Hi Orlando,
I would like to add some comments to what Bob explained already.
As Bob said, the absolute input voltage specification in the ADS1220 datasheet is only valid in case you disable/bypass the PGA. When enabling the PGA, ADS1220 has a similar common-mode voltage limitation as ADS1248.
Also as you already guessed, the common-mode voltage limit does in the end automatically limit the absolute input voltage range as well. However the absolute input voltage is dependent on the supply, gain, differential input voltage and reference voltage in this case. Therefore it is very hard to specify this in an easy way within the electrical specification table.
At gain of 1 and a supply of AVSS=0V, AVDD=5V, all analog inputs have to be >0.1V at all times.
You need to think of the PGA as a classical instrumentation amplifier. I attached a presentation that hopefully helps to clarify the whole concept.
In the end the output drive capability of the amplifiers that the PGA is composed of is what limits you.
Regards,