The RC filter between the amplifier and the ADC helps the amplifier drive the sampling capacitor during charging (acquisition) and discharging (conversion) times. During the acquisition time, if the amplifier has a load transient at the output, the time needed to recover (or settle) is commonly defined as the settling time. Typically, to achieve minimal distortion, the end value to settle is within ½ of the ADC least significant bit (LSB).

The specified settling time of the FDA is the time required for the amplifier to recover from transients caused at the THP210 output. Although the frequency response characteristics impact the settling time of the ADC application, these characteristics are not the key element to consider. The settling time of the FDA to react to load transients depends primarily on the output impedance of the amplifier at the required signal bandwidth. Equation 5 calculates the settling time, considering the time constant of the RC combination:

where:

• N is the number of bits in the ADC application
• τ equals R_F × C_F
• SET = 2 for a settling of ½ LSB, SET = 4 for a settling of ¼ LSB, and so on.

In order to verify whether the chosen RC filter combination fulfills the settling behavior, simulate the desired circuit with TINA-TI™ simulation software.