Figure 33 shows the small-signal equivalent model of the ADS816x analog inputs. The multiplexer input has a switch resistance (RMUX) and parasitic capacitance (CMUX). 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.
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 (tCYCLE – 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 34 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.
Averaging several output codes of a particular MUX input channel without switching the MUX achieves better accuracy and noise performance. The output of the multiplexer does not create a charge kickback as long as SDI is set to 0 (that is, as long as SDI returns the NOP command); see Figure 43 and Figure 45. The multiplexer does not switch during subsequent conversions except for the first time when a channel is selected. Thus high-impedance sources (such as the voltage from the resistor dividers) can be connected to the analog inputs of the multiplexer without an op amp.