Page 22 of the data sheet (top right) says with Vdrive>3.3V, SNR is reduce by ~1.5dB when reading during conversion. Is this delta about the same for Vdrive=2.5V? Also, is SNR generally improved for lower Vdriver?
Increasing the Vdrive greater than 3.3V affects the SNR performance of the AD7609. The larger swing of the digital interface incudes noise that affect the SNR parameter. Reducing the Vdrive doesn't guarantee that it will improve but typically, the nominal SNR will be 91dB for +/-10V input range and 90.5dB for +/-5V range with Vdrive = 2.3V to 5.25V.