## 3 Measurement of System Performance  
The power consumption of a PPG system is dependent on the magnitude of the peak LED current that is required for a signal with sufficient SNR, i.e. the amount of light that can be effectively modulated by the changes in blood volume. However, this dependence is minimized by optimizing the duty cycle of our system to 0.4% at a sampling rate of 200 Hz. Consequently, the total power consumption of 12 mW (4.8 mA of supply current) from the regulated supply of 2.5 V is dominated by the FSK transmitter drawing 2.9 mA of current. This is followed by the microcontroller at 1.55 mA. The microcontroller operates at a frequency of 4 MHz using the calibrated internal 8 MHz oscillator, and keeps the timer/counter module and ADC continuously running. This power consumption could be further optimized by utilizing the sleep mode of the microcontroller between sampling events, although our objective is to completely integrate its functionality into a single ASIC in our next iteration and obtain better efficiency and compactness altogether. Based on our pulsing scheme, the LED contributes an average current of 0.3 mA in the default experimental setup, while the signal conditioning blocks would account for a current of 0.05 mA (see Figure 7 for a graphical representation). We evaluated two different COTS pin-type non-rechargeable lithium batteries (BR425 and BR435 from Panasonic Corporation, Kadoma, Osaka, Japan) as the single-use power source. These are  
Figure 6: Screenshot of the GRC interface showing the different blocks that comprise the software-defined FSK receiver. Figure 7: Distribution of power consumption drawn from a 2.5 V source by the PPG readout system. two of the options that can be easily purchased and used in injectable form factor. The nominal capacity of these batteries (25 and 50 mAh in BR425 and BR435, respectively) is expected to allow for 5-10 h of continuous operation. However, the actual performance was not on par due to the peak current values that were higher than the nominal discharge rates (0.5 or 1.0 mA, respectively) and the apparent inability of these particular batteries to sustain short-but-high discharge pulses at this rate [21]. However, a smaller and rechargeable pin battery has been recently announced by the same company (CG-320B) with discharge characteristics that exceed our current requirements [22]. It has not been made commercially available yet, as of the publication of this paper, but we believe that it will enable further miniaturization of our system in the future. While we had shown the feasibility of leveraging the wireless power transfer in [13], further research for wireless power transfer is underway and is beyond the scope of this paper. We measured the input-referred noise of the AFE by placing the test board of our ASIC inside a Faraday cage, recording segments of 50 s (64,516 samples) of the SI output with an oscilloscope, and estimating its power spectral density (PSD) by calculating the FFT in MATLAB(r). In order to obtain the input current noise at different values of photocurrent and measure the effect of the shot noise of the PD, we placed the optical probe containing the LED and PD pair inside a opaque dark box, to minimize any external light interference and control the amount of incident light by tuning the LED driver. We also measured the actual gain of the circuit by comparing the change in the output when applying a step of 100 nA at the input with a setup of \(R_{f}=250\) k\(\Omega\), \(C_{f}=3.0\) pF for the TIA and \(C_{int}=25\) pF for the SI, resulting in a value of approximately 2 M\(\Omega\) as expected from Equation (1). The timing of the control signals has a direct impact on the performance of the circuit, because the integration time affects the AFE gain. Therefore, as our current implementation generates these signals through a microcontroller, the inherent jitter of the SI_EN signal has a major contribution on the noise of the readout circuit.