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AD7656 supply voltage

Question asked by appleman on May 22, 2014
Latest reply on Jul 1, 2014 by Rob.F



I want to use the AD7656 in my master thesis project. I have studied the
datasheet for a while, but there are some terms of the Absolute Maximum
Ratings (page 10) that caused me some headache:

Line 3 says:

V_DD to AV_CC: AV_CC − 0.3V to 16.5V

We can split this essay in 2 inequations:

V_DD - AV_CC > AV_CC − 0.3V
V_DD - AV_CC < 16.5V

Let's take a closer look to the first one, we can bring AV_CC to the
other side:
V_DD > 2*AV_CC - 0.3V

This would mean that V_DD has to be at least 2 times AV_CC minus 0.3V
which does not really make sense!?

The specifications (page 3) also say something different (AV_CC = 4.75V
to 5.25V, V_DD = 5V to 16.5V)

The next thing I am wondering about is Line 5:

DV_CC to AV_CC: −0.3V to AV_CC + 0.3V

which means:

DV_CC - AV_CC > −0.3V
DV_CC - AV_CC < AV_CC + 0.3V

DV_CC < 2*AV_CC + 0.3V

which also sounds a bit strange (but not impossible).

There is a third thing I want to know:

I want to use separate analog an digital supplys. The analog should be
+/-5V, and the digital 5V. I want to connect V_DD and AV_CC to +5VA,
V_SS to -5VA and DV_CC and V_DRIVE to 5VD. But as it is impossible to
start up the analog and digital supply exactly at the same time one has
to be before the other! The question is: Which one first? What does the
AD7656 tolerate?

I would assume that at first I power up 5VD, than after the µC has
booted it switches on the analog supply. Would this be an appropriate way?

As I want to use 2 AD7656 on my board and one costs about 20€ I don't
want to destroy them on the first try. This inconsistency in the
Absolute Maximum Ratings area makes me feel a bit unsafe. So what are
the real Absolute Maximum Ratings?

Thank you very much.