Shunts and amplifiers own the precision-current space, but they measure by getting in the way. When the measurement must not touch the circuit — high-voltage buses, motor phase currents, multi-hundred-amp rails — magnetic current sensing measures the field instead, delivering galvanic isolation for free. Here is how the technologies compare and where each wins.
The Physics: Field Strength Becomes a Measurement
Current produces a magnetic field proportional to its amplitude. Magnetic sensors measure that field — through a magnetic core concentrating it, or directly through air — and output a voltage or digital value. Three families dominate:
| Technology | Accuracy | Bandwidth | Best For |
|---|---|---|---|
| Open-loop Hall | ±1–3 %, drifts with temperature | 50–300 kHz | Cost-sensitive isolation, battery and motor monitoring |
| Closed-loop (compensated) Hall | ±0.5 % class, better linearity | 100–500 kHz | Servo drives, power quality, precision industrial |
| Fluxgate / TMR coreless | ±0.1–0.5 % class | kHz–MHz (technology-dependent) | Precision power measurement, high-end drives |
Open-Loop vs Closed-Loop: The Compensation Trade
- Open-loop sensors measure the field directly: simple, low power, small, cheap — but accuracy inherits the Hall element's temperature drift and the core's nonlinearity. Good enough for protection thresholds and coarse monitoring.
- Closed-loop sensors null the core flux with a compensation winding driven by the output: the sensor operates at zero field, cancelling drift and nonlinearity. The cost is a compensation output stage (supply current) and a larger package with the compensation coil.
- Coreless integrated sensors (Hall or TMR elements beside the busbar in a single IC) eliminate the core entirely — compact, fast, and surprisingly accurate with digital calibration. They have taken over EV traction-inverter and board-level DC monitoring designs.
The Specs That Decide in Practice
- Offset and its drift: unlike a shunt (whose signal grows with current), magnetic sensors carry a fixed offset error at zero current. Battery-management "sleep current" accuracy is an offset story — check µV-referred drift over temperature, not just the 25 °C figure.
- Bandwidth and delay: motor control loops and protection circuits need the measurement to arrive fast. Open-loop Hall delay of a few microseconds is fine for overcurrent protection, marginal for fast FOC loops — check step-response delay, not just −3 dB bandwidth.
- External field immunity and crosstalk: nearby busbars and motor windings inject fields. Differential sensing, core concentrating, and IC-level field-rejection features differ hugely between parts — test in the real magnetic environment.
- Isolation ratings: the headline benefit. Confirm working voltage, reinforced-isolation ratings, and creepage/clearance against the standard you certify to (UL/IEC 60747-17 for magnetic current sensors is the reference for EV traction applications).
Where Magnetic Sensing Beats the Shunt
- Galvanic isolation is required — high-side phase current on a 400/800 V bus, grid-side monitoring, medical patient-connected equipment.
- Current is large: shunt I²R loss above ~50–100 A becomes a thermal design problem; magnetic sensors add nearly zero insertion loss.
- Insertion impedance matters: no shunt voltage drop means no impact on the measured circuit — critical for efficiency-critical converters.
Where a shunt still wins: sub-amp precision, lowest cost, best linearity at DC, and simpler calibration. Many systems use both — shunts for precision DC buses, magnetic sensors for isolated phase currents.
Selection Workflow
- Define current range, bandwidth/delay requirement, isolation rating, and accuracy at temperature extremes.
- Pick the technology class from the table; shortlist by package (inline busbar vs PCB-trace primary).
- Check offset drift and external-field rejection at your real operating environment.
- Validate delay and step response in the actual control loop, not just on a signal generator.
JTDZ Tech supplies open-loop and closed-loop Hall sensors, coreless integrated magnetic current sensors, and the isolated amplifiers around them from Allegro, Melexis, TDK-Micronas, LEM, and TI. Tell us your current range and isolation requirement and we will quote the sensing approach that fits — contactless where it must be.
