Other Parts Discussed in Thread: AFE4403EVM
Hello,
I am trying to calculate blood oxygen (SpO2) values given data output from the GUI. I am familiar with the equations:
R = (ACrms of Red / DC of Red) / (ACrms of IR / DC of IR) and % SpO2 = 110 – 25 × R, as obtained from the slaa655 report and outside literature.
I am able to calculate R by the values obtained from “Scope Analysis” on the GUI. However, I am not confident that the coefficients in the % SpO2 equation are giving me accurate results. My concern arises from the variation in measured values that occurs whenever I change a parameter related to the LEDs (red and infrared). I have the ability to change LED current and gain specific to each LED, but doing so does not cause an equal change in the output voltage for each LED. If I change a parameter for one LED but not the other, does that completely throw off the SpO2 equation? Must the current and gain be the same for both the red and infrared LEDs?
I’m not having any trouble utilizing the product, but I am struggling with the data. I understand that some of the design guides (TIDA-00010, -00011, -01580) utilize a different GUI (HealthHub) which performs the calculation for the user, outputting a % SpO2 on the PC. Perhaps I could access the equation employed in HealthHub?
I also am using the AFE4403EVM and have the same concern with its calculations.
Additionally, I’m wondering about setting the “cancellation current”. From the datasheet section 8.3.1.2: “The receiver provides digital samples corresponding to ambient duration. The host processor (external to the AFE) can use these ambient values to estimate the amount of ambient light leakage. The processor must then set the value of the ambient cancellation DAC using the SPI.”
In the GUI itself, there is a dropdown menu where I can set the cancellation current myself. I am thus wondering if the microcontroller on the EVM is not taking care of the calculation for me, and if not, how I should know what to set the ambient cancellation value to be. I tried just incrementing it by 1 uA, and this caused the red LED voltage to become negative, which I don’t think is supposed to happen. Does this mean I don’t need any cancellation current? From equation 2 in the data sheet, it would seem that I_cancel = I_amb * (R_F/R_I).
In the AFE4403EVM GUI I believe that it performs these calculations for me through the "Enable Calibration Routine" option in the ADC Capture and Analysis Tab.
Thank you!
