To place the LMH32401-Q1 into a power-down mode, and thus help save system power, set EN high. Power-down mode puts the outputs of the LMH32401-Q1 internal amplifiers, including the differential outputs, into a high-impedance state. If a system consists of several photodiode and amplifier channels multiplexed to a single ADC channel, Figure 7-3 shows how this device feature can further save board space and cost by eliminating the need for a discrete high-speed multiplexer. 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. Multiplexing more than four channels in parallel degrades the performance of the enabled channel. When the amplifier is in power-down mode, the clamp circuitry is still active, thereby protecting the TIA input. The ALC loop is disabled when the amplifier is placed in power-down mode. When the LMH32401-Q1 is brought out of power-down operation, the ALC loop requires several time constants to settle. Figure 6-9 shows the low-frequency loop response, which in turn determines the time constant required for the loop to settle.

Figure 7-3 Configuring Two LMH32401-Q1 Devices in Multiplexer Mode to Drive a Single ADC