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Pulse Oxymetry can't be that hard, right?
“For the purposes of signal amplification, the photocurrent must be transformed into a voltage with moderate output impedance; this is achieved with the circuit shown in Figure 43, the op−amp being configured as a current−to−voltage converter. Because of the high junction resistance of the reverse−biased photodiode, the op−amp should be an FET type with a very high input impedance. Since the negative input of the op−amp acts like a virtual earth, the output voltage of the circuit is vo = −I RL. A very large feedback resistance may be used, values as high as several tens of MΩ being typical in practice.”