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I am designing a power meter based on ADE7758 and I'm using the official documentation for examples; I will also use an isolating IC for the communication between the ADE7758 and the microcontroller. I have some trouble understanding why is the ground of the +5V supply referenced to the neutral line in the diagram below: Power Meter Supply Design

The diagram is adapted from page 12 of AN-564 Application Note of Analog Devices and depicts a power supply design for a power meter.

The ADE7758 datasheet sheds some light on connection between digital and analog grounds, saying that these should be connected at only one point (page 9). What bothers me is that the voltage regulator's ground is supposed to be 0V for the digital circuitry, while the neutral rail may not be a good reference. I have not tested the circuit and before I go ahead and do that I would like to wrap my head around its working principle.

My other concern is what will happen if someone swaps the hot and neutral connections of the board; the microcontroller reference will be connected the the hot line and I want to avoid that.

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As a bare minimum the neutral needs to tie into the ground of the Metering Chip because of the need for the chip to also connect to the AC line hot side for the AC voltage monitoring. See circled areas in the diagram below. Without it the voltage measurement would be like trying to use a digital multimeter with just one lead.

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Picture Source (page 12)

Note that the reference board power supply showing connections of both sides of the transformer to the same side of the AC line is an unfortunate documentation error.

In many situations this type circuitry may very well have its power supply directly derived from the AC line without using the transformer coupled linear 7805 design used for the reference board.

In a single phase design the circuit would work just fine if the HOT and NEUTRAL were swapped. The product packaging and usage would have to take care of the safety aspects of this. On a three phase design on the other hand the NEUTRAL connection must be properly comprehended versus the three phase lines. Once again safety considerations are extremely important.

Final note: When working with any line attached circuitry at the design and engineering stage great care needs to taken to stay safe and not make any stupid mistakes.

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  • \$\begingroup\$ However, it should be possible to keep the setup isolated; just use a transformer for the V2P/N lines \$\endgroup\$
    – Richard the Spacecat
    Nov 22 '19 at 13:40
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    \$\begingroup\$ @RichardtheSpacecat - The metering chip needs to have a reference to the neural as I stated in my answer. So that part of the circuit cannot be isolated. The remainder of the circuit could be isolated (MCU, displays, serial EEPROM etc) easy enough by using isolators on the SPI connection to the metering chip and using a fully isolated transformer to power that other circuitry. \$\endgroup\$
    – Michael Karas
    Nov 22 '19 at 13:45
  • \$\begingroup\$ I doubt that a transformer on the lines that you mentioned would be able to maintain the 0.1% accuracy that the data sheet claims that the metering chip can achieve. \$\endgroup\$
    – Michael Karas
    Nov 22 '19 at 13:49

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