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This study presents a novel readout circuit that serves to evaluate quantity of skin melanin in a noninvasive way via utilizing a photoplethysmography (PPG) optical sensor. PPG sensors are widely used nowadays for noninvasive diagnosis due to their salient features such as low cost and easy-to-useness. PPG signal is known highly correlated with hum...
Citations
... Much effort was spent to solve this problem by adding DC photocurrent cancellation loops [3], [4], [7], [9]. Other designs use a DAC for DC current cancellation [5], [6], [8], [10]. Since the useful PPG signal is from 0.5 Hz to 8 Hz, a band-pass system is needed to reject the out-of-band noise, which requires large resistors and capacitors to achieve the low-pass corner frequency. ...
... Since the useful PPG signal is from 0.5 Hz to 8 Hz, a band-pass system is needed to reject the out-of-band noise, which requires large resistors and capacitors to achieve the low-pass corner frequency. Some designs, such as [5], [7], used off-chip capacitors to realize it, while [6] designed a low pass filter using switched-capacitor (SC-LPF), and current steering technique to build a LPF was adopted in [7], [9]. Another issue is the required low power operation for the PPG sensor portability. ...
This paper presents a low-power analog front-end (AFE) photoplethysmography (PPG) sensor fabricated in 0.35 μm CMOS process. The AFE amplifies the weak photocurrent from the photodiode (PD) and converts it to a strong voltage at the output. In order to decrease the power consumption, the circuits are designed in subthreshold region; so the total biasing current of the AFE is 10 μ A. Since the large input DC photocurrent is a big issue for the PPG sensing circuit, we apply a DC photocurrent rejection technique by adding a DC current-cancellation loop to reject the large DC photocurrent up to 10 μA. In addition, a pseudo resistor is used to reduce the high-pass corner frequency below 0.5 Hz and Gm-C filter is adapted to reject the out-of-band noise higher than 16 Hz. For the whole sensor, the amplifier chain can achieve a total gain of 140 dBμ and an input integrated noise current of 68.87 pArms up to 16 Hz.
This study presents an external temperature sensor assisted a new low power, time-interleave, wide dynamic range, and low DC drift photoplethysmography (PPG) signal acquisition system to obtain the accurate measurement of various bio signs in real-time. The designed chip incorporates a 2-bit control programmable transimpedance amplifier (TIA), a high order filter, a 3:8 programmable gain amplifier (PGA) and 2 × 2 organic light-emitting diode (OLED) driver. Temperature sensor is used herein to compensate the adverse effect of low-skin-temperature on the PPG signal quality. The analog front-end circuit is implemented in the integrated chip with chip area of 2008 μm × 1377 μm and fabricated via TSMC T18 process. With the standard 1.8 V, the experimental result shows that the measured current sensing range is 20 nA–100 uA. The measured dynamic range of the designed readout circuit is 80 dB. The estimated signal to noise ratio is 60 dB@1 uA, and the measured input referred noise is 60.2 pA/Hz½. The total power consumption of the designed chip is 31.32 µW (readout) + 1.62 mW (OLED driver@100% duty cycle). The non-invasive PPG sensor is applied to the wrist artery of the 40 healthy subjects for sensing the pulsation of the blood vessel. The experimental results show that for every 1 °C decrease in mean ambient temperature tends to 0.06 beats/min, 0.125 mmHg and 0.063 mmHg increase in hear rate (HR), systolic (SBP) and diastolic (DBP), respectively. Similarly, for every 1 °C increase in mean ambient temperature tends to 0.13 beats/min, 0.601 mmHg and 0.121 mmHg increase in HR, SBP and DBP, respectively. The measured accuracy and standard error for the HR estimation are 96%, and − 0.022 ± 2.589 beats/minute, respectively. The oxygen stauration (SpO2) measurement results shows that the mean absolute percentage error is less than 5%. The resultant errors for the SBP and DBP measurement are − 0.318 ± 5.19 mmHg and − 0.5 ± 1.91 mmHg, respectively.
