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Ad7915 is not working properly

Ad7915 is set as VDD=2.5V, VIO=3.3V, Vref=5V, in-grounding, and SDI is connected with 3.3Vin + as the analog voltage acquisition port. The test found that the collected data is normal when the simulation is greater than 3V, but the collected data is abnormal when the simulation is less than 3V, for example:  The actual voltage is 2.5V, and the digital signal is converted to an analog voltage of 1.7V.  May I ask what's going on here?