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Original question:

XTR116: DAC out to 4 to 20mA conversion.

XTR116: DAC out to 4 to 20mA conversion.

Sagar Rathod
Prodigy 115 points
Part Number: XTR116

Tool/software:

Hello All,

Please refer below link for the last communication for the same issue.

xtr116-dac-out-to-4-to-20ma-conversion

Now simulation is working fine with the given voltages of Vin: 0.1752V - 3.0998V ----  Iout: 4 - 20mA .

I have ordered XTR116 convertor module for the prototype, the block diagram is shown below.

The application is given below in block diagram.

I had tried this by using this convertor module and 2 wire current loop, and i have powered up The MCU  and other given ckt by XTR116 5V regulator output, and i got output prints from RS485 on Terminal window. (Very less current required for device to print the output)

And i manage to provide the voltage from DAC to XTR116 convertor to check 4 to 20 mA for DAC voltage 0.1752V - 3.0998V respectively. 

The resistors provided by last case i had mounted on the convertor board but still the value at lower voltage not getting proper. from  1.4884 to 2.95271V it gives proper current output as shown below.

DAC  Voltage (V) Output current (mA)
 From Convertor Loop		 Simulator Output
(mA)
0.172502 10.72 3.99
0.31894 10.86 4.79
0.46558 10.81 5.6
0.611771 10.72 6.4
0.7585 10.82 7.2
0.90438 10.75 8
1.050701 10.79 8.8
1.19626 10.86 9.6
1.34235 10.92 10.4
1.4884 11.07 11.2
1.63472 11.84 12
1.7806 12.63 12.8
1.92707 13.42 13.6
2.07333 14.20  14.4
2.21973 15 15.2
2.36657 15.89 16.01
2.51254 16.58 16.81
2.65968 17.38 17.61
2.8054 18.17 18.41
2.95271 18.96 19.22
3.09981 10.89

 

So the expected current output for 0.172502V is 4mA but when i start power supply the starting current shown on DMM is 10.89mA and it varies slowly and when i apply voltage in the sequence above in the table it gives the reading correct after 1.4884 to 2.95271V, why this is happened.

Is the right process or i am missing something?

I would appreciate if anyone help for the same.

Thank you.

 

  • 0
    TI__Mastermind 25360 points

    Hi Sagar,

    What is the resistive load that you are using in the output current loop? I noticed you have in your simulation 0ohms. 

    Sagar Rathod said:
    The resistors provided by last case i had mounted on the convertor board but still the value at lower voltage not getting proper. from  1.4884 to 2.95271V it gives proper current output as shown below.

    I'm a little confused. What resistors do you mean by this? 

    Best Regards,

    Robert Clifton

  • 0 in reply to Robert Clifton56
    TI__Mastermind 25360 points

    Hi Sagar,

    Also I'd like to confirm that the input DAC and MCU have their supply voltage and ground pins connected as such: 

    If not this might be causing some problems. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 115 points

    Hello Robert,

    Thank you for reply.

    The resistive load is 0 ohm to 250 i am varying so forget it. I am getting same output by changing the RL.

    I was talking about Ros and Rset resistors in simulation, I managed to set these resistors value on Development board, I have changed the R3 as 18.26K ohm Resistor and the Junction of the same connected to Pin 1 of IC (Vref) via 134.5K Ohm.  As shown in below image,  

    So the result not getting as i explained, i have connected 5V to MCU and other ckt. the ground (RED color) pin is connected from XTR116 board to MCU board. the Probes are connected to Ammeter.

    as shown in below Image. And i given DAC out voltage to XTR116 board. 

    Let me know if you need more information.

  • 0 in reply to Robert Clifton56
    Prodigy 115 points

    Hello Robert,

    I have connected the MCU ground Pin to Pin no 3 of IC, But still the issue is Persist.  Above is the image of connection.

    The RED pin now connected with Pin 3 of IC.

    Let me know if any more information is required.

  • 0 in reply to Sagar Rathod
    TI__Mastermind 25360 points

    Hi Sagar,

    Do you have a way to read the current running through this spot?

    This might help determine what's going on in this system. 

    Best Regards, 

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 115 points

    Hello Robert,

    The following are the current reading of the Output and and the I_In Pin no  2 current.

    I_IN (pin 2) Current mA Loop O/P Current mA
    0.045 10.925
    0.058 10.97
    0.065 11.025
    0.071 10.854
    0.078 10.911
    0.085 10.886
    0.092 10.928
    0.099 10.928
    0.106 11.025
    0.113 11.147
    0.120  11.879
    0.128 12.672
    0.136 13.468
    0.145 14.266
    0.153 15.064
    0.160  15.873
    0.169 16.667
    0.177 17.476
    0.185 18.272
    0.193 19.075
    0.201 19.881

    Let me know if any other information needed.

  • 0 in reply to Sagar Rathod
    TI__Mastermind 25360 points

    Hi Sagar,

    Thank you for providing this information! It's very informative. Just like with the data you provided before, the higher output currents seem to be accurate compared to the lower current. 

    This looks like a classic example of limitations being caused by the external circuitry consuming too much current. As described in the 2-Wire 4-20mA Sensor Transmitters: Sensor and Transmitter Current Consumption Limitations post, when you have over 3.8mA of current consumed by circuitry being powered by the VREG and/or VREF pins. 

    This theory also explains why the simulation didn't see it as you didn't add the input to the XTR in the simulation! 

    Next steps would be to confirm the quiescent current of your input circuitry and see if it consumes more than 3.8mA. Based on the fact that your circuit can't go below 10-11mA, I would suspect it might be consuming around 14mA. 

    Best Regards,

    Robert Clifton