ZHCSS23A april 2023 – august 2023 LMH32401-Q1
PRODUCTION DATA
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 (VREF). The ALC loop then outputs an opposing dc current (IDC) 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.