Are there any issues with the MAX86141 LED driver?

There are no known issues with the MAX86141 LED driver. What may be perceived as an issue is actually related to LED thermal drift, which can appear to affect photodiode performance. Specifically, ambient light intensity may drift over time on one channel of a photodiode due to LED thermal drift, expected within the first few minutes of operation, and changes in LED temperature and luminous intensity. Additionally, the opto-mechanical design might contribute to this problem. To isolate this issue, follow these steps: Test all devices in a dark room environment using optical enclosures or a black bag with no reflection. Validate signal quality. Test the device with and without the enclosure to determine if the enclosure is creating an offset. Ensure the inside of the enclosure is light absorbent to prevent internal reflection. Perform tests without human subjects to remove variables.

Is there a known issue with LED current drift?

LED thermal drift is normally expected within the first few minutes of operation due to the heating of the LED package, especially when driven with high current or long integration times (T INT ). As the LED temperature increases, its luminous intensity decreases over time, leading to a reduction in photodetector current readings. Consequently, even with a constant LED current driven by the AFE, the PPG reading (ADC counts) diminishes over time. To mitigate this, it is recommended to collect data for about 1 minute (or longer) and wait for the LED temperature to stabilize. Applying a first-order polynomial fit to the last 10 seconds of data can further reduce the effects of LED temperature drift when calculating the signal-to-noise ratio (SNR) of the application.

How PI parameter works for HR and SpO2 algorithms?

PI is one aspect of PPG signal quality. It is defined by AC/DC of PPG signal. The PI value varies for different subjects, diffent optical designs. Both HR and SpO2 algorithm likes higher PI PPG signal, which means the PPG signal is more high quality, more clear. However, if PI is good enough, saying over 0.05%, it is no longer an issue for HR algorithm. Things are different for SpO2 algorithm. SpO2 algorithm needs not only higher PI PPG signals. The higher, the better. SpO2 algorithm also need stable PI PPG signals. This is very important for stable and accurate SpO2 numbers output.

Why we need to do calibration for SpO2 algorithm? Is it workable without calibration?

SpO2 algorithm get SpO2 numbers based on the PI ration of IR and red channels. From the PI ratio, we could check a lookup table of SpO2 number vs PI ratio. That table varies from part to part, and varies between different optical designs. So a lab SpO2 calibration is a must. Without calibration, the SpO2 output numbers are completely no useless.

What is the requirement for algorithim to output stable SpO2 output?

First of all, no motion. Current PPG based SpO2 solution cannot support testing SpO2 in motion as the IR and Red channel PPG signals are too weak and unstable in motion. Secondly, PI should be reach the threshold. Algorithm calcuates SpO2 from IR and red channel PI ratio. If PI is too low, the ratio will be inaccurate. Thus the SpO2 accuracy cannot be garrenteed. Lastly, the singal quality should be OK. If the PPG signal is too noisy, the algorithm will consider the PPG signals as unqualified signals and output nothing waiting for good PPG signals.

Should I align the optical center of the LEDs for the wrist based SPO2 monitoring wearable design?

It is advised to align the optical center of the LEDs as SPO2 monitoring requires the RED and IR light to have similar optical paths.

What is the total input power requirement for the MAX86141?

The 3.1V - 5V requirement for VLED does need to be considered when looking at the power requirements for the MAX86140/1, alongside the 1.8V Vdd requirement. The VLED voltage is used to power the external LEDs which will be connected to the device. This voltage is generated external to the MAX86140/1.

Maxim bio sensor algorithm takes very long time to report HR/SpO2 data.

Maxim algorithm v1.9.3 needs around 20s-30s to get HR numbers. For SpO2, it costs around 45s if signal quality is good. Our algorithm team has plan to shorten that and will release new version later. We also could do some smart operations in the application layer: report the numbers earlier when the confidence level is low. And gradually converge to more reliable numbers.

For the HW design of MAX86141, can the Gsensor be connected to the main MCU instead of the sensor Hub MAX32664?

The Gsensor can be connected to the main MCU or the sensor Hub MAX32664, but the following points should be noted, 1. The Gsensor is connected to the main MCU and can read the data directly when it is used for step counting and other applications, so the MAX32664 does not need to transfer the data, so it saves more power. 2. Gsensor is connected to the Sensor Hub MAX32664. For HR and Spo2, Gsensor and BIO Sensor have better synchronization, which is more conducive to algorithm processing.

What SPI mode does the MAX86141 use for communication?

The MAX86140/MAX86141 uses SPI Mode 0 to communicate.

How can I use just one photodiode in the MAX86141?

Let the unused photo-diode input pin, PD1 (/ 2 ) _IN, float. Alternatively, if you just need one photodiode you can use MAX86140.

What does FDM/CDM fliter mean in MAX86141?

The FDM and CDM is the ambient light calulating method. FDM named Forward difference method, while CDM named Central Difference Method; FDM means only sample ambient light and LED exposure once per integration, CDM means sample ambient light twice, before and after LED exposure during one time integration.

For MAX86141,Which PD and LED does maxim recommend?

LED > Green Osram E1128, Osram LT PWSG > 3-in-1 (R+IR+G) SHF7013 PD > Ratio* 2:1 Vishay VEMD8080 > Ratio 1:1 (less recommended) Vishay VEMD5010, VEMD5080 Optical module: > (LED+PD) Osram SFH707