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Original question:

DAC161S997: unable to control the loop current through DAC161S997 IC

DAC161S997: No loop current variation in (4 - 20mA) DAC161S997

Adarsh Madhusoodanan
Prodigy 10 points
Part Number: DAC161S997

Hi  TI  community

We are using DAC161S997 for 4- 20 mA current transmission.

Below attached is the circuit schematics.

We have a problem with bring up of this circuit, that is, while we writing various hex values to the DACCODE register with SPI, the output loop current is not changing from 4 to 20mA.

We have the following observations.

1.The voltage across the loop resistor (250R) is 0.873 V, which means, the current passing through the resistor is appox.3.49 mA, may be the default value in ERR_LOW register. The loop voltage we have given is 24V.

2.We have tried setting the MASK_LOOP_ERR bit of ERR_CONFIG register by 1,and tried the same, but no change in the observation, if we seep the DACCODE Values, loop current was not changing.(we are measuring the current by reading the voltage across the loop resistance).

What could be the reason? Any suggestion for debugging this issue is appreciated.

Thank you in advance.

Warm regards,

Adarsh.

  • 0
    TI__Guru* 94285 points

    Adarsh,


    I'll need a little more information about how you have the device set up. Can you provide a little more information about how you power the device, and where the +24V return is? The negative connection of the +24V should not be the same as the ground/COMA/COMD of the DAC161S997. Here's a diagram from the datasheet:

    Here, LOOP- is not attached to the ground of the device. While the loop power is not completely isolated, it must be allowed to float with respect to the DAC supply. The LOOP- voltage is below ground by the loop current times 40Ω (from the diagram).

    Also, have you checked the timing for the SPI communication? I think the device sets up data on the falling edge of SCLK and the data is clocked in on the rising edge of SCLK.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 10 points

     Hi Joseph

    Many thanks for your response.

    the grounds of DAC and current loops are fully isolated.

    We have resolved the issue by writing 1 in the MASK_SPI_TOUT and MASK_SPI_ERR bits of ERR_CONFIG register. 

    Thank you for your prompt help.

    Adarsh.