On the AD7606? It is on page 11 of the datasheet:
http://www.analog.com/static/imported-files/data_sheets/AD7606_7606-6_7606-4.pdf
Abs max says that Vdrive (the digital supply) must not exceed
AVcc+300mV. So it looks like you have to make sure Vdrive doesn't come
on first and exceed AVcc, and use similar caution when powering down.
Otherwise, most likely some diode path or BE junction will come on and
only Analog Devices engineers would (maybe) be able to tell you whether
that ends in chip damage or just some lock-up scenario.
Looks like it is best to make Vdrive not come on until AVcc has come up
to spec'd value, and then also make sure that Vdrive never exceeds AVcc That makes sense.
later. I am not sure whether this is also critical during power-down and
I would ask Analog Devices. If it is critical you could, for example,
use a cheap POR/BOR chip (NCP3xx series, for example) to bleed off
Vdrive fast the instant AVcc falls below, say, 90% of its value. If you
do it in a clever fashion you can also let this POR/BOR chip handle the
power-up situation.
Well, the AD7606 does have two power rails, one for the analog parts
(several pins) and one for the digital part (pin 23), along with
prescribed conditional boundaries between the two. Abs max data is
usually something that needs to be taken seriously because it can mean
that permanent damage might result when exceeding values in that table.