Medical Applications Guide (Rev. B - Mouser Electronics

Diagnostic, Patient Monitoring and TherapyPulse Oximetry27➔OverviewThe pulse oximeter measures blood oxygenation by sensing the infraredand red light absorption properties of deoxygenated and oxygenatedhemoglobin.It is comprised of a sensing probe attached to a patient’s earlobe, toeor finger that is connected to a data acquisition system for calculationand display of oxygen saturation level, heart rate and blood flow. Lightsources, typically light-emitting diodes, shine visible red and infraredlight. Deoxygenated hemoglobin allows more infrared light to passthrough and absorbs more red light; highly oxygenated hemoglobinallows more red light to pass through and absorbs more infrared light.The oximeter senses and calculates an amount of light at those wavelengthsproportional to the oxygen saturation (or desaturation) of thehemoglobin.Because of the use of light in the absorbance measurement, thedesigner needs a true “light-to-voltage” conversion using current as theinput signal. The classes of photodiode amplifiers suitable for pulseoximetry applications are the classical resistor-feedback transimpedanceamplifier and the capacitor-feedback switched integrator. In eitheramplifier configuration, the resulting output voltage is read by ananalog-to-digital converter and serialized for MSP430 microcontrolleror TMS320 DSP for processing.Signal Acquisition ChallengesThe resistor-feedback amplifier circuitshown at right is the most commonbioelectric transimpedance circuit.10MΩWith the amplifier used in the invertingconfiguration, the light shining ona photodiode produces a small currentthat flows to the amplifier summingjunction and through the feedbackresistor. Given the very large feedbackPhotodiodeOPA353resistor value, this circuit is extremely sensitive to changesin light intensity. For example, an input light signal of just 0.001µWcan produce a full-swing output.Depending on design requirements, it can be very useful to achieveoutput swing down to or below ground. The auto-zero transimpedanceamplifier configurations shown on the next page will allow swing toground in Figure A and very close to ground in Figure B. A pull-downresistor tied to –5V will allow swing slightly below ground to minimizeerrors as the output gets very close to zero volts.I INA. Dual SupplyR1+2.5VREF3140V OAbsorption10660nm940nmR = I 940nmI 660nmR = Ratio of the Light Absorbanceat the two different wavelengths.HbO 2HbPhotodiode1MΩOPA340or OPA350–2.5VC1+2.5VR2OPA335–2.5VC2ADS83200.1600 700 800 900 1000Wavelength [nm]B. Single SupplyR1+5VREF3140With that, the oxygen saturation of the human blood:SaO 2 =[O 2 –Hb][O 2 –Hb] + [Hb]Photodiode1MΩC1OPA340or OPA350ADS8320(Usually between 94% and 98%) can now be calculated based on the law of Lambert Beer.I = Io • e— ε.xThat shows the resulting light (I) and its dependency of the light source (I o ), the coefficientof the extinction (ε, that results from pic) and the “amount” (χ) of each of the Hb typeseither oxygenated or deoxigenated.I IN–5V+5VOPA33540kΩ*R2C2The diagram shows the different absorption spectra of unloaded hemoglobin(Hb) and oxygen loaded hemoglobin (HbO 2 ).*Optional pull-down resistor toallow below ground output swing.Texas Instruments 2Q 2007www.ti.com/medicalMedical Applications Guide