## 3 Measurement of System Performance
Another main contributor to the noise of the AFE is the shot noise of the PD. Its effect is shown in Figure 8 by comparing the noise at a photocurrent of 3 uA and 10 uA, representing an increase of 590 pA\({}_{\text{RMS}}\) in the integrated current noise. Table 1 shows a comparison of our readout circuit with some other integrated PPG front ends, presented previously in the literature or available commercially. Our readout circuit provides a reasonable noise performance with low power consumption and comparable active area, which facilitates the integration of the complete system inside an injectable capsule. The noise could be further minimized once the digital core is embedded in the same ASIC and the jitter of the control signals is reduced. The readout circuit we present achieves a satisfactory duty cycle that directly benefits the power consumption of the system, as the high peak current of the LED is ON for a very short fraction of the sampling period. This will enable longer recording periods between battery charging cycles, which is critical given the very limited space available within the glass tube. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Literature**} & \multirow{2}{*}{**This Work**} & **TBCAS ’08** & **TBCAS ’10** & **TBCAS ’15** & **TBCAS ’13** & **AFE4490** \\  & & [23] & [24] & [25] & [19] & [26] \\ \hline Technology & 0.5 μm & 0.35 μm & 1.5 μm & 0.18 μm & 0.35 μm & NR \({}^{\star}\) \\ Supply Voltage & 2.5 V & 2.5 V & 5.0 V & 1.8 V & 3.3 V & 2-3.6 V \\ Sample Rate & 200 Hz & 100 Hz & 100 Hz & 165 Hz & 100 Hz & 20–5000 Hz \\ DC Current Rejection & 17.5 μm & 43.9 μm & NR & 100 μm & NR & 10 μm \\ Integrated Noise & 1 nA\({}_{\text{RMS}}\) & 3.53 nA\({}_{\text{RMS}}\) & NR & 600 pA\({}_{\text{RMS}}\) & NR & 50 pA\({}_{\text{RMS}}\) \\ Noise Bandwidth & 10 Hz & 6 Hz & 5 Hz & 10 Hz & 10 Hz & 20 Hz \\ Noise Photocurrent & 3 μm & NR & NR & 30 μm & NR & 5 μm \\ Active Area & 0.92 mm\({}^{2}\)1 & 0.55 mm\({}^{2}\) & NR & 0.84 mm\({}^{2}\) & 1.15 mm\({}^{2}\) & NR \\ Power Consumption 2  & 125 μm & 600 μm & 400 μm & 128 μm & 363 μm & 2.1 mW \\ LED Duty Cycle & 0.4\% & 10\% & 3\% & 0.7\% & 3–7\% & 1–25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of readout circuit performance to other integrated PPG front ends. In order to evaluate the performance of the wireless communication system, the dipole antenna was placed 60 cm away from the capsule and connected to the AirSpy R2 SDR receiver. Figure 9a shows the relative PSD recorded at the front end of the receiver. As the GNU Radio operates with many different hardware interfaces that are not strictly calibrated, obtaining the absolute power of the received signal [27] was not possible. However, we utilized a real-time spectrum analyzer (Model 3066, Tektronix, Inc., Beaverton, OR, USA) to record the absolute power levels. Figure 9b shows the superimposed plots of two such PSD measurements. To further investigate the quality of the communication link, we estimated the bit error rate (BER) of the received signal. BER of a binary signal is defined as the conditional probability of receiving an erroneous bit and expressed as,  
\[\text{BER}=\frac{1}{2}P_{0}(X>V_{th})+\frac{1}{2}P_{1}(X<V_{th}) \tag{2}\]  
Figure 8: Noise at the output of the switched integrator for (**a**) a photocurrent of 3 \(\upmu\)A, and (**b**) a photocurrent of 10 \(\upmu\)A, showing the effect of the PD shot noise. Figure 9: (**a**) Relative power spectral density at the front end of the SDR receiver. (**b**) Two superposed plots of power spectral density measurements recorded with a real-time spectrum analyzer to obtain the absolute levels of the received signal. where \(P_{0}\) and \(P_{1}\) are the probability density functions (PDFs) of the received signal levels for the symbols '0' and '1' respectively and \(V_{th}\) is the digitization threshold. For this purpose, we have analyzed \(5.4\times 10^{6}\) sample points to fit Gaussian curves to the PDFs of the symbols separated by \(V_{th}=0\) (Figure 10).