Oximeter_circuit_v.1.JPG (197726Bytes)
I am currently in the engineering phase to build a pulse oximeter. I mainly make use of the following references:
Normally a pulse oximeter analyzes the light absorption of two wavelengths from the pulsatile-added volume of oxygenated arterial blood (AC/DC) and calculates the absorption ratio:
SpO2 (O2 saturation value) is than simply computed by a look-up table stored in the memory of the MCU.
But there is fortunately another way to calculate SpO2. Only the AC level is used and SpO2 calculated by following formula:
where
(IAC)λ1=light intensity at 660 nm and
(IAC)λ2=light intensity at 940 nm.
We also know that photodiodes produce a leakage current that is directly proportional to the intensity of the light. The current is converted into a voltage by an according current-to-voltage-converter. The current is now again directly proportional to the voltage. This means, we can use the voltage instead of the intensity of the light to calculate SpO2.
We now firstly need to call a function which determines the maximum and minimum peak of the two signals in a certain time interval. We call it Vmin λ1/2 and Vmax λ1/2.
To remove the DC component from the signals, we compute:
The remaining root mean square voltage is then:
Now we can directly compute SpO2:
After figured that out, I have started working on the circuit diagram. A first version can be found attached. The amplifying and filtering stages are re-drawn from the Freescale page. As the values of the capacitors and resistors are not very clear, I had to re-calculate all the high-/low-pass-/Notch-filters and gains though.
I'll build a prototype the next days/weeks. If I get it working, I'll compare the data with a commercial oximeter. I'll update this blog as I make progress.
Update September 2, 2012
Currently the oximeter sensor is under construction.
I am still waiting for the pin photodiode to arrive in the mail.
Technical data of the photodiode:
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