By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed August 4, 2026
A 4-20 mA signal splitter takes one current loop input and produces two isolated 4-20 mA outputs, so a single transmitter can feed two receiving systems. The typical pair is a PLC plus a SCADA input, recorder, or display. A signal isolator does the same job for one output: it galvanically breaks the electrical path while passing the measurement through. Both devices exist for the same underlying reason, receivers that must not share grounds or loop power.
The devices themselves are simple DIN rail modules; what goes wrong in practice is the wiring around them. Voltage budgets stop closing once a splitter joins the loop, passive outputs get wired to passive inputs, and HART signals silently disappear. This guide works through the selection with real numbers. It assumes you know how a current loop carries a signal; if not, start with our 4-20 mA current loop guide.
Contents
- Splitters and isolators
- Why loops need isolation
- The voltage budget
- Powering the device
- Sinking or sourcing
- Three ways to split
- HART and fault signals
- Choosing the device
- FAQ
Splitters and isolators
Three closely related modules get mixed up on purchase orders, and the difference is only the output count and the scaling:
| Device | What it does |
|---|---|
| Signal isolator | One 4-20 mA in, one galvanically isolated 4-20 mA out. Breaks ground paths, passes the value |
| Signal splitter | One input, two (sometimes four) isolated outputs, each driving its own receiver independently |
| Signal converter | Isolator that also rescales, for example 0-20 mA to 4-20 mA, or current to 0-10 V |
Most splitters are three-way isolated: input, output, and power circuits are all separated from each other, and on a two-output unit the channels are also isolated from one another, so one output loop can be opened, shorted, or serviced without disturbing the other. That channel independence is the real argument for a splitter over simpler wiring tricks. For current-to-voltage conversion specifically, we cover the resistor and module options in the 4-20 mA to 0-10 V guide.
Why loops need isolation
A ground loop forms when a loop is referenced to earth at two points whose ground potentials differ. Plant grounds routinely sit hundreds of millivolts to a few volts apart. The difference drives a stray current through whatever path connects them. Through shared conductors, that current adds to or subtracts from the 4-20 mA measurement current.
The numbers are unforgiving. A 2 V ground difference across a 100 ohm shield path pushes 20 mA of stray current: larger than the entire 16 mA measurement span. Even 1 V appearing across a 250 ohm receiver input corresponds to 4 mA, a 25% of span error. Galvanic isolation removes the second reference entirely, so no stray path exists.
The second classic fault is bucking supplies: a 4-wire transmitter with an active output wired to a control system input that also sources 24 V. Two sources then fight over the same loop and the reading wanders or pegs. An isolator with a passive input and passive output sits between the two and lets each side power its own half. The third reason is protection: isolation blocks common-mode transients and keeps a fault on one system from reaching the other.
One caveat vendor pages rarely print: when the transmitter, the receiver card, and the power supply are all isolated already, adding an isolator buys nothing. Isolate when a real second ground reference or a real conflict exists, not by default.
The voltage budget
Every series device in a current loop consumes voltage, and a splitter’s input circuit is one more series burden. Datasheets state it either as input resistance or as a compliance voltage. Typical figures run 50 to 125 ohms, a 1.0 to 2.5 V drop at 20 mA; some passive designs take up to 5 V. The loop still has to close its budget:
Supply voltage ≥ transmitter minimum + all series drops at 20 mA
Example: 24 V supply, 2-wire transmitter with a 10 V minimum, 250 ohm receiver input (5.0 V), splitter input burden 2.5 V. Budget: 24 − 10 = 14 V available; drops total 5.0 + 2.5 = 7.5 V, leaving 6.5 V of margin. The loop works.
Run the same check whenever a splitter, isolator, or extra indicator joins an existing loop. Marginal 12 V systems and long cable runs are where it fails. The arithmetic and a resistance quick-check live in our 4-20 mA calculator. The receiving-side resistance limit formula is derived in the loop-powered devices guide.
The output side has its own budget too. Each splitter output must drive its receiver and wiring; output drive is specified as a maximum load, commonly 500 to 600 ohms at 24 V. A 250 ohm PLC input plus field wiring sits well inside that; two receivers chained on one splitter output may not.
Powering the device
Splitters and isolators come in three power architectures, and the choice decides the wiring:
| Architecture | How it works |
|---|---|
| Externally powered (4-wire) | Own 24 VDC or AC supply; active outputs that source current; lowest input burden; the default for splitters |
| Output loop powered | Takes operating power from the receiving loop; output is passive, so the receiver side must provide 24 V |
| Input loop powered | Takes 3 to 5 V from the measurement loop itself; no power wiring at all; adds the largest series burden |
The recurring point of confusion is the loop powered splitter. Its outputs are passive: each of the two receiving systems must supply its own loop voltage. The splitter runs on what it harvests from those output loops. A loop powered splitter cannot create two independent signals out of a single 2-wire transmitter loop with no other power anywhere.
Somebody still has to source every loop. If neither receiver can provide loop power, specify the externally powered 4-wire version and settle it.
Sinking or sourcing
Every 4-20 mA connection pairs one device that provides the loop voltage with one that regulates the current. An active, sourcing, output puts 24 V on its terminals and drives current out. A passive, sinking, output regulates current but expects the other side to provide the voltage. The wiring test is one question per wire pair: which end puts the 24 V across this pair?
