Current Loop Transmitter Circuit Design

A current loop carries a measurement as a current rather than a voltage, which is why it survives long cables, contact resistance and electrical noise that would ruin a voltage signal. The transmitter at the sensor end converts the measurement into a current between four and twenty milliamperes, and the circuit that does that has to work over a wide range of supply voltages and cable lengths.

Why a Current Loop Is Used

A voltage signal loses accuracy along a cable because the conductor resistance is in series with the source and varies with temperature. A current signal does not, because the same current flows through every element of the loop regardless of the resistance, so the receiver sees the transmitted value even after hundreds of metres of cable.

The four milliampere offset gives the loop a live zero. A reading of zero milliamperes means the loop is broken rather than that the measurement is at its minimum, which allows a fault to be distinguished from a valid reading. Values below four milliamperes and above twenty are available for signalling an alarm or a saturation condition.

The interface is also self powered in many installations, since the same pair of wires can carry the measurement and the energy to operate the transmitter. That combination of a simple two wire connection with inherent noise immunity is why the standard has survived for so long and is still specified in new installations.

The Transmitter Circuit

The measurement is converted to a voltage, and that voltage drives a circuit that sets the loop current. The conversion is usually done by a voltage to current stage built around an operational amplifier with a sense resistor in the loop return, so that the current is compared with the input and corrected by feedback.

Accuracy depends on the sense resistor as much as on the amplifier. A resistor with a low temperature coefficient and a tight tolerance keeps the scale factor stable, and a four terminal connection removes the effect of the solder joint and trace resistance from the measurement. The resistor value sets the voltage that the feedback loop regulates, so it should be chosen to give a level well above the amplifier offset.

The output stage has to work over the full compliance range. A transistor driving the loop dissipates power as the loop current flows through it, and that dissipation is highest when the loop resistance is at its minimum and the supply is at its maximum. The thermal design of that transistor is part of the transmitter design rather than an afterthought, and the package and copper area should be selected accordingly.

Loop Resistance and Compliance Voltage

compliance voltage is the maximum voltage the transmitter can develop across the loop while still regulating the current. It has to exceed the sum of the receiver resistance, the cable resistance and any other elements in the loop, with margin for the supply tolerance. If the compliance voltage is insufficient, the transmitter saturates and the current stops following the measurement, usually at the high end of the range.

loop resistance is the total series resistance that the supply sees, and it sets the voltage that the transmitter must produce. A 250 ohm receiver resistor plus a kilometre of a typical instrument cable adds up quickly, so the loop resistance should be calculated at the design stage from the actual cable specification rather than assumed.

The supply voltage is chosen to cover that requirement. A 24 volt supply is common, and it allows a loop resistance of several hundred ohms with a comfortable margin. Where the loop is long or the receiver has a high resistance, an isolated transmitter with a separate supply removes the constraint by allowing a higher voltage on the loop side.

Loop Powered versus Four Wire

A loop powered transmitter takes all of its operating current from the loop, which means the current it consumes must be less than the four milliampere minimum by a margin. That budget constrains the circuit: the amplifier, the sensor conditioning and any display must together draw less than about three and a half milliamperes, which rules out power hungry components.

A four wire transmitter has its own supply and uses the loop only to carry the signal. It can drive a higher loop resistance, use a processor with more capability and provide isolation without the power budget constraint. The cost is an additional pair of conductors, which is why the two wire version persists in retrofit applications.

isolation is worth its cost in an industrial installation. The measurement ground and the loop ground are often at different potentials, and an isolated transmitter breaks the path between them so that the difference does not appear as an error or as a circulating current. The isolation also protects the sensing circuit from faults on the loop wiring.

Protection for a Field Installation

A loop leaves the enclosure and travels through a plant, so it is exposed to surges induced by switching, by lightning and by faults elsewhere in the installation. Protection devices at the terminals limit those transients before they reach the transmitter, and a series element with the protection helps the clamp absorb the energy without diverting loop current during normal operation.

The protection has to be chosen so that it does not conduct at the maximum loop voltage, including the compliance voltage with margin. A device that starts to conduct at thirty volts would divert part of the loop current and produce an error at the top of the range, and the symptom would appear only when the measurement was near full scale.

Series resistance in the loop is not a problem in itself, because the current is unaffected by it within the compliance range, but the wiring and the terminals have to be sized for the current and for the voltage drop that the compliance calculation allows. The width required for a given current is covered in our guide to trace width and current, and the same reasoning applies to the terminals and the cable.

Layout of the Transmitter

Separate the sensing side from the loop side of the board, and keep the two return paths distinct until they meet at the intended reference point. A loop current that returns through the sensing ground introduces an error that changes with the measurement, which is the most difficult kind of fault to diagnose because the circuit appears correct on the schematic.

Place the sense resistor close to the amplifier inputs and use a four terminal connection where the package allows it. Keep the high impedance summing node small, and route the input pair from the sensor as a tight differential pair over a continuous plane. The approach is the same as for any precision front end, and the filtering and suppression methods in our article on EMI suppression design apply to the loop terminals as well.

Thermal layout deserves attention because the output transistor dissipates the difference between the supply and the loop voltage. Spread that dissipation over as much copper as the board allows, keep the temperature sensor or the reference away from it, and check the ambient rating of the components against the internal temperature rise of the enclosure.

Calibration and Field Faults

Calibrate the transmitter at four and twenty milliamperes with a precision ammeter in the loop, adjusting the zero and the span in that order. A two point calibration removes the offset and the scale error, and a check at twelve milliamperes confirms the linearity of the conversion.

A reading that sits at exactly four milliamperes when the process is at its minimum is correct, while a reading that drifts slowly downward usually indicates a leaking protection device or moisture on the terminals. A reading that is stuck above twenty milliamperes indicates an open loop or an overload, because the transmitter drives to its limit when it cannot close the loop.

Where the receiver shows a reading that corresponds to no plausible process value, check the loop resistance first. A corroded terminal or a partially open connection adds resistance that the transmitter may not be able to overcome within its compliance range, and the resulting saturation is often mistaken for a sensor failure. Measuring the voltage across the transmitter terminals while the loop is running separates the two cases immediately.

A conformal coating on the transmitter board is standard practice in humid plants, and the practices described in conformal coating apply to the high impedance nodes in particular. Coating does not replace a sealed enclosure, but it keeps surface leakage from adding a slow drift that appears only in the field.

FAQ

How do I calculate the maximum loop resistance? Divide the minimum supply voltage minus the transmitter minimum operating voltage by twenty milliamperes. Subtract the receiver resistance to find the cable allowance, then compare it with the actual cable resistance.

Why does my transmitter read high at the top of the range? Either the compliance voltage is insufficient and the output is saturating, or a protection device is conducting and diverting part of the current. Measure the voltage across the transmitter at full scale to distinguish them.

Can I share one supply between several loops? Yes, provided each loop has its own protection and the supply can deliver the sum of the currents. Sharing the return path is acceptable only if the voltage drops it creates are allowed for in the compliance calculation.