The LMH32404-Q1 device is optimized to work with photodiodes (PD) configurations that can source or sink current; however, the LMH32404-Q1 is optimized for a sinking current configuration. It is assumed that the LMH32404-Q1 device is being used with a PD that is configured with its cathode tied to the amplifier input and the anode tied to a negative supply voltage, unless stated otherwise.

The LMH32404-Q1 features two internal clamps, a fast recovery clamp and a soft clamp. The fast recovery clamp is the active clamp when the photodiode is sinking a photocurrent. The soft clamp is the active clamp when the photodiode is sourcing a photocurrent. Stray reflections from nearby objects with high reflectivity can produce large output current pulses from the PD. The linear input range of the LMH32404-Q1 is approximately 65 µA. Input currents in excess of the linear current range will cause the internal nodes of the amplifier to saturate, which increases the amplifier recovery time. The end result broadens the output pulse, which leads to blind zones.

To protect against this condition, the LMH32404-Q1 features an integrated fast recovery clamp that absorbs and diverts the excess current to the positive supply (V_DD1) when the amplifier detects its nodes entering a saturated condition. The integrated clamp minimizes the pulse extension to less than a few nanoseconds for input pulses up to 100 mA. The power-supply pins (VDD1 and VDD2) must have their own bypass capacitors to prevent large input pulses from affecting the differential output stage. The clamp circuitry is active when the amplifier is in standby mode and low-power mode, thereby protecting the TIA input.