ADE5569 AC voltage measurement and DC supply grounding

Question

Hello, 
 Im going to measure single AC line voltage (230V) using ADE5569. MCU is currently on a breadboard and is supplied with lab DC power supply. When DC supply is disconnected from the circuit and AC voltage is connected to ADE according to datasheet is seems OK. However, when DC is connect to the circuit residual-current device ( RCD ) disconnects the AC line. 
 According to UG-061 there is a direct connection of AC NEUTRAL to DC 0V (fig.9 R25). 
 So what might cause the problem? 
 Does it mean that the AC NEUTRAL and DC 0V are not on the same voltage level? So there was a current flowing from AC NEUTRAL to DC 0V and then to DC supply EARTH, or opposite? That would explain RCD action(current was not returning through phase but but through EARTH). 
 The same will happen when the serial line is connected to the ADE circuit. What seems to help is using supply that does not have EARTH connected. However connection to PC would still need isolation. 
 Can anyone explain what exactly is happening when connecting both DC supply/PC and AC line? 
 Thank you for advice, 
 Tomas

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ADIApproved · Answer

Hi saimot 
 Yes. It is recommended to ground the neutral line (when using a shunt sensor, phase has to be grounded). This is done to ensure that the ADE chip gets a stable ground reference and the common mode of the input signals is limited. But if you tie the neutral to earth ground (which will be at a different potential than the neutral), then there will be issues along the lines of what you are observing. What power supply circuitry are you using? Can you attach a schematic of your design? Is it possible for you to go for a power supply design taking neutral line as the ground reference? Or use a isolated power supply design? 
 And of course, if you are communicating with the PC, you will have to isolate it from the high voltage side of the board for safety purposes. 
 Refer to App note AN564: http://www.analog.com/static/imported-files/application_notes/an564.pdf for a reference design. (page 12 has a power supply design schematic) 
 Regards 
 hmani

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ADE5569 AC voltage measurement and DC supply grounding