A Low Power and Fast Tracking Light-to-Frequency Converter With Adaptive Power Scaling for Blood SpO$_2$ Sensing

This paper presents a monolithic low power and fast tracking light-to-frequency converter for blood SpO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> sensing. Normally, the tracking speed and the power consumption are two contradictory characteristics. However, different gain-bandwidth specifications for various ambient light intensities allow the dynamic optimization of the power consumption according to the light intensity. In this paper, the amplifier power consumption is adaptively scaled by the generated light-intensity-positively-correlated control voltage. Thus, the chip total power consumption at low light intensity is significantly decreased. Moreover, the proposed adaptive power scaling is achieved with a continuous analog domain, which does not introduce extra switching noise. The proposed light-to-frequency sensor chip is fabricated by using 0.35 <inline-formula><tex-math notation="LaTeX">${\rm {\mu m}}$</tex-math></inline-formula> CMOS technology with a die area of 1 × 0.9 mm<inline-formula><tex-math notation="LaTeX">$^2$</tex-math></inline-formula>. The measurement results show that the pulse light response for any light intensity is no longer than two new output square-wave cycles. The maximum total current consumption is 1.9 mA from a 3.3 V supply voltage, which can be adaptively scaled down to only 0.7 mA if the output frequency is about 25 KHz or lower. The minimum operational supply voltage of the proposed sensor chip is 2.5 V in the temperature range of –25 to 80 <inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>C with 4 KV ESD level (human-body model).