This DigiKey workbench demo used a TI XTR116 on a DKS-ADPT-01 adapter, a BD139 pass transistor, a 100 Ω burden resistor, and a Digilent Analog Discovery with differential oscilloscope inputs.
Key Takeaways
- The XTR116 shows how a two-wire 4-20 mA transmitter bridges a sensor-side circuit to a current loop.
- A two-wire transmitter is not an independent current source. Instead, it modulates the loop current by controlling the conduction of an external pass transistor.
- This is a transfer function implemented in hardware where IOUT = 100 × (V_IN / R_IN)
- The minimum loop current is never zero. This places a hard limit on the amount of current that may be drawn for the sensor and processing circuitry.
- The transmitter does not galvanically isolate the sensor-side from the loop-side circuitry. Do not inadvertently cross the grounds with a bench power supply or an oscilloscope probe ground.
- The transmitter is simultaneously powered by and sends a signal to the 4-20 mA loop. About 9 VDC of compliance voltage is required to keep the transmitter and sensor-facing circuit operational. Keep this in mind while troubleshooting analog systems.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Industrializing Microcontroller-Based Designs
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 01 May 2026
What functions are performed by an Analog 4-20 mA loop transmitter?
The Texas Instruments XTR116 shown in Figure 1 is used as a representative example of an analog current loop transmitter. Digitally controlled current loop transmitters are available and will be explored in a future article.
The analog 4-20 mA loop transmitter is highly optimized to reduce the need for external circuitry. The transmitter functionality is best viewed from the inside looking outward, first to the sensor-facing side and then to the loop output side. Refer to datasheet block diagram included for convenience as Figure 2.
Figure 1: TI XTR116 on a breadboard with supporting circuitry. The BD139 pass transistor is in the foreground.
Sensor-Facing Functionality
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5 VDC regulator output: This is a highly desirable feature that allows the sensor-facing side to be powered via the loop itself. This is the trick that allows low-power sensors to be connected with only two wires.
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Precision 4.096 VDC voltage reference: The UA variant of the XTR116 provides a typical accuracy of ±0.05 % (UA + SHE Variant) with a ±20 ppm/°C temperature coefficient. This is functionally equivalent to installing a precision voltage reference into the transmitter. For reference, an approximate equivalent is the LM4132.
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Operational Amplifier: The op-amp is part of the current sense loop. From Figure 2, we also see that the output current is calculated as 100 * (V_In / R_In).
Output-Facing Functionality
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Pass transistor: Note that the pass transistor cannot be mounted in the tiny SOIC-8 package as it dissipates too much heat. For example, assume a 24 VDC supply, a 100 Ω load resistance and a 20 mA current. The transistor will continuously dissipate 0.44 W calculated as 22 VDC @ 20 mA (the burden resistor drop 2 VDC). For a prototype, the BD139 or even a TIP41 is a reasonable solution. An SMD equivalent will most likely be used in production. But don’t forget this transistor is operating in its linear range and dissipates considerable power. As homework, I’ll leave it to you to calculate the worst case situation when the external power supply reaches 40 VDC.
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Note that external circuitry is required for surge protection. This typically includes a TVS and decoupling capacitor. A diode bridge may be used as it simplifies the sensor installation by removing the system’s polarity dependence.
Tech Tip: The 4.096 VDC is a power of two. It is a natural fit with a 12-bit DAC providing a nominal 0.001 volt per bit resolution.
Figure 2: Block diagram of the TI XTR116 transmitter.
Floating Ground on the Sensor-Facing Side
Careful inspection of the block diagram in Figure 2 reveals that the sensor-facing current return pin (pin 3) is not directly connected to the output-facing current output pin (pin 4). This strongly suggests that the XTR116 is designed for floating sensor-facing side of the circuitry e.g., not directly referenced to 24 VDC return, ground, or even the loop ground. This has strong test bench implications as we must not accidentally bridge the returns.
To be clear, the XTR116 does not provide galvanic isolation between the sensor and output sides. In Figure 2 we see that the 25 Ω resistor sits between pins 3 and 4.
Workbench Demonstration
The circuit shown schematically in Figure 3 is used to demonstrate the analog XTR116 current transmitter:
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The XTR116 is installed on a DigiKey DKS-ADPT-01 adapter board.
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An LED is connected to the 5 VDC regulator’s output to simulate a constant load of approximately 3 mA. This current represents the load drawn by the sensor and associated analog signal processing.
