To illustrate the V_OH/V_IH logic compatibility use an example case where the CD74HCT405x, with a V_IH minimum equal to 3.5 V, is driven by either a 5-V CMOS or 5 V TTL driver. Figure 1-1, shows that the 5-V CMOS driver can work because the minimum V_OH of 4.44 V is greater than the 3.5 V V_IH minimum of multiplexer. However, the 5-V TTL driver minimum V_OH of 2.4 V is less than the 3.5 V V_IH minimum and might not be high enough to register a logic HIGH on the mux’s logic control inputs under certain conditions.

Alternatively, by selecting the TMUX405x, a device with 1.8-V logic compatible inputs, we can see from the following how this V_OH/V_IH compatibility issue is resolved and the logic threshold required to register a HIGH on the mux’s inputs is met under all conditions. See 1.8-V Logic for Multiplexers and Signal Switches application note, for additional information on 1.8-V logic compatible inputs.