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DAC161S997: Increased HART Communication Errors Due to Earth Connection

Rohit6230
Intellectual 745 points
Part Number: DAC161S997

Tool/software:

Hello Support Team,

I am using the DAC161S997RGHT (WQFN-16) for the Current Output Section and the DAC8740HRGET (VQFN-24) for the HART section.

My device's output current can work in 3 configurations: 3-wire Source , 3-wire  Sink, 4 wire Isolated using switch selections. Refer schematic diagrams of my device for your reference

In a test given by  Field COMM Group, 2 million messages are transmitted from a hart test system master  to my device , with a maximum allowance of 20 errors (i.e., no more than 1 error per 100,000 messages).

However, I observed that when I connected my device's earth (GNDE) directly to the earth PIT  socket , the error rate exceeded 2-3  errors per 100,000 messages. In contrast, when I connected the GNDE to the earth terminal of a regulated power supply, I observed 0 errors per 100,000 messages. I'm attaching the schematic diagrams of my device for your reference.

In power supply section , 1nF/440-X1/ 250-Y2 Caps which are connected between Supply line(24V, GND) and Earth (GNDE)

PC18, PC19, PC21, PC22, PC20, PC5 PC6

In Current Output section ,  1nF/440-X1/ 250-Y2 Caps which are connected between Signal line (4-20mA) and Earth (GNDE)

IC9, IC10, IC28

What could be causing this issue? Any insights would be greatly appreciated.

 Kind Regards,

Yatin R

1-POWER SUPPLY SECTION.pdf2-HART-WITHOUT ISOLATION (2).pdf3- 4-20mA loop powered section,.pdf

  • 0
    TI__Mastermind 38498 points

    Hi Yatin, 

    Joe will review and get back to you shortly. 

    Best,

    Katlynne Jones

  • 0
    TI__Guru* 96165 points

    Yatin,


    I'm not sure of how the earth ground should be connected. There are lots of different ways that people recommend connecting the earth ground that have bases in ESD, EMI, and electrical safety. Generally, I think you want a good ground plane on the PCB and then a good connection to an earth ground, typically connected to the power ground.

    Looking at your circuit, you have your 24V input, but you have two earth ground connections near that input, but you also isolate it from the rest of the circuit through the transformer. For as difficult as it might be, I'd try experimentally trying to connect earth ground to the GND to see it it reduces the communication errors.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 745 points

    Hello Joseph,

    Any Updates?

    Regards,

    Yatin R

  • 0 in reply to Rohit6230
    TI__Guru* 96165 points

    Yatin,


    I'm not sure I have much more to offer on this thread. I still think the best way of debugging this problem is by experimenting on the circuit. You should definitely use an oscilloscope (with ground set at the GNDI) to see what noise is coming up through the connections.

    In the pdfs that you provided, there are connections to GNDE in both the current output and the power section. This might cause a ground loop voltage that interferes with the HART signal.

    Because the GNDE is causing the noise, I would start debugging by removing some of the direct connections from GNDE to the circuit. In both the power and the current output section, I would remove any of the common-mode capacitances that capacitively connect the GNDE. This would include PC21, PC22, PC5, PC6, IC9, and IC10. In the HART w/o isolation schematic, there are two mounting hole connections to the GNDE, but I would guess they don't have enough to couple noise into the schematic.


    Joseph Wu