Aug 19 2020 11 mins
This week's EYE on NPI takes a hint from the advice we're all getting to keep physically isolated - the ACS37002 current sensor (https://www.digikey.com/en/product-highlight/a/allegro-microsystems/acs37002-current-sensor) is a physically isolated sensor that takes advantage of the Hall effect to accurately measure power draw with full electrical isolation. Let's take a look! The ACS37002 is a 400kHz fully integrated Hall-Effect current sensor with very high accuracy, making it ideal for use in control loops of the most demanding power conversion applications. Current flows through the internal conductor coupling the magnetic field generated to the Hall sensor IC. As a result of no physical connection between the current and the IC, high voltage isolation is achieved. The current is sensed differentially by two Hall plates that subtract out interfering external common-mode magnetic fields. The sensor is factory-trimmed to provide high accuracy over the entire operating range without the need for customer programming. The sensor has an analog output and a reference output for signal integrity in noisy environments. A fast over-current fault output is included, having a user-configurable threshold via an analog input pin, providing short- circuit detection and enhanced system protection. The sensor also has four user-selectable gain options in the same device with two logic inputs without the need of extra components. This provides designers flexibility, and minimizes the overall bill-of-material for systems that need different current sensing ranges. At some point in an engineer's life, they have to measure the voltage, current or power of a circuit - for example, to make sure the circuit doesn't have a short or open, to verify that a battery is charging, or to adjust motor torque. Measuring current can be a little challenging - we can use low side sensing, where we stick a small resistance at the bottom of our circuit, and amplify the voltage difference from ground. (https://learn.adafruit.com/portable-solar-charging-tracker/analog-stuff) That makes a ground differential, which is often undesirable. Or you can have high side sensing, where you measure the difference between the highest voltage and a resistor drop. Sometimes this is challenging because the device you're measuring is much higher voltage than the circuitry doing the measurements, and the high common-mode makes that difficult. Or maybe you're measuring AC or high voltages and you want to keep your circuit isolated for safety! These Allegro current sensors are really nifty in that they are electrically isolated - there's no op amp connected to the sense resistor. How does this work? Instead of multiplying a voltage drop, we instead measure the magnetic field emitted by the current passing through the sense resistor. A hall effect sensor can measure the amount of field and then back-calculate the current passing by. This allow for very high voltage isolation. In the ACS37002 case, it's rated for 4.8kV isolation. Hall effect sensors are also very fast/responsive, so they can measure AC currents very easily, without introducing inductance like a current transformer would. The ACS712 (https://www.digikey.com/products/en/sensors-transducers/current-sensors/525?k=ACS712) is a famous chip from Allegro that was released a few years ago and is very popular for robotics, home automation and other high current measurement applications (https://www.digikey.com/en/product-highlight/a/allegro-microsystems/current-sensor-ics). This new sensor improves on the ACS712 - there's a 3.3V version of the ACS37002, the internal resistance is lower, the non-linearity is half as much, there's an adjustable range pinstrap and the current range is much higher - up to 66A! There's also a lot more configuration options, with gain ranging from 200 mV/A to 33 mV/A. This chip would make a great upgrade when you want better performance and comes in an easy-to-use SOIC 16 package. Want to upgrade your robot, scooter, or solar panel? You can order an ACS73002 today and have it in your hand tomorrow, pick a couple up today from Digi-Key stock by visiting https://www.digikey.com/short/zrb8p4 Also check out this Allegro video https://youtu.be/7XIXOHnmOSo?t=65 from 1:05 to 2:25 #EYEonNPI #DigiKey @DigiKey