Oximetry determines the arterial oxygen saturation levels by analyzing wavelengths of light that are absorbed by specific hemoglobin structure in the blood circulated in your fingertip. These light waveforms are transmitted by a semiconductor light emitting diode and detected by a photodetector. Extensive oversampling, filtering, and signal processing from the small signals are needed to eliminate noise such as movement artifacts. Lookup tables are used for calculation of oxygen saturation level, blood flow, and more. The circuit is very sensitive to light and requires precise data acquisition.
A preamplifier with low voltage noise, low current noise, and low power is critical. The AD8655 CMOS amplifier delivers precision offset voltage performance of 250 μV maximum over the full common-mode voltage range, low noise of 2.7 nV/√Hz at 10 kHz and low distortion of 0.0008%. The need for external transistor stages or multiple parallel amplifiers to reduce system level noise is eliminated. The AD8655 is available in an 8-lead MSOP or an 8-lead SOIC.
The ADC for this solution must offer low power, high performance, oversampling to reduce noise levels, and a small package size. The AD7691 PulSAR differential ADC features 18-bit resolution, a throughput rate up to 250 kSPS, and 1.5 LSB INL. It is available in a 10-lead LFCSP. The device’s low power scales with throughput and is pin-compatible with a family of products to facilitate easy upgrades.
The ADuC702x ARM7 family of precision analog microcontrollers or the Blackfin processor family can provide a basic processor solution for this application. Some oximeter designers, however, prefer the extended dynamic range offered by the floating-point capability of the SHARC® processor family.