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DAC161S997: Problem With two DAC161S997's in two wire loop powered ( 24V DC input , 2 DAC 4-20mA output)

rnd 5
Prodigy 160 points
Part Number: DAC161S997

DAC_SECTION.PDF Hello , 

My project is two wire loop powered 24V DC input & two 4-20mA output. In that I am using  two DAC161s997 to measure the two process values and accordingly to give 4-20 mA at output, you can get idea by schematic above the line. But the issue is I am unable to read 4-20mA as per process value . When I am using single DAC the circuit is working perfectly, but when I add second DAC in circuit , both DAC output's are showing highest current (i.e as per process value calculation if o/p1=10mA , o/p2 = 8mA but both showing same current op1's = 10mA & op2's = 10mA  ) . Please correct me if I need to change any in circuit. I referred TI's schematic attached below the line.

Thank you!!! 

tidrh13.pdf

  • 0
    TI__Expert 5820 points
    Hello,

    From looking at your schematic it appears you are trying have two loop powered DACs regulate the same loop. This is not possible as the power supply, transmitter, and load must be in a single series loop. LOOP-P for both devices is connected to the same power supply based on your schematic. Each device will require it's own loop and supply to regulate the current correctly.

    Perhaps you can provide some additional details on what you are trying to accomplish and I can make a recommendation. You are simply trying to represent two control variables? Do the transmitters need to be loop powered?

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Prodigy 160 points
    Thanks Garrett for quick reply ,

    Actually I am measuring temperature & humidity from the single sensor , and I will get two variable outputs from that sensor. By these readings I am trying to give two 4-20 mA output's . Is that possible? , because from my point of view they all are in loop. please correct me if I am wrong.
    Thank You!!!
  • 0 in reply to rnd 5
    TI__Expert 5820 points

    Hi,

    When you have two DAC161S997 devices sharing the loop supply they will both try to regulate the current in the loop. This will cause an error in the loop current value. If you are using loop powered devices you will have to have a separate power supply for each such that they are on separate series loops.

    Another option is to use a 3-wire configuration where the analog signal current is on a separate loop from the power supply. With this configuration you can have a single supply for multiple current loops. Refer to the blog below for an overview of the loop powered 2-wire transmitter and 3-wire transmitter. Let me know if you have follow up questions on this.

    https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2014/04/29/industrial-dacs-an-overview-of-analog-outputs-and-architectures

    Thanks,
    Garrett

  • 0 in reply to Garrett Satterfield
    Prodigy 160 points
    Thanks Garrett for giving your valuable time for us...!!!