Hi Sawamoto-san, As noted on page 13, AVDD is used as the reference for the ADS7049-Q1 . Having a stable reference is critical for getting good conversion results from any ADC (see Figure 35 and Table 1, pg 15). There are various graphs throughout the datasheet showing SNR & SINAD, THD, INL, DNL, Gain error, etc versus the Reference Voltage (the AVDD supply) that you can use to determine what performance issues you might run into when operating the device beyond 3.6V.
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