To solve the problem of high source impedance, insert a high-speed op amp between the signal source and the A/D input to reduce the source impedance. This eliminates the voltage divider effect and also minimizes the input bandwidth reduction caused by interaction with the input multiplexor capacitance.

When selecting an op amp, a general rule of thumb is that a slew rate of 2V / uS is required for each 100KHz of bandwidth or sample rate. The higher the slew rate, the higher the speed and the better the performance in high-speed sampling applications.