This reference design provides information about preparing and running the MAXREFDES280# low-noise PPG sensor band. This band uses ultra-low-power optical biosensor from Analog Devices in a wrist-worn design to capture biometric signals. The platform streams out the raw PPG data for post processing of vital signs like heart health and blood oxygenation based on the biosensor measurement.
The MAXREFDES280#, low-noise PPG sensor band allows for the quick evaluation of the MAX86171 optical analog front-end (AFE) for applications at various sites on the body, particularly the wrist. The MAX86171 supports both I2C and SPI-compatible interfaces. The MAX86171 has two optical readout channels that operate simultaneously. The MAXREFDES280# band allows flexible configurations to optimize measurement signal quality at minimal power consumption. The MAXREFDES280# band helps the user quickly learn about how to configure and use the MAX86171.
The MAXREFDES280# band consists of two boards. The MAXSensorBLE# is the main data acquisition board while the MAX86171_OSB# is the sensor daughter board for MAX86171. The MAXREFDES280# band is powered by a LiPo battery that can be charged using a USB-C supply. The MAXREFDES280# band comes with a MAX86171ENI+ in a 28-bump WLP.
Features & Benefits
Features
Quick Evaluation of the MAX86171
Supports Configuration Optimization
Facilitates Understanding of MAX86171 Architecture and Solution Strategy
Windows System with a USB port and Bluetooth 4 with BLE supported on its Hardware (Win BLE).
Procedure
The MAXREFDES280# is fully assembled and tested. Use the following steps to verify board operation: Visit the Design Resources tab to download the most recent version of the MAXREFDES280# software,
MAX86171GUISetupVxxx_Web.ZIP. Save the MAXREFDES280# software to a temporary folder and decompress the ZIP file.
Plug in the BLE dongle to one of the USB ports on the PC.
Open MAX86171GUISetupVxxx.exe and follow the instructions from the pop-up windows to install the PC GUI.
Press the power button for 1 second to power up the MAXREFDES280#. When powered on, the green LED toggles.
Note: To power off the device, press the power button for 3 seconds. The USB-C to USB-A cable is for charging the battery only. System performance might decrease if used while charging it. If the boards are not assembled, connect the two boards as shown in Figure 1.
Start the MAX86171 Heart-Rate Sensor Evaluation System software. Connect Device appears. Choose your device and press Connect as shown in Figure 2.
The GUI launches as shown in Figure 3 and Figure 4 .
Configure the band on the GUI and click on the Start button on the bottom left side to start the data acquisition.
When running, the LEDs on the band illuminate and the plots on the GUI stream with data as shown in Figure 5 and Figure 6.
Figure 1. PCB connect of MAX86171 sensor board to MAXSENSORBLE board.
The MAXREFDES280# includes one sensor PCB. It contains the MAX86171 optical AFE, a 3-axis accelerometer together with four photodiodes and eight LEDs (in four LED packages). The MAX86171_OSB# comes with four discrete photodiodes (Vishay VEMD8080), two Red/IR/Green LED packages (Osram SFH 7013) and two green LEDs (Osram CT DBLP31.12-6C5D-56-J6U6). The MAXREFDES280# allows raw optical and accelerometer data to be sampled and transferred to the GUI for both dynamic viewing and logging for later analysis. The MAXREFDES280# microcontroller PCB is used to do SPI-to-BLE communication, transporting the raw optical and accelerometer data to the PC through BLE.
Most of the MAX86171 functionality has been mapped to the GUI so the wide variety of applications supported by the MAX86171 can be rapidly explored. The following is a brief description of this functionality options.
Frame Rate
Frame rate defines how frequently a frame is repeated. Frame is a collection of measurements that can have a minimum of one measurement and a maximum of nine measurements. The frame rate can take on any value from 1fps to 2.9kfps.
Table 1shows the maximum supported frame rates (in fps) for the MAX86171 for the given number of measurements and use of accelerometer. The maximum frame rate is limited by the BLE protocol, not the AFE itself.
