Is it possible to connect Ade7763 directly to 8051 ??

 

If you can do an SPI interface then yes you can connect them.

 

http://www.analog.com/static/imported-files/data_sheets/ADE7763.pdf
for ADE7763 which mc has to use 8/16 bit ?

 

http://www.analog.com/static/imported-files/data_sheets/ADE7763.pdf
for ADE7763 which mc has to use 8/16 bit ?

I do not quite understand Is your question about if it is OK to use a 8 bit micro? From my quick read of the datasheet I would say yes

 

The ADE7763 uses an SPI interface. You can implement this on any mcu requiring only three connections, SCLK, DIN and DOUT. (CS' can be tied LOW.)

If the mcu already supports SPI then the task is made a bit simpler.