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XTR117 output current

john Ma
Intellectual 280 points
Other Parts Discussed in Thread: XTR117

1. when vin=0.4V, the output current is about 60uA, but the ideal value should be 100*0.4/20=2mA. Why?

2. Is there anything wrong about this schematic?

note: Q1 is BCX56

  • 0
    TI__Guru 62745 points

    Hello,

    The schematic looks appropriate.  However if the circuitry powered from the VREG voltage is consuming more than 2mA of current then the output current won't be able to get lower than 2mA because the VREG current is in series with the output current.  This subject is covered in the following blog post:  2-Wire 4-20mA – IRET < 4mA

  • 0 in reply to Collin Wells
    Intellectual 280 points
    Hi Collin,

    My system works well now, thanks for your support.

    BR.
    John Ma
  • 0 in reply to Collin Wells
    Intellectual 280 points

    Hi Collin,

    What's the range of R1 and R2? I do not find the specification in datasheet. R1 should  equal to 99*R2. But I get one part, R1=2.641Kohm, R2=29.53ohm. It seems this is an abnormal part. I hope to get your reply, thanks.

  • 0 in reply to john Ma
    TI__Guru 62745 points
    Hi John,

    The resistor values inside the XTR117 are trimmed for ratiometric, not absolute accuracy. This means that the ratio of R1/R2 is set to match the desired transfer function and gain error specified in the product datasheet. The values will be in the range shown in the product datasheet +/-20%.

    Measuring these resistances outside the package is not practical because the ESD protection elements and bond wires add additional resistances which are impacting your measurements. Typical protection cells used in these products have series resistances in the order of a few Ohms which is greatly impacting the measurement of the R2 resistor which is lower in value.
  • 0 in reply to Collin Wells
    Intellectual 280 points
    Hi Collin,

    Thank you for your detail reply. My system works well now.

    BR.
    John Ma