What’s the secret about the restriction in the abs. max. ratings of AD7927: “VDRIVE to AGND must be −0.3V to AVDD + 0.3V” and what happens if you don’t take care, e.g. if VDRIVE is powered up before AVDD? Could this damage the ADC?
The Abs max rating table has an implicit way to properly power up the AD7927. The "VDRIVE to AGND must be −0.3V to AVDD + 0.3V” means that the AVDD must be power up first before Vdrive, this sequence must be follow to properly operate the part. Powering the Vdrive first prior AVdd, the performance of the part cannot be guarantee and there is a high chance that the part might be damage. It is always recommended to follow the abs ,max rating.
the meaning of the AMRs is quite clear to me. I want to find out what really happens in case of violating this restriction. In the AN_932 I found some explanation for the other restrictions but not to this one. Why is this dependancy between digital and analog voltage?
https://ez.analog.com/thread/96902 mentions for the AD7266, which has a similar restriction: "If Vdrive goes above 5.3V (with AVDD=5V) then you can turn on an internal protection diode and part will be in an invalid state.". Could this be the same scenario, an what is this diode for?
The supplies for analog and digital circuitry in the IC are separate. There are circuitry inside the pat that needs to power up first before the other circuit. Like in the case of the AD7927. Need to power up the AVdd then the Vdrive.
Each pin has a ESD protection diode. An overvoltage to the pin will forward bias the diode. This was explained on the AN932. This diodes are ESD protection diodes to protect the part from voltage spikes or an overvoltage on the pins. When this happen the part will be in unknown state. The performance of the part is not guaranteed at this state and high possibility that the part will be damaged when exposed for longer period of time.