Figure 6-4 shows the small-signal equivalent model of the ADS816x analog inputs. The multiplexer input has a switch resistance (R_MUX) and parasitic capacitance (C_MUX). The parasitic capacitance causes a charge kickback on the MUX analog input at the same time as the ADC sampling capacitor causes a charge kickback on ADC inputs.

Figure 6-4 Synchronous and Timed Switching of the MUX and ADC Input Switches

In conventional multichannel SAR ADCs, the acquisition time of the ADC is also the settling time available at the analog inputs of the multiplexer because these times are internally connected. Thus, high-bandwidth op amps are required at the analog inputs of the multiplexer to settle the charge kickback. However, multiple high-bandwidth op amps significantly increase power dissipation, cost, and size of the solution.

The analog inputs of the ADS816x provide a long settling time (t_CYCLE – 100 ns), resulting in long acquisition time at the MUX inputs when using a driver amplifier between the MUX outputs and the ADC inputs. Figure 6-5 shows a timing diagram of this long acquisition phase. The low parasitic capacitance together with the enhanced settling time eliminate the need to use an op amp at the multiplexer input in most applications.

Figure 6-5 Early Switching of the MUX Enables a Long Acquisition Phase