Why does my 2-wire 4-20mA transmitter output not reach the full 20mA? When I change the load resistance or decrease the supply, the output changes.
What can I do to fix this?
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The XTR has a minimum compliance voltage (usually around +7.5V). The voltage drop across the load and resistive losses in the cables cause the supply to decrease. Your loop supply needs to be at least VCOMPLIANCE + IOUT_MAX*RMAX. The IOUT_MAX is 20mA, and RMAX is the sum of your load and cable resistance. If the device does not have a sufficient supply, it will lose its ability to regulate the output current. As a result, the output current will not reach 20 mA and will typically become non-linear as the input circuitry loses power.
Increasing the load resistance or decreasing the supply could be violating the compliance voltage of your system
The attached article goes into more detail on 2-wire transmitter compliance voltage.