AD7708 absolute ratings

Hello there,

I have a question about the absolute ratings of the AD7708.

According to the datasheet:

AVDD to DVDD . . . . . . . . . . . . . . . . . . . . . . . . –5 V to +5 V

Typically, in my design: the AVDD is a 5.7V and DVDD at 3.3V; within the above limits.

The   thing is that at power up, AVDD is provided before DVDD and theoritically, it
could have 5.7V between them. 

When I look at the voltages during startup,  the difference is limited to around 1.8V.

I don’t know what is limiting it; is it something internally? 

I am also planning to delibarately shut-down the DVDD for  power savings

and I would like to verify what are the constraints.

Please advise,

Teodor

  • 0
    •  Analog Employees 
    on Mar 19, 2015 12:15 AM

    Hi Teodore,

    The Absolute Maximum Ratings are a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device.

    The specifications section in the AD7708 data sheet specifies the AVDD supply maximum to be 5.25 V. So, you are operating the part outside of spec by using a 5.7 V supply and functional operation of the part is not guaranteed nor are the devices specifications because this maximum supply specification is being violated.

    Operating the part at the absolute maximum ratings for a very short period should not cause you any issue. For example, during startup if you reach the maximum ratings briefly then this should not cause any issues. However, in your case you will violate the maximum ratings because your AVDD supply is 5.7 V. You mention that in practice this appears to be limited to 1.8 V but without further information about your schematic and power supply configuration it would not be possible to say how or why this is being limited.

    For power savings the ADC has a power down mode which is the recommended method to reduce power. It's not clear to me right now what, if any, side affects could occur by power down only the DVDD supply and not the AVDD supply. In your configuration the obvious effect is that you will violate the absolute maximum ratings and most likely damage the ADC. Is their any reason why you wouldn't use the power down mode which is described in the data sheet ?

    Regards,

    Rob

    FormerMember