The datasheet of AD7688 recommends adding a buffer to the reference source to meet source's low impedance requirements. Fig 33 on page 16 shows an example. I'm wondering whether that curcuit will work as proposed. Especially in regard to the power supply of the AD8031 (5V) and the input Level of the positive input (5V). Datasheet of AD8031 explains the saturation behaviour (typ 20mV below VCC + voltage drop across Rc). AN931 explains that the Pulsar-ADs produce higher dynamic current spikes while converting and recommend a buffer.Thus will the voltage provided by the AD8031 suffice the requirements of AD7688.
Correct the Output Voltage of AD8031 swings below 5V. The Circuit shown in page16 figure33 still suffice to drive the AD7688. This still with in the range of reference voltage.Please see table3 on page4 of the AD7688 datasheet.