If I enable the AD7768 analog input buffers do I still need an external ADC driver opamp?
The precharge buffers are not full buffers and do not replace the external driver opamp(s). The analog input precharge buffers provide the initial charging of the internal switched capacitor network during the first ~25% of the sampling phase. For the remaining ~75% of the sampling phase, the buffers are automatically bypassed, and the fine accuracy settling charge is provided by the external driver. This eases the driving requirement of the external opamp(s), and in some cases allows lower power opamps to be used.
When enabled, the AD7768 internal sampling capacitors are charged close to the input voltage by these on-chip analog input precharge buffers, before the external driver is connected to the sampling capacitors. This eases the driving requirement of the external network. The analog input precharge buffers reduce the switching kickback from the sampling stage to the external circuitry and reduce the average input current. They make the input current more signal independent, to reduce the effects of sampling distortion.