The MAX86173 from Analog Devices is an ultra-low-power, dual-channel optical analog front-end (AFE) designed specifically for next-generation wearable health and fitness devices. It integrates transmit and receive signal chains into a single compact die, enabling developers to build clinical-grade optical sensors for pulse oximetry, heart-rate monitoring, and multi-parameter biometric tracking without the complexity of discrete component designs.

If you are evaluating optical AFEs for a wrist-worn pulse oximeter, an ear-clip SpO2 module, or a medical-grade fitness tracker, this guide covers the MAX86173 architecture, key electrical parameters, application circuits, and supply-chain considerations you need to make a sourcing decision.

What Is the MAX86173?

• Nine LED driver outputs generated from three independent 8-bit current-programmable LED drivers.
• Two low-noise charge-integrating receiver channels, each with its own 20-bit ADC and best-in-class ambient-light cancellation (ALC) circuit.
• A 256-word FIFO that offloads burst data from the host microcontroller.
• Dual I2C and SPI interfaces with autonomous operation capability.

Why It Matters for Wearables

• Complete Dual-Channel Optical Data-Acquisition System
• Ultra Low-Power Operation for Wearable Devices
  • Low-Power Operation, Optical Readout Channel < 11µA at 25fps
  • Exposure Integration Period Ranging from 14.6μs to 118.2µs
  • Low Shutdown Current < 1µA
• Excellent Top-End Dynamic Range > 97dB in White Card Loop-Back Test (Nyquist Sample-to-Sample Variance)
• Extended Dynamic Range up to 115dB (Averaging and Off-Chip Filtering)
• Supports Frame Rates from 1fps to 2.9kfps
• High Resolution 20-Bit Charge Integrating ADCs
• Supports Four PD Inputs for Multi-Parameter Measurements
• Supports Nine LED Driver Output Pins Generated from Three 8-Bit LED Current Drivers
• Low Dark Current Noise of < 50pARMS(Sample-to-Sample Variance in 118.2µs Integration Time)
• Excellent Ambient Range and Rejection Capability
  • 200µA Ambient Photodiode Current
  • > 70dB Ambient Rejection at 120Hz (Burst Average > 2)
• Miniature 2.78mm x 1.71mm, 7 x 4, 0.35mm Ball Pitch WLP Package
• -40°C to +85°C Operating Temperature Range

The MAX86173 is an ultra-low-power optical data acquisition system with both transmit and receive channels. On the transmitter side, the MAX86173 has nine LED driver output pins, programmable from three high-current, 8-bit LED drivers. On the receiver side, the MAX86173 has two low-noise charge integrating front-ends that each includes independent 20-bit ADC and best-in-class ambient light cancellation (ALC) circuits, producing the highest performing integrated optical data acquisition system in the market today.

Due to its low power consumption, compact size, ease and flexibility of use, the MAX86173 is ideal for a wide variety of optical sensing applications such as pulse-oximetry and heart-rate detection.

The MAX86173 operates on a 1.8V main supply voltage and a 3.1V to 5.5V LED driver supply voltage. The device supports both I2C and SPI interfaces in a fully autonomous way. The device has a large 256-word built-in FIFO.

The MAX86173 is available in a compact WLP package 2.78mm x 1.71mm with 7 x 4, 0.35mm ball pitch.

Block Diagram Description

1. LED Transmit Subsystem
2. • Three independent 8-bit LED current DACs.
   • Each DAC feeds a multiplexed driver network that routes to nine physical output pins.
   • This allows time-division or multi-wavelength illumination sequences (e.g., red → IR → green) without external analog switches.
   • LED current is programmable per pulse, so the host can implement adaptive illumination algorithms that dim the LED in high-perfusion conditions and boost it in low-perfusion conditions.
3. Optical Receive Subsystem (×2)
4. • Each channel contains a low-noise transimpedance amplifier (TIA) followed by a charge integrator.
   • The 20-bit ADC samples the integrated charge at the end of each exposure window.
   • Two channels mean you can connect up to four photodiodes (two per channel in differential or single-ended configurations) for spatial diversity or multi-site measurement.
5. Ambient Light Cancellation (ALC)
6. • The ALC circuit subtracts ambient photocurrent before integration, preventing the ADC from saturating under bright sunlight or indoor lighting.
   • The spec claims >70 dB rejection at 120 Hz when burst averaging is>2, which is critical because 120 Hz is the dominant flicker frequency of fluorescent and LED indoor lights in many regions.
7. Digital Control and FIFO
8. • A state machine manages LED sequencing, ADC timing, and FIFO writes.
   • The 256-word FIFO stores samples until the host requests them via I2C or SPI.
   • Autonomous mode means the MAX86173 can run measurement loops while the host MCU sleeps, waking it only when the FIFO threshold is reached or a timer expires.

Form-Factor Suitability

• Wrist-worn devices: Smartwatches and fitness bands where PCB area is limited to a few square millimeters.
• Finger clips: Medical pulse oximeters that require dual-channel red + IR measurement in a disposable or semi-reusable clip.
• Earbuds: In-ear PPG is gaining traction because the ear has good blood perfusion and less motion artifact than the wrist. The 2.78 mm × 1.71 mm package fits comfortably inside compact earbud PCBs.
• Rings and small patches: Emerging health-monitoring form factors where every millimeter of PCB matters.

Clinical vs. Consumer Trade-Offs

• Calibrated optical coupling between LED, tissue, and photodiode.
• Validated signal-processing algorithms (e.g., motion-artifact rejection, perfusion-index gating).
• Biocompatibility and mechanical design appropriate for the measurement site.
• Regulatory documentation from the finished-device manufacturer, not the chip vendor.

