Different challenges came up during various phases of the project development. Some of the key challenges have been highlighted below.

• System Characterization Phase
  • Identifying appropriate system specifications and circuit architectures was a significant challenge during the initial phase of the project's development. Certain parameters, such as the frequencies of interest were know, but in order to ascertain the gain required for the filtering blocks, the hysteresis width and the filter roll-off, literature studies for photocurrent outputs and empirical calculations for filter transfer functions were conducted. This data then helped in zeroing in on the architecture of individual blocks.
• Measurements
  • Significant challenges were posed during the measurements period. Several iterations of LED and photodiode biasing had to be carried out to obtain the input signal at the desired AC intensity. Further, the intensity of light reflected varies depending on skin tone, tissue density of the finger and a variety of other factors dependent on the subject of the measurements. The solution developed to resolve this issue involved addition of variable resistances in series with the feedback resistance of the transimpedance amplifier. The feedback resistance was made programmable up to 530K Ohms in the final design. While having all chip level pins accessible at the PCB level was extremely convenient in helping realise this circuit, a more sophisticated implementation could be realised using Programmable Gain Amplifiers (PGAs).