4 Hardware

As shown in Figure 4-1, it shows the connection structure of the SAC inside FR235x.

• SAC0 as a TIA
• SAC2 as the gain stage
• ADC will capture SAC0 and SAC2 output
• SAC1 and SAC3 use to drive IR and RED LED.

From the structure in Figure 4-1, you can get the voltage of SAC0 output (Vo1):

The voltage of SAC2 output (Vo2):

In order to improve the SNR of the analog-to-digital converter (ADC) signal, the output signal of SAC2 needs to be adjusted at ADC VREF/2. Therefore, the value of DAC2 needs to be adjusted in real time according to the output of SAC0.

In the schematic (Figure 4-2), you can find that:

• Two SACs are used with transistors to drive RED*IR LEDs
• SAC0+SAC2 are connected internally to amplify photodiode signal
• A button is used to wake up the system and start to measure
• A SPI interface is used to drive the organic light-emitting diode (OLED) screen (include driver)
• An SBW and BSL inter-integrated circuit (I2C) mode program debugging port
• An universal asynchronous receiver/transmitter (UART) serial port used to communicate with GUI