r_on is the resistance of the switch when turned on. r_on should be as low as possible to reduce signal loss and to reduce propagation delay. Propagation delay of the switch depends on the RC time constant, which is made up of the switch r_on and the load capacitance. For applications in transmission-line environments, r_on should be less than, or equal to, the line impedance to minimize unwanted signal reflections. For digital applications where the switch is connected to a resistive load, the switch resistance and the load resistance form a voltage divider. Therefore, in this case, r_on should be as low as possible to maintain a valid input logic high (for example, V_IH) of the downstream devices. r_on not only should be small, but also should be flat across the input voltage range to maintain a linear signal change from input to output. Signal distortion depends on the flatness of the r_on versus V_I curve, that is, equal to 20log∆r_on /RL, where RL is the load resistance. So, to keep signal distortion minimum as the signal amplitude varies, r_on should be kept flat over the whole input signal range. In NMOS series switches, special gate voltage-boost circuitry is needed to keep r_on flat over the V_CC range. In NMOS/PMOS parallel switches, r_on is fairly constant and may have multiple peak values within 0 V to V_CC input voltage range. The shape of the r_on versus V_I curve depends on the threshold voltages of NMOS and PMOS (see Figure 3-4).