Designers use lot of techniques in their circuit design for measuring the current passing in the circuit. Generally, called a current sensing approach, there are several methods that have been used,
1. Using Op-amp based approach
2. Using Current Transformer
Both the above options help in only current sensing and if you want to measure the power you have to sense the voltage and multiply current and voltage. while this occupies lot of space on the board and is not good for modern day dense circuits. Also, adds up to the cost if voltage and current are sensed individually. Now, if you consider ACS71020 from Allegro Micro systems in your design, you can measure voltage and current using a small form factor chip.
The below functional block diagram from ACS71020 clearly shows the mechanism used for voltage and current sensing.
While the voltage sensing uses a sense resistor followed by operational amplifier based approach, the current sensing uses hall-effect based approach. ACS71020 has a copper conductive path inside the chip set with IP+ as input IP- as output. the current flowing between these pins generates a magnetic field which is sensed to provide current reading in digital form. ACS71020 provides a SPI interface for interfacing to the micro controller. Irrespective of the voltage that is to be sensed, the chip power supply input can be same as the micro controller supply. So, there is no need of additional power supply for ACS71020.
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