Hi Kai Thank you very much for reply.
As Vref must not be greater than Vcc, it could be wise to gain Vref from Vcc. Why not adding a little LC filter, with ground terminal of this C connected to analog ground? This would heaviliy dampen digital noise on Vref and guarantee, that Vref isn't greater than Vcc.

I waited for your reply and Today I have already given few boards for manufacturing and in those boards I am deriving Vref from Analog Vcc and the ADC Chip PCF8591 is power supplied through Digital Vcc. I am using LM336-5.0 for Vref generation. Will it cause any problem. These boards were pilot lot. In the final production I will certainly make the correction as per your suggestion.

Yes, but only for DC currents! There IS a connection between both windings at higher frequencies. And this will result in a ground loop for digital currents!
Interwinding capacitance between both windings can be several hundreds of pF, have a look yourself by the help of a C-meter.
Assume an interwinding capacitance of 300pF. Then this gives an impedance of about 1 / (2 x pi x 10MHz x 300pF) = 53 Ohm at 10MHz. At 100MHz this impedance is only about 5 Ohm!

Ok, but then whats the correct way of feeding the two Supplies i.e. Analog supply and Digital Supply, without a ground loop.

Yes, but 10 Ohm is too small. Something in the range of 1kOhm would be better.

I will put a 1k in series with the Zero cross signal that is coming from the Analog signal and entering the digital section.

This can also be a soft ferrite.

If ferrite is used in series wont it eat up the signal itself if the signal is a high frequency one. [ A wild thought may be I am wrong].

Keep in mind, that everything I said is the more valid the higher the precision of ADC is. So, what might be neglected for a 8bit converter becomes relevant when you switch to a 12bit converter, for instance. And I remember, that you told us to eventually switching to higher precision ADC, didn't you?

I am carefully noting down all the important points coming out from this discussion. I will prepare a list of these points and refer to them whenever I design mixed stuff and very soon I am migrating to 12 bit design.

Thanks & Regards,
Prahlad Purohit