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A BD139 NPN transistor is used as the pass element.
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A Digilent Analog Discovery is used as both signal generator and oscilloscope. The 0 to 2.5 VDC drive signal is connected to TP1 while the output is monitored on TP2.
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The test signal is injected into the input current pin (pin 2) via a 10 kΩ resistor.
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A 100 Ω load resistor is used to convert the current to a voltage. The 100 Ω resistor develops 100 mV per mA of loop current e.g., 20 mA yields a 2.0 VDC drop.
Tech Tip: The Digilent Analog Discovery is equipped with differential oscilloscope inputs. These are useful for this experiment as measurements can be made without inadvertently shorting the XTR116 current return (pin 3) and the current-loop output (pin 4). Refer to Figure 1 for probe placement (orange for channel 1 and blue for channel 2).
Figure 3: Schematic of the author’s XTR116 test circuit.
DigiKey Workbench Findings
The results are shown in Figure 4. Observe:
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The signal generator is configured to provide a 0 to 2.0 VDC sawtooth waveform.
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The experiment supports the datasheet claim that the output current is 100 * (V_In / R_In).
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The input and output waveforms are linear except for the lowest current readings. The discontinuity at low input levels is caused by the constant LED current of approximately 3 mA. This design choice prevents the loop current from going to zero.
Tech Tip: Note that the XTR116 contains a linear regulator similar in function to the 7805. In this application, the voltage is sent to the LED. The return path for the LED is to the current return leg (pin 3). This LED current is additive to the current through the pass transistor.
The minimum loop current is determined by the XTR116 quiescent current plus any current flowing through the regulator and the voltage reference (pins 8 and 1). Loop current is never zero.
Test for Minimum Compliance Voltage
The XTR116 requires a minimum overhead voltage to operate because of the 5 VDC linear regulator. The datasheet identifies a minimum compliance voltage of approximately 9 VDC.
To test this aspect, the input voltage was set to produce a constant 20 mA loop current. The 24 VDC supply was then lowered while simultaneously monitoring the 5 VDC output (pin 8) and the 20 mA output current itself. As the low voltage was reduced, the 5.0 VDC regulator was the first parameter to drop when the loop supply reached 9.5 VDC with the current loop malfunctioning when the supply voltage reached 8.0 VDC.
Note that this test ignored the voltage drop across the load resistor in Figure 3. This is important as the results are highly dependent on the value of the burden resistor located in the PLC. It is also somewhat dependent on the voltage drop in the connecting wires which are effectively in series with the burden resistor.
Implications for the Field
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The transmitter is not an independent source of current. It modulates the loop current through the pass transistor. Without the external 24 VDC supply the device is useless.
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Do not expect a zero loop current even if the sensor is measuring what should be zero. Remember that the sensor and amplifiers scavenge power from the loop.
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If the loop cannot reach 20 mA, first verify that the source voltage remains steady at a nominal 24 VDC. If this is good then continue by searching for the voltage drops associated with every node. An excessive voltage drop is associated with either a burden resistor or a high impedance connection. Finally, consider using a process meter in place of the circuit to verify that the problem is not with the sensor or physical system being measured.
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Be sure to view this guide to industrial troubleshooting.
Figure 4: Input and output waveforms for the XTR116U.
Continued Learning
The same TI XTR116 is available on the MIKROE Click board shown in Figure 5 (left socket).
While this is outside the scope of this article. I encourage you to experiment with the board to learn more about the signal chain from the SPI input to 4-20 mA current loop output. This includes:
- Analog Devices ADuM1411: Quad-channel digital isolator
- Microchip MCP4921: 12-bit DAC with a SPI interface
- Texas Instruments XTR116: 4-20 mA Current-loop transmitter
Also, be sure to examine the output section as shown in Figure 6. This includes necessary protection such as the filter capacitor and TVS diode. It also includes an optional bridge diode. This is convenient as the two-wire current loop is no longer polarity dependent.
Figure 5: Collection of MIKROE 4-20 mA boards hosted on a Microchip curiosity platform.
Figure 6: Schematic of the MIKRO 4-20 mA Click board’s output section.
Continue Exploring Industrial Control Systems
If this discussion was helpful, you may also want to explore:
DigiKey Navigation
- Full Catalog: Industrial Control & Automation
Related Foundational Articles
- When to Use PCB-Mounted Relay Sockets
- DIY PLC Components: Key Attributes of a 24 VDC Output Driver for Industrial Automation
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.