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What is the meaning of the absolute input voltage
The AD7864 is a replacement for the TI part ADS8321 so the product line has put the “Absolute Input Voltage” spec in the general specifications section of the datasheet (as in the ADS8321 datasheet) rather than in the "Absolute Maximum Ratings" section as is the ADI standard. See the following clarification provided by the AD7684 applications engineer: The "Absolute Input Voltage" under the specs table is simply a copy of what the major competitor TI is doing. This is traditionally not the ADI standard. "Absolute Maximum Ratings" are in their own section and you should refer the customer to this section. The operating input range on IN+ for all small package PulSAR ADCs is (AGND - 0.1)V to (Vref + 0.1)V. For single ended ADC, the operating input range on IN- is (AGND - 0.1)V to (AGND + 0.1)V. For differential input ADC, the operating input range on IN- is (AGND - 0.1)V to (Vref + 0.1)V. For the absolute max on these pins, refer to the "Absolute Maximum Ratings" section in the datasheet. In any event, all small package PulSAR ADC will be VDD + 0.3V, GND - 0.3V since there is an ESD structure on VDD and AGND on all of our ADCs inputs. The common mode input range for IN+ and IN- on the differential input ADCs is also (Vref/2 - 0.1) to (Vref + 0.1)V. Operating beyond the common mode operating point, introduces non linearity's. You cannot tie IN- to GND and swing IN+ from 0 to Vref on these ADCs with differential inputs.