O2 Sensing Circuit Case Study


Project

Medical advances such as pulse oximetry have been around for decades, but recent improvements in these tools increases the ability to provide better healthcare. SIGMADESIGN provided design, electrical and firmware engineering services for a client requiring the enhancement of a pulse oximetry device. The challenge was to develop custom configurability of data collection by the medical sensing device which determines a number of different vital signs including:

  • Heart rate
  • Oxygen saturation (SpO2)
  • Blood pressure
  • Respiration rate
  • Cardiac output
  • Many others

Challenge

SIGMADESIGN Electrical and Firmware Engineers worked together to provide the desired functions on a bench-top printed circuit board (PCB) that interfaced with the sensor to allow testing and characterization. Effectively, everything on the circuit was programmable which required a unique set of integrated systems to function together. The board contains the following major sections: microcontroller, LED drive and analog front end (AFE).

Solution

The microcontroller generates the control signals for the hardware and all timings. The microcontroller amplifies and digitizes the analog signals produced by the analog front-end (AFE) hardware. The LED driver receives signals from the microcontroller and drives the LEDs in the sensor. A photodiode on the sensor detects transmitted (or reflected) light at each wavelength. Light variations affect the measurement so calculations adjust for disparities. Under firmware control, LED pulse can be varied to compensate for skin transmissivity and other differences.


Electrical Engineering Services:

  • Analog signaling
  • LED driving
  • Computer-generated circuit simulations (for testing effectiveness of device)
  • Programmable low-pass filters (LFP)
  • High pass filter (HPF)
  • Current-to-voltage (I/V) Converter
  • Synchronous Demodulator
  • ADC anti-aliasing Filter
  • ADC Multiplexer (MUX)