When I supply power, Vdd and Vdrive, to the AD7091R the reference voltage output level is not always the expected 2.5V. What am I doing wrong?
There are two possibilities,
Ensure after power up that a software reset is carried out. This is performed as outlined in the data sheet on page 18.
To issue a software reset is issued complete the following sequence after power is applied,
1. Start a conversion.
2. Read back the conversion result by pulling /CS low after the conversion is complete.
3. Between the second and eighth SCLK cycles, pull /CS high to short cycle the read operation.
4. Start another conversion and at the end of this conversion, the software reset is executed.
Alternatively check the input voltage level to the analog input pin Vin. The analog input range is 0V to Vref. There are internal parasitic diodes between Vin an Vref and if Vin exceeds Vref by >0.3V these diodes can start to conduct thus forcing the voltage level on Vref to rise.