For multiplexed systems, the observed THD(+N) performance is a function of the On-Resistance (R_ON), R_ON flatness (variation of the mux channel R_ON across the applied signal range) of the device and the load resistance, R_L. As the ratio R_L/R_ON increases the THD+N performance improves. This is due to the creation of an amplitude-dependent voltage divider, as shown in Figure 4-2, through the combination of R_on and R_L. This follows then that the observed harmonic distortion of the mux output is minimized when R_L is larger, such as a high impedance input of a non-inverting amplifier.

Figure 4-2 Voltage Divider Created by MUX R_ON and R_L

Typically, for applications where THD matters, the operational peak-to-peak voltage range can be a smaller range than the full voltage swing the mux is capable of. Therefore, this is important to verify that the operating voltage of a device does not fall in a region where the R_ON of the mux varies greatly. Some R_on curves are flat across the entire region, while others can have optimized performance across a specific input voltage region. Figure 4-3 shows how the same device can have different THD performance, given different operational peak-to-peak (V_pp) voltages.

Figure 4-3 R_ON Curve With Optimized (Green) V_pp Region vs Unoptimized (Red) V_pp Region

To optimize THD performance, the recommendation is to use the R_ON region of the multiplexer that is the flattest. Figure 4-4, shows the optimized THD Regions for common of R_ON curves for analog multiplexers. Figure 4-5 highlights how an ultra-flat R_ON optimizes THD performance in the TMUX4827.

Figure 4-4 Best Region (Green) of V_IN for Optimized THD Performance

Figure 4-5 TMUX4827 Achieves Optimized THD Performance Through Ultra-Flat R_ON