The LO input is a high speed, fully differential analog input that
responds to differences in the input levels (and not logic levels).
The LO inputs can be driven with a low common-mode voltage
amplifier, such as the National Semiconductor® DS90C401 LVDS
driver. The graph in Figure 22 shows the range of common-mode
voltages. Logic families such as TTL or CMOS are unsuitable
for direct coupling to the LO input.