| Light-to-frequency converter(LFC),which is used to detect light irradiance,it's output is frequency signal.A high dynamic range light sensor with a linear response is a key component for modern medical electric system.For example in the oximeter,a light irradiance converter is adopted that must be sensitive to lights with wavelengths of both 660 and 940 nm.It's difficult to achieve high dynamic range and high stability.The light-to-frequency converter with low cost and high performance attracts more and more attention in the field of medical electronics.This paper presents a high linear dynamic range light-to-frequency converter(LFC)with integrated photodiodes(PDs).The whole system consists of four modules,which are PD array,bandgap reference,current detect circuit and pulse frequency generate.At beginning,the working principle and performance index of LFC were analyzed.The high dynamic range with linear response is achieved by three techniques.At first,the N-Well/P-Substrate PD is regulated to almost zero voltage drop,in order to minimize the bias-dependent leakage current under a low light irradiance.Secondly,a replica amplifier is implemented to monitor the offset voltage of the regulator against process-voltage-temperature variation,to assure offset voltage of PD is zero.Finally,the use of delay circuit,which is not sensitive to process-voltage-temperature(PVT)to design the pulse frequency module(PFM)to ensure linearity and stability of output frequency.The proposed LFC chip is fabricated using 0.35-?m 2-poly-3-metal non-epitaxy CMOS technology with a die size of 0.85 mm2,proving up to 126-d B linear sensing range with the light irradiance as low as 0.5 nW/cm2 and less than 0.4-Hz output dark frequency under 125 ?,where the temperature-dependent dark current suppression enjoys almost 50 times improvement,when compared with the state-of-the-art industry products.The proposed LFC chip can be applied in a high accurate oximeter system. |
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