The LMH32401 device is designed to work with photodiode (PD) configurations that can source or sink current; however, the LMH32401 is optimized for a sinking current configuration. It is assumed that the LMH32401 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 LMH32401 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 LMH32401 device is approximately 65 µA in the high-gain configuration and 650 µA in the low-gain configuration (PD sinking the photocurrent).

Input currents in excess of the linear current range cause the internal nodes of the amplifier to saturate, which increases the amplifier recovery time. The end result is a broadening of the output pulse leading to blind zones in the system response. To protect against this condition, the LMH32401 features an integrated 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 ns for input pulses up to 100 mA. The power-supply pins (VDD1 and VDD2) must each have their own bypass capacitors to prevent large input pulses from affecting the differential output stage. When the amplifier is in low-power mode, the clamp circuitry is still active, thereby protecting the TIA input.