A1421, A1422, A1423: High Precision Hall Effect AC-Coupled Differential Sensor ICs with Integrated Filter Capacitor

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The A1421, A1422, and A1423 are AC-coupled Hall-effect sensor ICs which include monolithic integrated circuits that switch in response to changing differential magnetic fields created by rotating ring magnets or, when coupled with a magnet, by ferrous targets. This family of devices also includes an integrated capacitor that provides the high accuracy of analog sensing without an external filter capacitor. This reduces cost and components, while improving the reliability of the final sensor solution.

Magnetic field changes affect the two integrated Hall transducers and then are differentially amplified on the chip. Differential design provides immunity to radial vibration, within the device operating air gap range, by rejection of this common-mode signal change. Steady-state system offsets are eliminated using an on-chip differential bandpass filter with integrated capacitor. This filter also provides relative immunity to interference from electromagnetic sources. The device utilizes advanced temperature compensation for the high-pass filter, sensitivity, and Schmitt trigger switchpoints to guarantee optimal operation to low frequencies over a wide range of air gaps and temperatures.

Each device includes: a voltage regulator, two Hall transducers, temperature compensating circuitry, a low-level amplifier, bandpass filter, Schmitt trigger, and an output driver. The on-board regulator permits operation with supply voltages from 4.0 to 26.5 V. The output stage can switch 20 mA over the full frequency response range of the device, and is compatible with CMOS logic circuits.

The devices in this family differ from each other in their switchpoint specifications and their switching polarity. The A1421 has a small hysteresis and asymmetrical switchpoints, with one switchpoint at the zero-crossing. The A1422 has a small hysteresis and symmetrical switchpoints, both near the zerocrossing. The A1423 offers high vibration immunity, by means of its larger hysteresis that establishes symmetrical switchpoints further from the zero-crossing. The output polarities are shown in the Product Selection Guide table.

This variety of options provides flexibility for achieving solutions for a wide range of applications, including automotive transmission and crankshaft speed sensing.

The device package has an operating ambient temperature range –40°C to 150°C , and is provided in a 4-pin plastic SIP. Each package is available in a lead (Pb) free version (suffix, K-T) , with a 100% matte tin plated leadframe.

  • Integrated tracking capacitor
  • Used for sensing motion of ring magnet or ferrous targets
  • Wide operating temperature range
  • Operation with magnetic input signal frequency from 20 Hz to 30 kHz
  • EMI/ESD-resistant
  • Large effective air gaps
  • 4.0 to 26.5 V supply operating range
  • Output compatible with CMOS logic families
  • Reverse battery protection
  • Resistant to mechanical and thermal stress
  • Accurate true zero crossing switchpoint (A1421 only)
  • High vibration immunity, in running mode (A1423 only)

Typical Applications

  • Crankshaft
Allegro MicroSystems K Package (4-Pin SIP)

Each device is available in a 4-pin plastic SIP package. Each package is available in a lead (Pb) free version (suffix, K) , with a 100% matte tin plated leadframe.

K STP Model Documentation

Part Number Specifications and Availability

Documentation and Resources


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All Types Application Note Selection Guide
Documents Found: 32
Document Type Document Name
Application Note
Fundamental Properties of Speed Sensors
Selection Guide
Allegro Product Selection Guide