We have an existing design that uses several of the ADC128S102 analog to digital converters. It has recently come to our attention that it is possible that the power sequencing requirement that the analog supply, VA, come up before the digital supply, VD, may not be met. It is possible for the VD to be applied for an indefinite time (minutes to hours or more) before the VA is applied. Both are nominally 3.3V when active, but are derived from separate regulator circuits. To date, we have not experienced any unusual operation of the devices when the digital supply was powered on first, but need to decide whether the design needs to be modified to ensure the power sequencing requirements are met. Can you please provide additional information on the reasons for the power sequencing requirement in the datasheet, and under what conditions it may result in impaired performance or failure of the part? We do have the ability to prevent initiating any conversions until the analog supply is present without any hardware modifications. Modifying the hardware at this stage is possible but difficult.
Based on this statement from the datasheet, the ESD diodes appear to only be turned on if a conversion is initiated while VA is powered down:
"To avoid turning on the ESD diodes, the digital supply (VD) cannot exceed the analog supply (VA) by more than 300mV, during a conversion cycle"
Is this the case? If so, would this risk be reduced as long as no conversions are initiated while VA is low? Are there any other risks we should consider?
Thank You.
