To improve the SpO $_2$ 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 ( $g_{m}$ -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- $\mu$ m CMOS process. The overall chip area is 1 × 0.9 mm $^2$ 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 $_2$ sensor could be as low as 0.06% with 2-percentage-point error of SpO $_2$ . The proposed chip is suitable for portable low-power high-performance blood oximeter devices especially for hypoperfusion applications.

中文翻译：

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用于低于 0.1% 灌注指数血液 SpO2 传感的降噪光频率转换器。

改善 SpO $_2$针对低灌注（低灌注指数）应用的传感系统性能，本文从两个方面提出了一种低噪声光频率转换器方案。首先，提出了一种低噪声光电流缓冲器，通过跨导升压来降低放大器本底噪声（ $g_{m}$ -boost) 电路结构。其次，提出了一种脉冲频率占空比调制的数字处理单元，通过限制最大输出频率来最小化后续定时器中的量化噪声。所提出的光到频率传感器芯片的设计和制造采用 0.35- $\mu$ m CMOS 工艺。芯片总面积为 1 × 0.9 mm $^2$室温下 3.3V 电源的典型总电流消耗约为 1.8mA。测量结果证明了输出脉冲占空比调制的建议功能，而典型 10 kHz 输出频率的 SNR 为 59 dB，与之前的设计相比提高了约 9 dB。其中，2-3dB SNR的提升源于gm-boosting，其余来自版图设计。系统内实验结果表明，使用建议的血液 SpO 的最小可测量 PI $_2$ 传感器可能低至 0.06%，SpO 有 2 个百分点的误差 $_2$ . 所提出的芯片适用于便携式低功耗高性能血氧仪设备，尤其适用于低灌注应用。