I would like to use AD7995.
So, I have a question.
Data is transferred by pulling up VDD at 4 V and SCL / SDA at 3.3 V.
In this case, does the SCL / SDA start running after the 4 V power supply rises to 3.3 V?
The AD7995 is controlled using I2C interface. after power up, then digital lines can have a signal. You can check page 22 of the datasheet about the serial interface of the AD7995. The AD7995 can be powered up with Vdd from 2.7V to 5.5V supply.