Allegro MicroSystems Current Sensors

Hall-effect current sensing uses the Hall effect to measure current without physical contact. When current flows through a conductor, it generates a magnetic field proportional to the current. A Hall sensor placed near the conductor measures this magnetic field and outputs a voltage proportional to the current. Allegro's integrated sensors combine the current conductor and Hall sensor in one package. Key advantages: galvanic isolation (no electrical connection between current path and sensor), bidirectional sensing (measures current in both directions), low power loss (conductor resistance < 1mΩ), and fast response (< 2μs). The Hall voltage is V_H = (I × B) / (n × t × e), where I is current, B is magnetic field, n is charge carrier density, t is thickness, and e is electron charge.

Use Hall-effect sensors for isolated, bidirectional current measurement with fast response and low power loss.

Power dissipation in Allegro current sensors comes from the conductor resistance: P = I² × R_conductor. Allegro sensors have very low conductor resistance (0.3-1.5mΩ typical), minimizing power loss. Example calculations: ACS712 (1.2mΩ): at 10A, P = 100 × 0.0012 = 0.12W

at 30A, P = 900 × 0.0012 = 1.08W. ACS37610 (0.1mΩ): at 100A, P = 10000 × 0.0001 = 1W

at 200A, P = 40000 × 0.0001 = 4W. Thermal considerations: Junction temperature Tj = Ta + (P × θja), where Ta is ambient temperature and θja is thermal resistance (typically 40-60°C/W for SOIC packages). Ensure Tj stays below maximum (usually 150°C for automotive grade). For high-current applications, provide adequate copper area on PCB for heat spreading. Some high-current sensors include thermal shutdown protection.

Calculate P = I² × R; ensure Tj < 150°C with proper thermal design; use low-R sensors for high-current applications.

Current sensor bandwidth selection depends on application requirements: DC applications (battery monitoring, power supplies): 0-10kHz bandwidth sufficient

AC line monitoring (50/60Hz): 1-10kHz bandwidth

Motor control (PWM switching): 50-100kHz to capture PWM ripple

Inverter output monitoring: 10-50kHz for fundamental frequency

Fast fault detection: > 100kHz for overcurrent protection

High-frequency power conversion: 500kHz-1MHz for GaN/SiC converters. Allegro offers sensors from DC to 1MHz bandwidth. Higher bandwidth provides faster response but may introduce more noise. For most motor control applications, 50-100kHz is adequate. For fast protection circuits, choose > 100kHz. The ACS37010 offers 1MHz bandwidth for high-frequency applications. Always verify that sensor bandwidth exceeds your signal bandwidth requirements.

Use 50-100kHz for motor control; > 100kHz for fast protection; 1MHz for high-frequency power conversion.

Allegro current sensors offer multiple output options for microcontroller interfacing: Analog voltage output (ACS712, ACS720): Output is proportional voltage (e.g., 185mV/A for ACS712-05B), connect directly to ADC input

use voltage divider if ADC range is lower than sensor output. PWM output (ACS37800): Duty cycle proportional to current, measure with timer/capture input. Digital I2C/SPI (ACS37800, ACS37010): Direct digital interface, read current value via serial bus. Key considerations: Reference voltage - sensors may output ratiometric to Vcc or fixed reference

ADC resolution - calculate LSB size to ensure adequate current resolution

Sampling rate - sample at > 2× bandwidth for accurate capture

Filtering - add RC filter at sensor output to reduce noise (typically 1-10kHz cutoff). For high-noise environments, use differential ADC inputs or add external instrumentation amplifier.

Use analog output for simple applications; digital I2C/SPI for noisy environments; PWM for isolated digital interface.

Allegro current sensor accuracy varies by product and conditions: Total error at 25°C: ±1% to ±5% depending on device

Total error over temperature (-40°C to 150°C): ±2% to ±8%

Offset error: ±5mV to ±30mV depending on device

Sensitivity error: ±1% to ±3%

Nonlinearity: ±0.5% to ±2%. Factors affecting accuracy: Temperature - all sensors have temperature drift, use devices with internal compensation

External magnetic fields - shield or orient to minimize interference

Power supply variation - use stable Vcc for best accuracy

PCB layout - follow guidelines for optimal performance. For highest accuracy applications (energy metering), use ACS720 with ±1% total error. For general monitoring, ACS712 with ±2-3% is sufficient. Always check datasheet for specific accuracy specifications at your operating conditions.

Use ±1% sensors for metering; ±2-3% for monitoring; consider temperature drift for automotive applications.