I am using AD7476SRT and it is fed by a Rail to Rail o/p Opamp as a buffer. I am seeing a stuck/saturation in Rail to Rail opamp's output only when the voltage close to the rail (3.3V and output stucked at 2.8V), the opamp is rail to rail (3mV output drop) for 100K load resistance. I am feeding DC signal to ADC via Opamp, even though I am not reading the ADC, I think the ADC's input impedance in Track mode is pretty low and it is loading my opamp. In data sheet it is saying it is having 100 ohm impedance for AC? but for DC how much it is ? I am getting close to 10K from my simulations, is it true? how to achieve rail to rail performance for this ADC?