transformer ground problem and attenuation network

[gehan_s]

Hi all,

I am designing a 3 phase power meter using the ade7758. To power the IC there is a transformer step down stage (See page 11 of a similar application note). There the neutral and the secondary of the transformers are shorted (both of them are named AGND). Can this be done ???? (In a way it is needed because the IC is sampling the AC voltage directly from the mains and it needs a reference point. But is it safe ??)

Secondly what is the point of having R20 to R36 for them are short circuited ???

Thanks in advance !!!!!!!!!!!!

 

[chuckey]

Can't see this on this data sheet :- https://www.analog.com/en/analog-to...rgy-measurement/ade7758/products/product.html
Frank

 

[FvM]

Can't see this on this data sheet

The AN641 link is pretty hidden in the post. Found it after reading the datasheet and the post a third time. :-(

Secondly what is the point of having R20 to R36 for them are short circuited ???

The voltage divider calibration concept (shorted resistors) is explained in the application note. Don't need to retell it, I think.

But is it safe ?

Energy meters have usually direct mains connection. Safe insulation is required. The external data interface has an opto coupler of sufficient voltage rating. During development, safety transformers or suitable differential probes are required.