Why Magnetic Sensor Selection Deserves Care

A magnetic sensor turns a physical position into an electrical signal, so its sensitivity, package and accuracy set how well the system knows where something is. Choosing the wrong sensitivity wastes resolution or saturates the output, and a poor magnetic circuit adds error that no later tuning can remove. This guide walks through a repeatable method for selecting an Allegro linear Hall-effect sensor IC.

Step 1: Estimate the Field

Start with the magnetic circuit. The field at the sensor depends on the magnet material and size, the air gap and the geometry, so estimate or measure the field across the travel range. A small neodymium magnet at a short gap can produce a few hundred gauss, while a larger gap reduces the field quickly. Knowing the maximum and minimum field tells you the sensitivity you need, because the output must stay within its linear span at both ends.

Field versus Range

The usable field range of a linear sensor is set by its sensitivity and the output span around the quiescent point at half the supply. A high sensitivity reaches the output limit at a smaller field, and a low sensitivity tolerates a larger field. Estimate both the maximum field at the closest position and the field at the farthest position before you commit.

Step 2: Choose the Sensitivity

The Allegro A1324/5/6 family offers sensitivities of 5.0, 3.125 and 2.5 mV/G. Choose the highest sensitivity that keeps the output in range at the maximum field, because a higher sensitivity gives a larger signal and a better resolution for the ADC. For a small field and a short gap, the A1324 at 5.0 mV/G is ideal; for a medium field, the A1325 at 3.125 mV/G balances the two; and for a large field or a generous gap, the A1326 at 2.5 mV/G keeps the output in range.

Ratiometric Output

The output is ratiometric, so it scales with the supply and matches an ADC that shares the same reference, and the quiescent output sits at half the supply at zero field. This lets a positive or a negative field move the output above or below the midpoint, which makes a bidirectional measurement straightforward.

Step 3: Match the Package and Supply

Choose the package for the board: the 3-pin SIP is easy to prototype with and suits a through-hole design, and the 3-pin SOT-23 suits a compact production board. Both operate from a 4.5 V to 5.5 V supply, so a standard 5 V rail works, and both are immune to mechanical stress and stable after temperature cycling. Confirm the operating temperature against the application, because the parts cover -40 °C to +150 °C.

The Magnetic Circuit

A consistent air gap is the key to a repeatable measurement, so fix the gap with a mechanical stop or a bushing and keep the magnet and the sensor aligned. For a rotary measurement, two sensors placed in quadrature give the angle and the direction, and the field should vary smoothly with the angle for the best accuracy. Model or measure the field, then choose the sensitivity to match.

Step 4: Plan Accuracy and Noise

The device uses a dynamic offset cancellation technique to remove the offset drift and the low-frequency noise, which gives a low-noise output and a stable offset over temperature. To realize it, keep the supply clean with a bypass capacitor, keep the output trace short and free of switching noise and keep the sensor away from stray fields such as a nearby inductor or motor. Confirm the signal-to-noise ratio for the resolution you need.

Getting Help

If you send your travel or angle range, the magnet and the air gap, the supply and the accuracy target to our FAE team, we will propose a sensor and a sensitivity and support the design-in. BeiLuo holds mainstream Allegro magnetic sensors in regional stock and ships them with an import declaration, a certificate of origin and a RoHS compliance file, and our engineers will review the choice with you before you commit to production.