ZHCSCL7C May 2014 – April 2021 AFE4403
PRODUCTION DATA
Refer to LED Configurations for different ways to connect the LEDs to the TXP, TXN, and TX3 pins. The photodiode (shown in Figure 9-2) receives light from both the Red and IR phases and usually has good sensitivities at both these wavelengths.
The photodiode connected as shown in Figure 9-2 operates in zero bias because of the negative feedback from the transimpedance amplifier. The signal current generated by the photodiode is converted into a voltage by the transimpedance amplifier, which has a programmable transimpedance gain. The rest of the signal chain then presents a voltage to the ADC. The full-scale output of the transimpedance amplifier is ±1 V and the full-scale input to the ADC is ±1.2 V. An automatic gain control (AGC) loop can be used to set the target dc voltage at the ADC input to approximately 50% of its full-scale. Such an AGC loop can control a combination of the LED current and TIA gain to achieve this target value.