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DAC8740H: DAC8740H Compatibility with the XTR115

Nicholas Krouglicof
Prodigy 10 points
Part Number: DAC8740H
Other Parts Discussed in Thread: XTR115, XTR111, XTR300, DAC8741H, DAC8742H

Tool/software:

Hello,

We're currently using the XTR115 current loop transmitter in several of our products. We're now in the process of implementing HART in several of our loop-powered RTD transmitters by adding the DAC8740H modem IC. Integrating the modem IC with the XTR115 appears to be straightforward. I'm just wondering if there's anything I should be concerned about.

Also, I noticed there's also the DAC8742 IC. I don't see much of a difference other than the package. Are there other differences I should be aware of.

Thanks, Nick

  • 0
    TI__Guru* 94285 points

    Nick,


    The XTR115 should be ok to use with implementing HART for the 4-20mA loop. For some of the other XTR devices (XTR111 and XTR300) are implemented with a current chopping mirror that may throw extra noise at about 9-10kHz. This extra noise might disrupt HART signals, so we generally do not recommend those devices for HART.

    I haven't used the XTR115 much, but most of the documentation in the datasheet has a 10nF capacitance at the 4-20mA loop output. This is too high for the HART implementations, and you would want to reduce this to about 1nF to pass the HART receive impedance test. For the DAC8740H, the MOD_OUT sends HART signals through a capacitance to a series resistor into the XTR IIN pin, while the MOD_IN pin receives the signal at LOOP+ through another capacitance.

    The DAC8740H is part of a family of HART modem devices. The different versions refer to the interface on the data transmitted and received from the microcontroller.

    DAC8740H - UART
    DAC8741H - SPI
    DAC8742H - UART and SPI


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 10 points

    Thanks for the quick reply. This is helpful. 

    Nick