Table 1. MAX86171 Max Frame Rates (fps)
Maximum Frame Rate
With Acceletometer
Without Accelerometer
Number of Measurements
1 PD
2 PD
1 PD
2PD
1
500
500
1000
1000
2
500
250
1000
500
3
250
125
500
250
4
250
125
500
125
5
125
125
250
125
6
125
62.5
250
125
7
125
62.5
125
62.5
8
125
62.5
125
62.5
9
125
62.5
125
62.5
Picket Fence Configuration
Under typical situations, the rate of change of ambient light is such that the ambient signal level during exposure can be accurately predicted and high levels of ambient rejection are obtained. However, it is possible to have situations where the ambient light level changes extremely rapidly. For example, when a car with direct sunlight exposure passes under a bridge and into a dark shadow. In these situations, it is possible for the on-chip ambient light correction (ALC) circuit to fail and produce and erroneous estimation of the ambient light during the exposure interval. The optical controller has a built-in algorithm, called the picket fence function, that can correct for these extreme conditions on the ALC circuit.
Refer to the MAX86171 data sheet under the Picket Fence Detect-and-Replace Function section for details.
System Control
There is option to power down one of the PPG readout channels or use dual PPG channels simultaneously. When dual PPG channels are used, the data log will show data from both PDs for each configured measurement.
Photodiode Bias
The MAX86171 provides multiple photodiode biasing options. These options allow the MAX86171 to operate with a large range of photodiode capacitance. The PD BIAS values adjust the PD_IN bias point impedance to ensure that the photodiode settles rapidly enough to support the sample timing. PD BIAS is configured depending on the capacitance of the photodiode used.
Accelerometer Configuration
The on-board accelerometer can be enabled or disabled by using the GUI. Supported accelerometer full-scale ranges are ±2g, ±4g, and ±8g. The output data of the accelerometer can also be configured from 15.63Hz to 2000Hz when used with Sync Mode of External Clock or External Frame.
Measurements Enable
Measurement Enable specifies the data acquisition sequence that the internal state machine controller will follow and indicates where the converted data will be mapped into the FIFO.
Each FIFO field can be applied to one measurement. Acquired data can be from LED1~9 (optical exposure from LED1~9) that are illuminated independently. The other options are Ambient (optical data with no exposure, just ambient illumination) or Disable (skip this acquisition).
The MAX86171 supports up to nine measurements per frame. Each of the nine measurements are configured in the MEAS1 Setup to MEAS9 Setup registers. Any measurement can be enabled and the measurements do not need to be contiguous. If direct ambient needs to be measured, it should always be the last enabled measurement in the frame.
The enabled measurements sequence will repeat for each frame. Each measurement, if enabled, will be plotted in the Meas x (x = 0...9) tabs, respectively, as shown in Figure 6.
Refer to the MAX86171 data sheet under System Control and MEASx Setup sections for details.
Integration Time
The MAX86171 supports exposure integration times of 14.8μs, 29.4μs, 58.7μs, and 117.3μs. The exposure pulse width is a critical parameter in any optical measurement. Longer exposures allow for more optical photons to be integrated and obtain better noise performance but also increase system power and reduce ambient rejection capability.
Measurement Average
The MAX86171 can complete sample averaging of 2 ~ 128 samples internally. This feature is useful if more optical energy is needed to make a low perfusion measurement but the data rate across the interface or the processing power in a host micro is not desirable.
PPG Range
Each MAX86171 optical channel has four ADC full-scale ranges, which include 4μA, 8μA, 16μA, and 32μA.
PPG PD Select
There are two PPG readout ADC channels. These two PPG readout channels can support up to four photodiodes. PPG channel 1 can be muxed to PD1 or PD3 and PPG Channel 2 can be muxed to PD2 or PD4.
LED Driver Configurations
In each measurement, the three LED drivers have a range. There are four full-scale range settings 31mA, 62mA, 93mA and 124mA.
