Yes, provided there is enough voltage headroom to accommodate the signal conditioner’s burden voltage.
At first glance, the 4-20 mA signal conditioner looks like a distance repeater. This undersells the technology and how it may be applied in your systems. Instead of simply repeating a signal (4-20 mA in and 4-20 mA out), a signal conditioner may be used as a system interface. It provides galvanic isolation. It also provides a way of series connecting (electrically tapping) into a 4-20 mA line and repeating an analog signal.
The testbench shown in Figure 1, with wire diagram in Figure 2, is used to explore the operation of the Schneider Electric RMC1AABD signal conditioner.
Key Takeaways
- The signal conditioner is to analog signals as the solid-state relay is to digital signals.
- These interposing decoupling devices allow signal transfer between two galvanically isolated systems with minimal circuit disturbance. For example, a sensor can be powered from a different source than the PLC.
- The signal conditioner may also be used to tap into an existing current loop to produce an isolated replica of the current. This derived signal can then be passed to an add-on edge data collection device without significantly modifying the existing system.
- Insertion into an existing loop consumes voltage headroom. Calculations are required to determine if there is sufficient voltage margin for the series-connected signal conditioner.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Analog
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 17 Aug 2026
Figure 1: Testbench for the signal conditioner using a Fluke ProcessMeter as the source.
Figure 2: Wire diagram for the testbench setup.
How far can a 4-20 mA signal travel?
The absurd 1 km length is used to make a point about wire resistance. It provides a worst-case voltage drop for our voltage budget calculations. Note that Siemens recommends the sensor be installed no more than 200 m from the featured ET 200 SP analog module.
Tech Tip: In practice, most system designers would convert the sensor output to a digital representation and then use Modbus TCP, PROFINET, or related networks to transfer the signal to the PLC. These digital methods are less susceptible to noise. For increased noise immunity, consider using a fiber-optic backbone. Note that fiber may be run adjacent to power conductors without risk of electromagnetic interference. The cost of the fiber may be offset by the reduced number and complexity of cable trays required to isolate the power and network cables.
Can we tap into a current loop?
This section examines the circuit operation to determine if there is enough voltage headroom to tap into an existing 4-20 mA current loop. To answer this question we must first calculate the voltage margin. We can tap into the system if the signal conditioner’s burden voltage is less than the original system’s voltage margin.
Equipment Assumptions
To better understand the nature of the 4-20 mA system, let’s anchor our discussion in real-world components. Let’s also assume the sensor is 1 km away from the receiver. We will illustrate the technique using the following representative equipment:
- Phoenix America 13973 145 PSI pressure sensor with a minimum operating voltage of 10 VDC for the 4-20 mA loop
- Siemens 6ES71346GF000AA1 ET 200SP input module with a 100 Ω input resistance
- 1 km of shielded Belden 8762 060U1000 20 AWG with a datasheet specified resistance of 11.2 Ω/1000 ft (36.7 Ω/km)
Calculating Voltage Margin in a 4-20 mA System
Account for the wire’s resistance for both conductors:
R_total = 2 x 36.7 Ω = 73.4 Ω
Calculate the voltage drop in the wire:
V_wire = 20 mA * 73.4 Ω ≈ 1.5 VDC
Calculate the voltage across the PLC’s analog input:
V_PLC = 20 mA * 100 Ω ≈ 2 VDC
Evaluate the voltage budget assuming a 20 V supply (see Tech Tip) with a 20 mA current loop:
| Loop Element | Voltage Budget |
|---|---|
| Wire | 1.5 VDC |
| 4-20 mA sensor / transmitter | 10 VDC |
| PLC analog input | 2 VDC |
| Total required | 13.5 VDC |
| Available margin | 6.5 VDC |
Tech Tip: Many industrial control systems are powered using a nominal 24 VDC. Do not use this value for voltage budget calculations. This article shows that an industrial voltage buffer maintains a 22 VDC ride-through voltage. We then subtract a few volts, leaving 20 VDC as a conservative worst-case design minimum.
Calculating the Burden Voltage of the Signal Conditioner
Figure 1, with accompanying wire diagram in Figure 2, shows the signal conditioner on the author’s workbench. This device has a 4-20 mA input and an isolated 4-20 mA output.
- The Fluke 789 is supplying a 10.1 mA signal.
- The Fluke 87-V displays the isolated current replica. Note that the signal conditioner has an active (powered) current source.
- The DC power supply is powering the signal conditioner. As will be shown, the signal conditioner contains circuitry to galvanically isolate the 24 VDC supply from the conditioner’s input and output circuitry.
- A third voltmeter (not shown) is used to measure the voltage across the signal conditioner’s input.
The RMC1AABD signal conditioner has a measured input voltage drop of 1.8 VDC in response to a 20 mA signal. This corresponds to a 90 Ω equivalent series resistance on the input of the signal conditioner.
Suitability
Based on the previous calculations, the measured 1.8 VDC is less than the calculated worst-case voltage margin. Consequently, the signal conditioner may be inserted in series with the existing current loop as shown in Figure 3. This example shows the ET 200SP as the original input module with a second (isolated replica) current loop for a data collection system. Note that the signal conditioner performs the same function as a solid-state relay. Instead of a digital signal, it allows an analog signal to be isolated and sent across domains.
Figure 3: Wire diagram of the signal conditioner that is series connected into an existing control loop.
Unexpected Workbench Discoveries
Originally, I had assumed the signal conditioner’s output was associated with the +24 VDC supply. The voltmeter showed that the output is completely independent (floating). In fact, experiments show that the conditioner’s output terminal (pin 2) may be referenced to either the +24 VDC supply or to the return (ground). Like the negative terminal of a battery referenced to a node, the results are consistent with a floating circuit.
On closer inspection, the signal conditioner’s datasheet describes the isolation as:
3 kV AC between the inputs and the supply circuit
between the outputs and the supply circuit
between input and output
To understand how this was done, I removed the signal conditioner’s cover. The isolated sections show clear delineation between the input, output, and power supply sections. Figure 4 shows the measures Schneider engineers took to achieve the signal isolation. This is one of two isolation modules. It appears to transfer the signal from the input to the output sections. The other device contains a transformer to provide power to both the input and output sections.
Figure 4: One of two isolators used in the Schneider signal conditioner.
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.