ANT+ Protocol Guide: Fitness Sensors & Diagnostics

Core Technical Specifications

center Freq 2.457 GHz (fixed frequency)

modulation GFSK

data Encoding 8-byte payload per message

operating Range 5–30 m

antenna PCB trace or ceramic chip

tx Power 0 dBm typical

bandwidth 1 MHz

channel Spacing Fixed single frequency

Real-World Devices & Applications

Fitness & Sports

Heart rate chest straps (Garmin, Wahoo, Polar)
Cycling speed and cadence sensors
Cycling power meters (Stages, Quarq, Garmin)
Running foot pods
Fitness equipment (treadmills, bikes with ANT+ display)

Reference Circuits & Schematics

ANT+ Module — nRF52832/nRF52840

Nordic nRF52 series chips support ANT+ alongside BLE via the SoftDevice protocol stack (S332/S340). Garmin’s D52 module is a pre-certified ANT+ solution.

Pin	Description
`VCC`	1.7–3.6V supply
`GND`	Ground
`ANT`	PCB trace antenna
`SWD`	Debug/programming
`GPIO`	Sensor inputs (HR, cadence, power)

Circuit Walk-Through

ANT+ uses adaptive isochronous channels — each sensor transmits at fixed intervals (e.g., 4 Hz for heart rate). The protocol is managed: device profiles define exactly what data each device type sends. HR profile sends 1-byte heart rate + R-R interval. Power meter sends instantaneous power, accumulated energy, and cadence. The ANT+ network key is publicly known (0xB9A521FBBD72C345) and required for all ANT+ communication.

Signal Structure & Waveform Analysis

ANT+ Channel Configuration

ANT+ uses a star topology with sensors broadcasting on assigned channels.

Channel Type

TX or RX

Sensor = TX (master), Display = RX (slave)

Network Key

8 bytes

Public ANT+ key: 0xB9A521FBBD72C345

Device Type

1 byte

e.g., 0x78 = HR, 0x79 = Speed/Cadence, 0x0B = Power

Message Period

Varies

HR: 8070 counts (4.06 Hz), Power: 8182 (4.00 Hz)

Timing Notes: ANT+ messages are 8 bytes fixed payload. Channel period in 32768 Hz counts (e.g., 8070 = ~4.06 Hz). Transmission type includes device number for pairing. ANT+ is NOT compatible with BLE — requires dual-protocol chip (nRF52) or separate radio.

Analysis & Capture Tips

Use ANT+ USB stick with Garmin ANT+ simulator software
nRF Connect SDK includes ANT+ profile examples
Check device number and transmission type for pairing
Verify network key is set correctly (public ANT+ key)

Bench Troubleshooting & Repair Guide

1. Sensor Not Found

ANT+ display device / nRF Connect

Check: correct device type in search, ANT+ network key configured, sensor battery OK, sensor within range (<5 m for initial pairing). Some sensors need to be ‘woken up’ by movement.

2. Dropouts

Wi-Fi Analyser

ANT+ uses a single fixed frequency (2.457 GHz). Heavy Wi-Fi traffic on channel 11 (2.462 GHz) can cause interference. Move away from Wi-Fi routers during use.

3. Dual ANT+/BLE

nRF Connect SDK / SoftDevice docs

nRF52 chips can run ANT+ and BLE simultaneously using S332/S340 SoftDevice. If BLE works but ANT+ doesn’t, check SoftDevice version and ANT+ license key.

ANT+ (2.4 GHz)