Hallo Raza,

please read the first 7 pages of this document, which explains ports' performance of AT89C51:

http://www.atmel.com/dyn/resou...OC0499.PDF

I remember a guy having problems with ADC ICL7135, it had to do with analog ground and digital ground. I told him he should read datasheet, this would explain everything. When he told me, that he could not find information I was surprised and finally re-read datasheet again. I must confess, that the guy was right. Datasheet was so much interested to tell, that both grounds have to be separated, that there was no hint, that they both have to be connected somewhere!
What I want to tell is, for a newbie a datasheet looks totally different than for an experienced person. For me and the writer of datasheet it was absolutely clear, that boths grounds have to be connected somewhere. So clear, that the writer was not writing about it and we both could read it between the lines. But the newbie had lots of trouble.

Raza, analog and digital ground are separated on your chip, because noisy digital currents running through digital ground pin would heavily contaminate analog ground, when there was only one ground connection to die. So, for digital currents there is an additional ground terminal, preventing analog ground from being contaminated.

Have a look at the following schematic for understanding what I mean:



Assume you have a chip which is sitting in DIL package, and there is one, and only one ground connection to die. Then, every ground current must flow over this connection. If ground current consists of digital currents, then inductivity of this only one ground connection makes trouble. High frequency content of digital current causes lots of voltage drop accross inductivity of this only one ground connection. As consequence, ground reference point on die is also shifted relative to that ground potential, which can be measured at ground terminal of DIL package. In fast digital logic chips this voltage drop accross ground connection, which is so important that it got an own name, namely 'ground bounce', can be in the range of many, many hundreds of millivolts!
If your die's circuitry is now containing an ADC, and there would be a direct connection between analog ground and digital ground ON DIE, then analog ground reference on die would also be shifted relative to an analog reference point of PCB. But circuitry on die cannot know, of course, what's happening on it's ground connection inside DIL package, but thinks, that this ground bounce belongs to analog input signal. So, what ADC sees (and converts!) is analog signal + ground bounce. The result would be a desaster, more than 4bit resolution is hardly to achieve.
Most ADCs circumvent this problem, by a special ground routing scheme: For digital ground and analog ground there are two different and separated connections to die. One for the noisy digital currents and the other for ground referencing of analog section of die. Now, analog ground of analog section of die can be directly connnected to clean analog ground of PCB. Digital currents are no longer flowing along this ground connection. ADC on die does not see any longer analog signal + ground bounce, but only analog signal.
Of course, on die, analog ground and digital ground are not identical. Potential difference is just identical to ground bounce. But this doesn't matter. Circuitry on die is designed and fabricated in such a manner, that this potential difference makes no problems. But, do always connect analog ground pin and digital ground pin with shortest connection on PCB, because otherwise potential difference can drastically increase to values, which can no longer be accepted.
If you forget to make a connection between both grounds on PCB, analog section and digital section on die will float relative to another and really strange things will happen... Floating is limited by some parasitic pn-junctions on substrate, but circuitry will not work correctly, at all.

Keep in mind, that distinguishing of an analog and digital ground connection to die is no guarantee for ideal performance! It's really neccessary to prevent digital currents from flowing along analog ground routing ON PCB, also. So, there should also be a certain separation of analog and digital circuitry on PCB. The only connection is ideally to be made at ADC chip, by making a short and wide connection (as free from inductivity and resistivity as possible) between analog ground pin and digital ground pin. No analog signal is allowed to be routed over and to be connected to any digital circuitry and no digital signal is allowed to be routed over and to be connected to any analog circuitry. It's the best to have two ground planes, one analog for analog circuitry and one digital for digital circuitry and to have them connected at only one point, namley to the connection of analog ground pin and digital ground pin at ADC.

There could be written much more about this topic. But it's enough to focuse on one important point: Do everything to prevent, that any digital current flows over any analog ground routing! Even the most sophisticated grounding, filtering and screening schemes are only the consequence of this simple rule...

Bye,
Kai