Hi,
The DC oparating point of the OPAMP is V_Ref, which is 2.0V.
But the sensor maybe has any other operating point.
Therefore you need to seperate both operating points. This is done by blocking DC.
A HPF is the usual way to do this.
Now why 0.5Hz.
Heart rate is appriximately 40 beats per minute (sportsman doing nothing) up to 200 beats per minute (heavy working).
40 beats per miunte is 40 bpm / (60 s/min) = 0.666 bps = 0.666Hz.
You want fc below the lowest frequency of interest. 0.5Hz is well chosen.
Gain of 48:
The input signal needs to be amplified to best fit to an ADC (or whatever the signal processes).
The signal amplitude of the input is unknown, the ADC input range is also unknown. At least to us.
But the developers will know.
3.4Hz:
200bpm / (60s/min) = 3.33bps = 3.33Hz.
Also well chosen. They just focus on the heartbeat rate.
10k pot:
You could use 1k ... 100k.
1k gives less noise, while 100k gives less power consumption.
10k is a good compromise.
Klaus