIBUS, OPA188

Hi I am planning/designing a device whiche should next to generation of V and  mA also have a function to use  the hart Hart, FF and PA protocol seperately. In studying the documentation of the DAC8742HEVM I did nowhere see how to connect the board to a 4-20mA loop. The loop is powered by an external supply.

I plan to use the DAC8742 in combination with a DAC8771 to generate V and mA . I noticed the DAC8771 has a Hart_In pin.

Do I connect the Hart_in pin of the DAC8771 with the OUT of the DAC8742HEVM board en the IN of the DAC8742HEVM board just to the loop+? That would be simple and hopefully not interfere with seperate V and mA generation with the DAC8771. I see the

Will this connection then properly with FF and PA?

Can you please explain, preferably with a little sketch.

Your help is appreciated.

Joost

  • 0
    TI__Mastermind 18546 points

    Hi,

    Due to US holiday, our responses will be delayed. We will get back to you tomorrow. 
    Thanks for understanding !!!

    Regards,

    AK

  • 0
    TI__Guru* 92715 points

    Joost,


    The DAC8742HEVM wasn't designed specifically to be placed inside a 4-20mA loop. I would note that the device operates on a 6V maximum supply, so if it is in a loop, the power supply should probably be independent. In the loop, the DAC8742H can be set up similar to the Figure 34 in the DAC8742H datasheet.

    The MOD_OUT connects to the HART driving point in the circuit, while the MOD_IN is capacitively coupled to the loop supply. Figures 32 and 33 in the DAC8742H datasheet offer a few more details about these connections.

    As for the connection to a DAC8771, I think you could use the DAC8771EVM and connect it up as a 3-wire current transmitter. The connection should be straightforward. In a sense, the coupling if the HART FSK should be similar to this application note:

    http://www.ti.com/lit/an/slaa572/slaa572.pdf

    This application note shows the connections to the DAC8760 instead of the DAC8771. However they should be similar in operation. The DAC8771 should be a little simpler because the current control is more integrated, without an ISET-R connection. I would definitely recommend testing this with the DAC8742HEVM with the DAC8771EVM.

    As for connections for Fieldbus and Profibus, I don't have much experience with that standard. However, on the EVM there are certain jumper settings that are used to select different filters going into and out of the DAC8742HEVM. These jumper settings are listed in Tables 11 and 12 of the DAC8742H Evaluation Module User's Guide. Also, I did find an E2E thread about using the DAC8740H for supporting Fieldbus and Profibus with some extra collateral on that. You can read through it here:

    https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/663222/dac8740h-profibus-fieldbus-schematic-with-the-tm4c123

    Again, I would recommend getting the DAC8741HEVM and the DAC8771EVM just to test these connections.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Thank yoy , Joseph for your reply. I am still puzzled. I came across a very nice example which I hoe will work with the TI components as well. Can you please comment. I hope it is readable, as I could not find a way to add file.

    Note;  Iout and Vout are shorted. While the OPA188 takes care of the voltage Out with the VSense+ (output OPA188) and VSense- connected to one end of the loop and GND (have a look at CN-0278).

    Joost

  • 0 in reply to J.C. Geluk
    TI__Guru* 92715 points

    Joost,


    Yes, the DAC8771/DAC8742H combination would be put together in the same way. I would note that the DAC8771EVM does have the IOUT and VOUT tied together (through some small resistance), but it does not use an op-amp buffer to connect VSENSE.

    Also, I would note that the HART connections for the DAC8742H are slightly different than the AD5700. I think there are some subtle changes in capacitance/resistance values in the connection to the HART input and output for the DAC8742H. I think the version that you clipped out is similar to Figure 33 in the DAC8742H datasheet. Regardless, it should be similar to Figure 33 or 34.

    I think that using the two EVMs, you should be able to test the connections. 


    Joseph Wu

  • +1 in reply to Joseph Wu
    Prodigy 20 points

    Thaank you Joseph,

    Noticed the IOUT and VOUT in the DAC8771 myself too. Just after I sent the message :(.

    Yes I will check connections and performance with both EVMs. On paper it should work. Proof of the pudding is the eating...

    Thanks again

    Joost