The photo-diode generates a current from the received light. This current signal is amplified by a transimpedance amplifier SAC0, one of the four built in SAC, is used to amplify this signal. Since the current signal is very small, it is important for this amplifier to have a low drift current. The large DC component is caused by the lesser oxygen bearing parts of the body tissue and scattered light. This part of the signal is proportional to the intensity of the light emitted by the LED.

The small AC component is made up of the light modulation by the oxygen bearing parts such as the arteries plus noise from ambient light at 50/60 Hz. It is this signal that needs to be extracted and amplified. The LED level control tries to keep the output of SAC0 within a preset range via control SAC1 and SAC3. The Normal Red and IR LEDs are controlled separately to within this preset range. Effectively, the output from both LEDs matches with each other within a small tolerance.

The extraction and amplification of the AC component of the SAC0 output is performed by the second stage SAC2. The DC tracking filter extracts the DC component of the signal and is used as an offset input to SAC2. As SAC2 would only amplify the difference it sees between the two terminals, only the AC portion of the incoming signal is amplified. The DC portion is effectively filtered out.

The offset of SAC2 is also amplified and added to the output signal. This needs to be filtered off later on.