The LMH32401-Q1 has an integrated, dc, ambient light cancellation (ALC) loop that cancels any voltage offsets as a result of incidental ambient light. ALC mode only works when the PD is sinking the photocurrent. To enable ALC mode, set IDC_EN low. Incidental ambient light on a photodiode produces a dc current that results in an offset voltage at the output of the LMH32401-Q1 TIA stage. Section 7.2 shows how the ALC loop senses the low-frequency dc offset at the output of the TIA stage and compares the offset against the internal reference voltage (V_REF). The ALC loop then outputs an opposing dc current (I_DC) to compensate for the differential offset voltage at the device input. The ALC loop has a high-pass cutoff frequency of 100 kHz. ALC mode is disabled when the amplifier is placed in power-down mode.

The shot noise current introduced by the ALC loop increases the overall amplifier noise; therefore, if the ambient-light level is negligible, disable the loop to improve SNR. The ALC loop helps save PCB space and system costs by eliminating the need for external ac-coupling, passive components. Additionally, the extra trace inductance and PCB capacitance introduced by using external ac-coupling components degrade the LMH32401-Q1 dynamic performance.