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 !!!!!!!!!!!!

 

[Mike odom]

Those are the voltage inputs. The 0 ohm resistors are stuffed or remove to set the attenuation of the voltage channels. Read the calibration procedure a few pages in front of the schematic. If you're building the circuit as a prototype, you can stuff only the resistors you need, and not have to short out any of the ones you don't need, just by not putting them in. But if you are taping a board, they did that to make it flexible for whatever the actual input voltages are (the design example shows the calculation for a 240V system).

The neutral has to be shorted to the AGND as this is the reference for the three phases, and thus the reference for the power meter chip. Safe? As long as you don't 'lick test' it while it's hot. The output data looks like it goes through an optoisolater, so it should be ok.

 

[gehan_s]

Those are the voltage inputs. The 0 ohm resistors are stuffed or remove to set the attenuation of the voltage channels. Read the calibration procedure a few pages in front of the schematic. If you're building the circuit as a prototype, you can stuff only the resistors you need, and not have to short out any of the ones you don't need, just by not putting them in. But if you are taping a board, they did that to make it flexible for whatever the actual input voltages are (the design example shows the calculation for a 240V system).

thank you very much !!!!!!!!!!!

if i want to achieve complete isolation from the mains then i shouldn't use the attenuation network directly. a good way would be to use step down transformers as i am using them anyway to power the circuit. how can i go about doing this ???

The neutral has to be shorted to the AGND as this is the reference for the three phases, and thus the reference for the power meter chip. Safe? As long as you don't 'lick test' it while it's hot. The output data looks like it goes through an optoisolater, so it should be ok.

thank you very much !!!!!!!!!!!

if i want to achieve complete isolation from the mains then i shouldn't use the attenuation network directly. a good way would be to use step down transformers as i am using them anyway to power the circuit. how can i go about doing this ???

 

[panic mode]

you put three of them, one on each phase. then on secondary you use the attenuation in a same way to calibrate each phase.