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DAC161S997: How to protect the DAC agenst shortage between LOOP N and GND

Ahmad Jasim
Prodigy 20 points
Part Number: DAC161S997

Hi,

We have desigend a self powered 4-20mA transmitter. Using the DAC161S997. We used an isolated 24V loop power supply and it is working fine.

We recognised that the DAC been permenantly damaged if by any misstake you shorting the LOOP N to the GND of the DAC circuit and when it comes to industrial usage it could happen.

My quistion is how to protect it agenst that?

See the schematic please.

 

After a one time shorting we saw that the value of mA signal has an offset. At low error sends 4.2 mA instade of 3.375 mA

At 20 mA almost 24mA.

  • 0
    TI__Guru* 93035 points

    Ahmad,


    I don't have any specific methods of preventing a connection between LOOP- and GND. The LOOP- connection will pull the loop current and adding any series resistance or diodes to the connection path may affect the operation of your board.

    As is shown in the block diagram from page 8 of the datasheet, the LOOP- node is lower than ground by as much as 0.80V. This comes from the 40Ω resistor at COMA sourcing the almost all of the loop current. Because of this, LOOP- must be allowed to float below the ground.

    All of that being said, have you checked the voltage from COMA to LOOP- just to make sure that it is the expected voltage? Also, have you replaced the device from the damaged board to see if this is a problem internal to the DAC161S997? I think damage is possible, but I would have first guessed that it wouldn't damage the device, but rather it would shut off the loop, pulling the minimum current for the components to operate.

    Post back and let me know if you looked at any of these things.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Hello Joseph,

    I did not measure the voltage between COMA and LOOP- because I chose to replace the device. cause I know i did a short circuit between ground and LOOP-.

    I did the same mistake for the second time and same permanent damage happened.

    4.2mA att 3.375mA (ERROR LOW). Second one also replaced and workes now. So thats why I thought it needs a protection against that.

  • 0 in reply to Ahmad Jasim
    TI__Guru* 93035 points

    Ahmad,

    I'll need to check to see if anyone else has come across this. As I mentioned, I didn't know of any particular protection circuit that can be used for a LOOP- to GND connection. 

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 93035 points

    Ahmad,

    I did check, and I wasn't able to find anyone that had a problem with this connection. However, I just wanted to check, is there a reason that you brought out the ground node for an external connection? Normally I would have expected just a LOOP+ and LOOP-, while the ground node is seen as an internal node on the board.

    Again, we would have thought this would reduce the current in the loop, instead of destructively pulling extra current.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Dear Joseph,

    Thank you for following up the case.

    Can you please specify 'problem with connection'? I tested two devices and get same permanent damage. How do you want me to prove the issue?

    With issue, I mean that the device been permanently damaged.

    The reasone we have a ground (GND) connection presents because the instrument we developing has other signals out.

    Ahmad Jasim

  • 0 in reply to Ahmad Jasim
    TI__Guru* 93035 points

    Ahmad,

    When I said that I couldn't find anyone with a problem with this connection, I had meant that I had looked to see if any previous customers that had asked about this device had a similar destructive problem. I have not seen any other similar destructive problems with this device. It's possible that other designs do not have an external connection to the device ground.

    At the moment, I don't have access to the DAC161S997EVM to look at it. I will however be able to look at it in a couple of days to check the basic schematic, and see if I get the same damage.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 93035 points

    Ahmad,


    I was able to get a DAC161S997EVM and test the connections. On the EVM, I connected the board ground to the LOOP- pin and the board immediately pulls as much current as possible from the supply. In my setup I have the supply current limited to 30mA. However, I'm sure that if the supply wasn't limited, I would almost certainly damage the board.

    At this point, I don't know of a method of protection for this type of short circuit. Neither the LOOP- nor ground are designed to be limited in the amount of current they source or sink. I'm not sure if you're able to add a series resistance to ground from the board connection, but that may limit some current going being pulled from the supply.

    Joseph Wu