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Measuring currents is one of the oldest tasks of metrology…
Measuring currents is one of the oldest tasks of metrology. The knee jerk reaction involves adding a resistor to the current path and then analysing the voltage drop. Hall effect sensors provide a variety of benefits ranging from simplicity to galvanic insulation.
Allegro’s ACS723 sensor comes in a handy SOIC8 package, and provides a low resistance of just 0.65 milliohms. System design is simple – simply look at figure 1, which shows how to get the Allegro chip into your design.
Instead of having to play around with a differential amplifier, simply supply between 4.5 and 5.5 V to the component. Then, change the current to be measured to the device using pins one, two, three and four. Allegro provides two pins each in order to simplify layout and reduce voltage drops on the PCB and the interconnect.
The actual current output is then dependent on the selected version of the component: as of this writing, Allegro provides a total of seven different versions handling current ranges from 5 to 40 Ampere. In all cases, however, an analog voltage is put out which can be used without further meddling.
A small trap…
The only important consideration is the minimum input impendance of the attached amplifier, which is set to 4.7 Kiloohms. Most modern ADC parts do not have a problem meeting this – in the worst case, a simple operational amplifyer can be added to provide voltage buffering.
Whilst traditional resistor-based sensors need careful placement, a Hall effect sensor does not care about voltage potentials. You can set the sensing loop close to supply or close to ground – no special considerations need to be taken when designing the measuring loop.
Furthermore, resistors heat up significantly as currents raise. Our Allegro chip impresses with relatively constant thermic behaviour. Another very nice aspect of the product involves the presence of a bandwidth selection. Allegro provides you with a configurable filter, limiting the bandwidth of the filter to either 20 or 80 kHz. That way, unwanted noise and transients can be blocked out without having to perform additional filtering of the output signal.
Does it pay?
Feeding our price comparison engine yields a price range from £2.79 to £1.56 depending on the current range. Given the severe simplification and PCB space savings, going Hall effect definitely makes sense…