The LMH34400 can be placed in low-power mode by setting EN high, which helps in saving system power. Enabling low-power mode puts the outputs of the internal amplifiers in the LMH34400 in a high-impedance state.

Figure 6-1 shows how this device feature can further save board space and cost by eliminating the need for a discrete high-speed multiplexer, if a system consists of several photodiode and amplifier channels multiplexed to single time-of-flight detector circuit. The disabled channel outputs are not an ideal open circuit; therefore, as the number of multiplexed channels increases, the disabled channels begin to load the enabled channel. An additional isolation resistor helps to reduce the impact of reflections from disabled channels. Multiplexing more than four channels in parallel degrades the performance of the enabled channel.

When the amplifier is in low-power mode, the clamp circuitry is still active thereby protecting the TIA input. The ambient light cancellation loop is disabled when the amplifier is placed in power-down mode. When the LMH34400 device is brought out of power-down operation, the ambient light cancellation loop requires several time constants to settle. The time constant is based on the 400-kHz cutoff frequency of the loop.

Figure 6-1 Configuring Two LMH34400 Devices in Multiplexer Mode