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If I enable the analog input buffers on the AD7768 (or AD7768-4, AD7768-1New, AD7761) 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). Some further details on how to choose a suitable driver amplifier in combination with the enabled or disabled precharge buffers is available in this application note on design of a modular DAQ subsystem: Modular Data Acquisition (DAQ) Footprint Using the AD7768/AD7768-4
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.
The amount of time that the precharge buffer is active is ~15ns in full power mode, ~29ns in median power mode, and ~116ns in low power mode. So this equates to approximately 25% of the sampling period, when the modulator is used at the maximum rate, for each power mode. If the modulator is run at a lower rate then the precharge buffer is still enabled for the same length of time, which becomes a smaller portion of the overall sampling period.
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.
The net effect of the precharge buffers is to reduce both the average current into the analog input pins, and to reduce the amplitude of the "kick" that the external amplifier experiences. Both of these effects lead to an input current and voltage profile which is less dependent on the signal, and therefore leads to less distortion introduced at the input stage.