• Wearable Devices for Fitness, Wellness and Medical Applications
• Clinical Accuracy
• Suitable for Wrist, Finger, Ear and Other Locations
• Optimized Performance to Detect:
• Optical Heart Rate
• Heart-Rate Variability
• Oxygen Saturation (SpO2)
• Body Hydration
• Muscle and Tissue Oxygen Saturation (SmO2and StO2)
• Maximum Oxygen Consumption (VO2max)

MAX86173 vs. MAX86171

MAX86173 vs. MAX86174 / MAX86175

• Lower power modes.
• Improved ALC algorithms.
• Broader LED driver voltage range.

Competitive Landscape

Optical Mechanical Layout Considerations

1. LEDs: Typically red (660 nm), infrared (940 nm), and optionally green (530 nm) for HR and SpO2. Match LED peak wavelength to the photodiode spectral response.
2. Photodiodes: Silicon PIN or PN photodiodes with high responsivity at the LED wavelengths. The dual-channel architecture allows you to place two PDs at different distances from the LED (differential spacing) to improve motion artifact rejection.
3. Optical barriers: Black epoxy or physical shrouds between the LED and PD prevent direct optical crosstalk without passing through tissue.
4. Cover lens: Medical-grade glass or acrylic with anti-reflective coating. Thickness and refractive index affect optical coupling efficiency.

Firmware Integration Tips

• FIFO threshold tuning: Set the FIFO almost-full interrupt to a value that matches your MCU's preferred DMA burst size. A common starting point is 128 words (half the FIFO).
• LED current calibration: Per-user skin tone and perfusion vary widely. Implement an auto-ranging algorithm that starts with a low LED current and increases it until the PPG signal amplitude exceeds a noise-floor threshold.
• Sampling rate selection: For standard pulse oximetry, 25–100 fps is sufficient. For HRV analysis, 100–250 fps may be preferred. For research-grade high-resolution tissue oxygenation, explore higher rates and post-processing averaging.
• Ambient tracking: Use the ALC status registers to monitor ambient rejection performance. If you see frequent saturation flags, increase the integration time, reduce the LED current, or improve mechanical shielding.

If MAX86173 Is Unavailable or EOL in the Future

Migration note: If you must swap the MAX86173 for a non-Analog Devices part, budget engineering time for register-map translation, optical recalibration, and algorithm retuning. The PPG signal path is sensitive to ADC quantization noise, integration time, and ALC behavior, so even parts with similar headline specs may require weeks of re-validation.

Q1: What is the MAX86173 used for? 

The MAX86173 is an optical analog front-end for wearable health sensors. It enables pulse oximetry (SpO2), optical heart rate monitoring, heart rate variability, and multi-parameter biometrics by driving LEDs and digitizing photodiode signals with high resolution and low power.

Q2: How many photodiodes can the MAX86173 support? 

It supports up to four photodiode inputs across two independent receive channels. Each channel can be configured for single-ended or differential operation, giving flexibility for multi-site or spatially diverse optical sensing.

Q3: What is the dynamic range of the MAX86173, and why does it matter? 

The MAX86173 achieves > 97 dB dynamic range in white-card loop-back tests and up to 115 dB with averaging and off-chip filtering. High dynamic range matters because it allows the AFE to resolve weak PPG signals from low-perfusion tissue (e.g., cold fingers, dark skin, or wrist motion) without saturating under bright ambient light.

Q4: What is the power consumption of the MAX86173? 

The optical readout channel draws less than 11 µA at 25 fps. Shutdown current is below 1 µA. Total system power depends heavily on LED drive current, which is application-dependent, but the AFE itself is optimized for 24/7 wearables.

Q5: Does the MAX86173 include LEDs and photodiodes? 

No. The MAX86173 is an AFE only. You must provide external LEDs (typically red, IR, and green) and photodiodes, along with the optical-mechanical design that couples light into tissue and back to the sensor.

Q6: What interface does the MAX86173 use? 

It supports both I2C and SPI. I2C is ideal for low-pin-count, shared-bus designs. SPI is preferred for high-throughput streaming at frame rates above a few hundred fps.

Q7: Is the MAX86173 qualified for automotive applications? 

As of the latest datasheet revision, the MAX86173 is not listed as AEC-Q100 qualified. It is rated for an industrial temperature range of –40 °C to +85 °C. For automotive or safety-critical designs, contact Analog Devices or your distributor to discuss custom qualification or alternative AEC-Q100 parts.

Q8: What package does the MAX86173 come in? 

The MAX86173 is available in a 2.78 mm × 1.71 mm wafer-level package (WLP) with 28 balls (7 × 4 array, 0.35 mm ball pitch). This is a chip-scale package designed for compact wearable PCBs. Assembly requires board-shop capability for fine-pitch WLP or WLCSP handling.

Q9: How does the MAX86173 compare to the MAX86171? 

The MAX86173 offers dual receive channels, 20-bit ADC resolution, and a 256-word FIFO, while the MAX86171 has a single channel, 18-bit ADC, and 128-word FIFO. The 73 is positioned for premium and clinical-grade wearables; the 71 targets cost-optimized consumer fitness trackers.

Q10: Where can I buy the MAX86173?

The MAX86173 is available from authorized distributors such as DigiKey, Mouser, and Arrow. For competitive pricing, volume quotes, and local inventory checks, you can also contact Welllinkchips directly.