ACS712ELCTR-05B-T

The ACS712ELCTR-05B-T from Allegro MicroSystems is a fully integrated Hall-effect current sensor that provides galvanically isolated, linear current measurement in a compact SOIC-8 package. The device uses Allegro’s patented Hall-effect technology to measure AC or DC current through an internal conductor, providing a voltage output proportional to the current with 2.1 kV RMS galvanic isolation between the current-carrying pins and the sensor electronics.

The ACS712 is one of the most widely used current sensors in the hobbyist and maker communities, as well as in industrial and automotive applications. Its simplicity of use (5 V supply, analog voltage output) and low cost have made it the de facto standard for non-invasive current measurement in Arduino and Raspberry Pi projects.

The 05B variant is the most sensitive version, with a measurement range of plus or minus 5 A and a sensitivity of 185 mV/A. Other variants in the ACS712 family include the 20A version (ELCTR-20A-T, 100 mV/A) and the 30A version (ELCTR-30A-T, 66 mV/A). The trade-off is between measurement range and resolution: the 05B version provides the finest resolution (smallest detectable current) but the narrowest range, while the 30A version provides the widest range but the coarsest resolution.

The galvanic isolation is a critical safety feature. The current-carrying conductor (IP+ to IP-) is electrically isolated from the sensor electronics (VCC, GND, VIOUT, FILTER) by the Hall-effect IC’s monolithic construction. The 2.1 kV RMS isolation rating allows the sensor to measure high-voltage circuits (up to 2.1 kV RMS above the sensor supply ground) safely, without any direct electrical connection. This is essential for measuring mains-powered loads, motor drives, and other high-voltage applications.

The ratiometric output means that the output voltage is proportional to the supply voltage (VCC). At zero current, the output is VCC/2 (2.5 V with a 5 V supply). At plus 5 A, the output is VCC/2 + 5 A x 185 mV/A = 2.5 V + 0.925 V = 3.425 V. At minus 5 A, the output is 2.5 V – 0.925 V = 1.575 V. This ratiometric architecture means that any noise on the VCC supply appears proportionally on the output, which can be cancelled by using VCC as the ADC reference voltage.

The 1.2 mOhm internal conductor resistance is extremely low, resulting in minimal power dissipation and voltage drop even at full-scale current. At 5 A, the power dissipation is only I squared x R = 25 x 0.0012 = 30 mW, and the voltage drop is only 6 mV. This makes the ACS712 suitable for measuring current in high-efficiency power circuits where insertion loss must be minimized.

The FILTER pin is a unique feature that allows the user to adjust the sensor bandwidth and noise performance by connecting an external capacitor from FILTER to GND. The bandwidth is approximately 80 kHz with no capacitor. Adding a capacitor reduces the bandwidth (and noise) according to the formula: Bandwidth = 1 / (2 x pi x 1 kOhm x CFILTER). For a 1 nF capacitor, the bandwidth is approximately 160 kHz (increased filtering). This allows the designer to trade response speed for noise reduction based on the application requirements.

The ELC package (SOIC-8) provides a compact, surface-mount solution. The TR suffix indicates tape and reel packaging for automated assembly. The 100 percent matte tin leadframe is compatible with lead-free soldering processes.

The ACS712ELCTR-05B-T operates as a Hall-effect linear current sensor using Allegro’s monolithic integration of a Hall element, signal conditioning, and current conductor.

Hall Effect Principle: When a current-carrying conductor (the IP+ to IP- path inside the IC) generates a magnetic field proportional to the current, a Hall element positioned adjacent to the conductor detects this magnetic field. The Hall voltage produced is proportional to the product of the magnetic flux density and the bias current through the Hall element. Since the magnetic flux density is proportional to the current being measured, the Hall voltage is directly proportional to the measured current.

Internal Conductor Path: The current to be measured flows through the IP+ pin (pins 1-2) to the IP- pin (pins 3-4) through an internal copper conductor integrated into the IC package. This conductor has a resistance of only 1.2 mOhm, minimizing power dissipation and voltage drop. The conductor is positioned directly above the Hall element, ensuring strong magnetic coupling and high sensitivity.

Signal Conditioning: The raw Hall voltage is processed through several stages of analog signal conditioning: (1) a high-gain amplifier boosts the small Hall voltage; (2) a chopper-stabilized offset cancellation circuit removes the DC offset and drift of the Hall element; (3) a factory-trimmed calibration circuit adjusts the gain and offset for the specified sensitivity (185 mV/A) and zero-current output (VCC/2); (4) the output buffer drives the VIOUT pin with a low-impedance voltage output.

Ratiometric Architecture: The output voltage is ratiometric to the supply voltage (VCC). This means that the zero-current output is always VCC/2, and the sensitivity scales with VCC. If VCC is 5.0 V, the sensitivity is 185 mV/A; if VCC is 4.5 V, the sensitivity scales to approximately 185 x 4.5/5.0 = 166.5 mV/A. To eliminate the effect of VCC variation on the measurement, the ADC that reads the VIOUT pin should use VCC as its reference voltage, so that both the sensor output and the ADC reference scale together.

FILTER Pin Operation: The FILTER pin provides access to an internal node in the signal path where an external capacitor can be connected to GND to create a low-pass filter. The internal resistance at this node is approximately 1 kOhm. The filter bandwidth is BW = 1 / (2 x pi x 1 kOhm x CFILTER). With no external capacitor, the bandwidth is limited by internal compensation to approximately 80 kHz. Adding CFILTER reduces the bandwidth and the output noise. This is useful for DC and low-frequency current measurement where noise must be minimized.

Galvanic Isolation: The current-carrying conductor (pins 1-4) and the sensor electronics (pins 5-8) are separated by the IC’s monolithic isolation barrier. The isolation barrier is designed to withstand 2.1 kV RMS (60 Hz, 1 minute) between the two domains. This isolation is achieved through the IC’s construction, where the conductor and the Hall element are separated by an insulating layer within the silicon die. No external optocouplers or isolation amplifiers are required.

Magnetic Hysteresis: The Hall element and the magnetic circuit are designed for nearly zero magnetic hysteresis. This means that the sensor output does not depend on the previous current history, ensuring accurate and repeatable measurements regardless of whether the current has been high or low before the measurement.