What’s the Difference Between Sinking and Sourcing? 4-20mA Current Loop Isolators

By Mike, Inside Technical Sales Representative

Sinking, sourcing, active, passive, powered, resistive, two-wire, four-wire, what’s the diff?!

One of the most called for products in the process I/O field are our 4-20mA current loop isolators for interfacing field device to control device. There’s a need for galvanic isolation, as well as matching up I/O sinking/sourcing requirements of the input to output devices. I receive two or three calls weekly for 4-20mA isolators/splitters where the caller is not clear on the sinking/sourcing aspects of the devices at hand. Additionally, different manufacturers tend to use different terminology. Sinking, sourcing, active, passive, powered, resistive, two-wire, four-wire, what’s the diff?!

Types of Current Loop Isolators

A current loop circuit needs a DCV power source to create a voltage potential sufficient to cause electrons to flow (current). No power source means no current flow. Analog input circuits, such as mA input, have a resistive load (or impedance) value. This will cause a predictable voltage drop from the power supply in the current loop circuit. Predominately, current loop isolators use a 4-20mA input/output range (although some still operate in the 0-20mA I/O range). As can be seen in the graphic below, there are two main types of current loop isolators used today; two-wire loop powered and four-wire DC/AC powered isolators, as well as 4-20mA splitters.

4-20mA Current Loop Isolators: Sinking or Sourcing, What's the Diff?

2-wire Loop Isolators

Two-wire loop powered isolators are named such because only the input and output are wired via two signal wires; one wire powers the isolator, thus no extra wiring is required to provide DCV power to the isolator. Two-wire isolators (and signal converter/transmitters) have grown in popularity due to the time savings over wiring one extra set of power leads. Looking at the graphic above, both the Acromag 651T and 671T two-wire isolators are sinking (or passive) on the input. However, the 672T is sourcing (powering) the output loop side, while the 651T is also sinking on the output. The curved arrow in the graphic symbolizes the source of the loop power and direction of current flow.

The 651T output loop powered isolator is most popular by and large. It is sinking (passive) on both the input and output. This alleviates the common problem caused when an input field device and output control/monitor device are both powering the loop circuit; fighting each other as a result. The industry commonly refers to this as “bucking loop power supplies.”

4-wire DC/AC Powered Isolators

Acromag’s 631T DC powered isolator represents four-wire DC or AC powered 4-20mA isolators in the graphic. The 631T has a separate set of power connection terminals independent and isolated from the input/output circuits. Four-wire 4-20mA loop isolators (and splitters) will source (or power) the output loop circuit.

The TT336 four-wire mA input transmitter can be software-configured to output, either sourcing a 4-20mA or a 0-10VDC signal. Likewise for the SP336 mA input splitter, including software configuration for mixed outputs.


Signal Isolation: What it is and Why it’s Important. This whitepaper describes some of the signal quality problems in instrument and control circuits. Acromag signal isolators and isolated signal transmitters help prevent these issues.

 Electrical Grounding Rules. This 3-part whitepaper series discusses proper circuit wiring/shielding and grounding.

Whatever your signal I/O isolator or transmitter requirement is, call on Acromag. We’re one of the widest signal I/O products manufacturers in the market, and backed by over sixty years of quality manufacturing. Made in Michigan, USA.