Reference Circuit for using AD5700 in HART Master Mode

In order to acquire readings from one or more HART-enabled Loop-Powered Smart Transmitters, we are planning to use AD5700 in HART Master Mode. It would be of immense help, if one can give us a reference circuit using AD5700 in HART Master Mode. At high-level the functional diagram is as shown below.  Thanks a lot !

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  • +1
    •  Analog Employees 
    on Aug 28, 2020 5:17 PM 3 months ago

    Hi corvid,

    Please take a look at the following reference designs: CN0414, CN0418, or CN0435 (PLC/DCS system example).

    Regards,
    Mihai

  • Hi Mihai,

    Thank you for the quick response. We went through all the reference circuits. It gave us a fair bit of idea. It helped us to understand that the circuit given in page #17 of AD5700/AD5700-1 data sheet, may also fit our requirement.  

    Few doubts:

    1. Can we use the circuit in Fig 28 for interfacing one or more HART Field Instruments in Multi-drop mode ?
    2. In Fig 28, Can we use SMPS output voltage for V.loop ? 
    3. Is it possible to use the circuit in Fig 28 for interfacing normal 4-20mA based non-HART field instruments as well ?
    4. Which one of the circuit is recommended for harsh industrial or outdoor environments ?
    5. Are there any standard guidelines we need to know for using these circuits in harsh industrial or outdoor environments ?

    Thank you
    Corvid Team

  • +2
    •  Analog Employees 
    on Sep 1, 2020 1:55 PM 2 months ago in reply to corvid

    Hi Corvid Team,

    1. Yes, you can use the circuit in Fig 28 to interface one or more HART Field Instruments in Multi-drop mode if all HART Field Instruments communicate only through HART.

    2. Yes, you must use an SMPS output voltage for V.loop to power the field instruments and facilitate the communication.

    3. You can use any type of field instrument (sensors in this case), but only one 4-20 mA field instrument can be connected at a channel. One filed instrument can incorporate multiple sensors, one primary measurement transmitted through a 4-20 mA loop (e.g. temperature) and one or multiple secondary measurements transmitted through a HART interface (e.g. pressure, flow rate...). A good example is CN0267.

    4. For harsh industrial or outdoor environments, it is recommended to add an external bandpass filter for AD5700(-1) instead to use the internal one, like in Fig 28.

    5. Depends on your application. For example, you will need to isolate your field instruments from your MCU to not be damaged by a surge. The previous recommended reference designs are a good example. An FAE can advise you with more guidelines. Also, check out this https://www.fieldcommgroup.org/sites/default/files/technologies/hart/ApplicationGuide_r7.1.pdf.

    Best Regards,
    Mihai

  • Hi Mihai,

    We have 2 follow-up questions:

    1. We trying the above circuit using AD5700 evaluation board (EVAL-AD5700-1EBZ). We noticed that in the evaluation board, there is a 22nF capacitor instead of 10nF near the HART_OUT pin. Is it fine to use 22nF in place of 10nF as is without changing rest of the circuit components ? Please advice.

    2. Our MCU has an in-built 12-bit ADC and it works with 3.3V. So we are using 165Ω resistor instead of 250Ω resistor. Do we need to change 20kΩ resistor and 10μF capacitor as well ? Please advice.

    Thank you
    Corvid Team

Reply
  • Hi Mihai,

    We have 2 follow-up questions:

    1. We trying the above circuit using AD5700 evaluation board (EVAL-AD5700-1EBZ). We noticed that in the evaluation board, there is a 22nF capacitor instead of 10nF near the HART_OUT pin. Is it fine to use 22nF in place of 10nF as is without changing rest of the circuit components ? Please advice.

    2. Our MCU has an in-built 12-bit ADC and it works with 3.3V. So we are using 165Ω resistor instead of 250Ω resistor. Do we need to change 20kΩ resistor and 10μF capacitor as well ? Please advice.

    Thank you
    Corvid Team

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