Exactly one end must. Two active devices on one pair fight each other, the bucking supply problem again; two passive devices leave a dead loop that reads zero.
Splitter datasheets mark each output as active or passive, and better units let you configure per channel. Match them. An active splitter output feeds a passive PLC input directly; a passive one needs the PLC card, or an external supply, to power that pair. If a reading sits at zero after installing a splitter, check this before anything else.
A passive output wired to a passive input is the most common commissioning fault with these modules. Terminal-level wiring conventions are covered in our 2-wire, 3-wire and 4-wire wiring guide.
Three ways to split
A dedicated splitter is not the only route to feeding two receivers, and it is worth knowing all three before buying one.
Receivers in series. Current is equal everywhere in a series loop, so two receiver inputs wired in series both see the true signal; the cost is burden, since the drops add. With a 24 V supply and a 10 V transmitter minimum, the loop tolerates up to (24 − 10) / 0.020 = 700 ohms. Two 250 ohm inputs plus 50 ohms of wiring total 550 ohms and work; a third receiver would push it to 800 ohms and fail.
The larger objections are operational. There is no isolation between the systems. Worse, an opened input, an unplugged card, or a blown fuse on either receiver kills the signal to both.
A second output from the transmitter. Many converters and transmitters already provide dual outputs: analog plus pulse, relay, or RS485. That solves the problem at the source, with no extra module and no added burden.
A dedicated splitter. The clean general answer when both receivers need an analog 4-20 mA, ownership of the two systems is separate, or the second system may be serviced independently. Each side gets its own isolated, individually driven copy of the signal.
Application example
Test facility, India. A flow bench on DN200 and DN400 water lines needed a local reading at the pipe and a digital feed to the PLC from the same measurement. Rather than splitting an analog loop, we proposed magnetic flow meters whose converters provide the local display and a separate digital output for the PLC. That is the second-output route: solved at the transmitter, with no extra modules in the loop.
HART and fault signals
Two things ride on a 4-20 mA loop besides the value, and both deserve a check before a splitter goes in the middle.
HART superimposes its digital data as 1200 and 2200 Hz tones on the analog current. An ordinary analog splitter or isolator low-pass filters the signal, with bandwidth commonly in the tens of hertz. The tones are stripped, and a configurator downstream of the module no longer sees the device. If you need HART access, either connect at the transmitter side of the splitter, or specify a HART pass-through model, which several manufacturers offer.
Our HART pressure transmitter page covers what travels over that digital layer.
Fault signaling should propagate too. NAMUR NE43 devices report failure by driving the loop to 3.6 mA or below, or 21 mA and above. The receiving system’s diagnostics only work if those levels arrive intact. Check that the splitter’s specified range and current limit pass them.
Units limiting output at 25 to 30 mA reproduce a 21.5 mA upscale fault faithfully, and good designs track the input below 4 mA too. A module that clamps its output at exactly 4 to 20 mA would convert a deliberate fault signal into a healthy-looking reading, which defeats the diagnostic. Fault level conventions are detailed in the loop-powered devices guide.
Choosing the device
Six datasheet lines decide whether a splitter or isolator fits the loop:
| Check | What to look for |
|---|---|
| Input burden | Resistance or compliance voltage; confirm the source loop’s budget still closes with it added |
| Output drive | Maximum load per output, commonly 500-600 ohms; must exceed receiver input plus wiring |
| Isolation rating | Read both numbers: a test withstand such as 1500 VAC for 60 s, and the continuous working voltage, often 250 VAC. The working figure is the one that matters day to day |
| Response time | Milliseconds to tens of milliseconds; fast enough for monitoring; for control, verify against the loop’s required response |
| Accuracy and drift | 0.05-0.1% of span is normal; the module’s error adds to the transmitter’s |
| Fault and HART behavior | NE43 levels pass through; HART pass-through only if specified |
One boundary worth stating: a signal isolator is not an intrinsic safety barrier. Hazardous area loops into Zone 0/1 need certified IS barriers or galvanic IS isolators selected under the plant’s protection concept, not a general purpose splitter. And the field side of all of this is a transmitter: sizing the loop starts with its supply requirements, covered on our pressure transmitters pages.
FAQ
Can a 4-20 mA signal be split?
Yes, three ways. Wire two receivers in series when the voltage budget allows. Take a second output from the transmitter. Or install a signal splitter that produces two galvanically isolated 4-20 mA copies; that is the standard answer when the receivers belong to separate systems.
How do I duplicate a 4-20 mA signal?
Install a signal splitter: one loop input, two isolated outputs, each driving its own receiver up to the specified load, commonly 500 to 600 ohms. Confirm the input burden still fits the source loop’s voltage budget. Then match each output’s active or passive mode to the receiving input.
What is the purpose of a signal isolator?
It galvanically separates the sending and receiving sides of a current loop while passing the measurement through. That eliminates ground loop errors between points at different ground potentials. It also resolves two devices fighting to power one loop and blocks common-mode transients from crossing between systems.
What is the difference between signal isolator and signal converter?
An isolator repeats the same signal type across an isolation barrier, 4-20 mA in and 4-20 mA out. A converter isolates and also changes the signal, for example 0-20 mA to 4-20 mA or current to 0-10 V. Most catalog families offer both functions in the same housing series.
Request a quote
Tell us the transmitter type, supply voltage, and what each receiving system needs. We will confirm whether the loop needs a splitter, an isolator, or just a second output at the instrument. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.