Each of the three LED drivers has an LED Pulse Amplitude Current setting. Each measurement can drive one, two, or all three LED drivers. This configuration of LED driver and LED mux is highly flexible, allowing for any of the nine LED driver pins to sink current from one LED driver (or two LED drivers for LED1, LED2, LED3). Each LED driver has an 8-bit current source DAC. The Peak LED Current box allows for an actual current to be entered. The nearest available DAC current is selected and displayed in the field.
LED Settling Time
The LED Settling Time is the time prior to the start of integration time that the LED is turned on. There are four settlings: 24μs, 18μs, 12μs and 6μs. This time is necessary to allow the LED driver to settle before integrating the exposure photo current.
Ambient Light Cancellation
The on-chip ambient light cancellation incorporates a proprietary scheme to cancel ambient-light-generated photodiode current, which allows the sensor to work in high ambient light conditions.
PPG Offset
Each optical signal path also incorporates a two-bit offset DAC for extending the optical dynamic range. This allows for a larger convertible exposure range by sourcing some of the exposure current from the offset DAC.
Start/Stop Button
The Start button is used to start data acquisition from the demo. The Start button will only be effective when the MAXREFDES280# is connected and detected. Once Start/ has been pushed, the Stop button appears, which can be used to stop the acquisition. Once the acquisition has started, all settings are locked. Terminate the acquisition to change any setting.
Reset Button
The Reset button will clear out all register settings back to the programs start up.
Data Logging
Raw optical and accelerometer data can be logged from the Logging pulldown menu item. There are two options available: Data saved to file or in the flash. When File data logging is selected, the GUI asks for a folder location where the logging file will be saved. Create a new folder or accept the default. Data logging will start on the next Start button and will continue until the Stop button is pressed. The final file write is only done when the File pulldown menu item is accessed and the data-logging button is pressed.
Flash logging allows raw sensor data to be stored to the integrated 32MB flash memory chip in a binary file format. The max duration for flash logging is dependent on the following: frame rate, number of optical channels, and use of accelerometer.
The GUI enables/disables flash logging. The GUI can be disconnected while flash logging to allow for remote operation (PPG Plots not available). Preparing the flash memory can take up to 30s after enabling. If the flash memory fills or the battery power drops too low, flash logging will automatically stop and the file will close. Only one file can be saved at a time. The file must be downloaded since it will be erased on the next log request.
If a log has completed, a binary file will be found on the device. The binary log file must be downloaded through the USB-C cable; it cannot be downloaded through BLE. When the device is plugged into the PC, it enumerates as a USB mass storage device. However, the file can only be copied from this device. No other operations (such as deleting or saving other files) will work on this device.
Copy the file to a local PC volume and then run the Parse Bin File under the Logging menu to generate a CSV file.
Register Map Access
Under the Register Tab, the user can access the sensor register map as shown in Figure 7. Press Read, to read all the register values currently configured in the optical AFE. Bolded font bits are logic one. Normal font bits are logic zero. Click on the bits to toggle their value and click on W to write the value to the device. The register value does not change until W is clicked. Click R to read the register value to verify the write.
The onboard tri-color LEDs are used as status indicators.
LED Green:
Toggling (1Hz 50% duty cycle) = BLE advertising Toggling (1Hz 10% duty cycle) = BLE connected LED
Red:
USB-C cable connected to charger
On = charging
Off = charge complete
Flash Logging:
On = busy preparing the flash memory or flash memory is full
Toggling (synchronously with the green LED) = logging
Off = not logging
Note that flash logging indication takes precedence over the charging indication (i.e., if the device is plugged into a charger, the red LED indicates charge status). If flash logging is enabled while plugged into the charger, the red LED indicates flash log status.
Power Switch
Press the power switch (SW) to turn on/off the device. When powered on, the green LED will toggle per the LED indicator section. When powered off, the green LED will go out. The red LED may light temporarily, indicating that the flash log is closing. Plugging in the USB-C cable will also power up the device.
Battery/Charging
Use the USB-C cable to charge the integrated single-cell LiPo battery. The integrated PMIC initiates and stops charging automatically. Charge status is indicated through the red LED and GUI.
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