Budget the Burden Resistor in a 4–20 mA Receiver

A larger receiver resistor produces a larger voltage for the ADC, but it also consumes more of the current loop’s available voltage and dissipates more power. Selecting it solely to fill an ADC range can leave the transmitter unable to maintain the requested current under a low supply or long cable condition.

Calculate the normal operating values

For an illustrative 150 ohm resistor, 4 mA produces 0.6 V and 20 mA produces 3.0 V. Power at 20 mA is 0.06 W from current squared times resistance. These are nominal values; they do not establish an appropriate resistor power rating, fault tolerance or ADC input margin.

Include the specified signal overrange and fault conditions in a separate calculation. Check the resistor’s tolerance, temperature coefficient, working voltage and derating information. Self-heating can change its resistance and therefore the indicated current. The relevant temperature is the component’s operating temperature in the assembled product.

Complete the loop voltage budget

List the transmitter’s required operating voltage, receiver drop, cable resistance, protection components and any additional series devices. Evaluate the combination at the conditions that demand the greatest voltage. Texas Instruments’ explanation of two-wire transmitter compliance discusses how insufficient loop voltage can prevent the intended current from being maintained.

Confirm whether the product supplies loop power or expects an externally powered transmitter. The wiring and available voltage are different questions from the resistor’s current-to-voltage conversion. Label connection instructions clearly enough that the test setup represents the intended field arrangement.

Review the ADC interface

Check input range, common-mode conditions, reference tolerance and protection behavior. A 3.0 V nominal maximum is not automatically suitable for every ADC powered from 3.3 V. Include resistor tolerance and expected overrange before deciding the headroom is sufficient.

Keep voltage-sense routing tied to the intended resistor terminals so unrelated return-path drops do not enter the measurement. Include filter loading, input leakage and protection leakage in the error assessment. A protection device can be electrically quiet under one condition and contribute more error at elevated temperature.

Test loss of compliance explicitly

Use an appropriate controlled loop source and vary supply voltage and representative series resistance. Check low, high and overrange currents, then confirm how an open circuit and relevant wiring faults are reported. Preserve raw ADC readings as well as displayed engineering units.

The released design record should connect the resistor choice to both measurement accuracy and loop operation. A receiver that converts current accurately on a short bench cable still needs evidence that the complete intended loop can supply that current.