A Noise-Reduced Light-to-Frequency Converter for Sub-0.1% Perfusion Index Blood SpO$_{\text 2}$ Sensing

To improve the SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> sensing system performance for hypoperfusion (low perfusion index) applications, this paper proposes a low-noise light-to-frequency converter scheme from two aspects. First, a low-noise photocurrent buffer is proposed by reducing the amplifier noise floor with a transconductance-boost (<inline-formula><tex-math notation="LaTeX">$g_{m}$</tex-math></inline-formula>-boost) circuit structure. Second, a digital processing unit of pulse-frequency-duty-cycle modulation is proposed to minimize the quantization noise in the following timer by limiting the maximum output frequency. The proposed light-to-frequency sensor chip is designed and fabricated with a 0.35-<inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m CMOS process. The overall chip area is 1 × 0.9 mm<inline-formula><tex-math notation="LaTeX">$^2$</tex-math></inline-formula> and the typical total current consumption is about 1.8 mA from a 3.3-V power supply at room temperature. The measurement results prove the proposed functionality of output pulse duty cycle modulation, while the SNR of a typical 10-kHz output frequency is 59 dB with about 9-dB improvement when compared with the previous design. Among them, 2–3 dB SNR improvement stems from the gm-boosting and the rest comes from the layout design. In-system experimental results show that the minimum measurable PI using the proposed blood SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> sensor could be as low as 0.06% with 2-percentage-point error of SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula>. The proposed chip is suitable for portable low-power high-performance blood oximeter devices especially for hypoperfusion applications.