Embodied robots rely on a diverse sensor suite — IMUs, force-torque sensors, tactile arrays, and joint encoders — to perceive their environment and their own body state. The sensor fusion PCB that aggregates and processes these signals must preserve microvolt-level analog signals in an electrically hostile environment: 20–40 PWM motor drivers switching 48V at 20A within centimeters of the sensor board. This article covers the analog signal integrity challenges unique to robotics.

Sensor Modalities and Signal Characteristics

• IMU (Inertial Measurement Unit): 9-axis (3-axis accel + gyro + mag). Digital output via SPI at 1–8 kHz. MEMS sensors (e.g., Bosch BMI270) integrate ADC on-chip — the PCB's job is maintaining low-noise SPI communication, not analog signal conditioning

• Force-Torque (F/T) sensor: Strain-gauge Wheatstone bridge. Excitation: 5–10V, output: 0–20 mV full-scale (at 2 mV/V sensitivity). Requires precision instrumentation amplifier (e.g., AD8422, gain 100–500) with <1 μV/°C offset drift and >120 dB CMRR. Output: 0–5V to ADC

• Tactile sensor array: 8×8 to 16×16 taxel matrix of capacitive or piezoresistive elements. Scanned at 100–500 Hz. Each taxel produces pA–nA level signals. Requires transimpedance amplifier (TIA) or charge amplifier per taxel — or multiplexed readout with analog switches

• Joint encoders: Absolute magnetic encoder (14–18 bit resolution). Digital output via BiSS-C or SSI over RS-485. The PCB's role is differential line reception and termination, not analog processing

Analog Front-End Design

• In-amp selection: For F/T sensors, the instrumentation amplifier must have <10 nV/√Hz input voltage noise, >100 dB CMRR at 50/60 Hz (for mains-coupled noise), and <1 μV/°C offset drift. The AD8422 (gain 100, BW 80 kHz) is a common choice

• Anti-aliasing filter: 2-pole Sallen-Key low-pass filter at 500 Hz before the ADC. Prevents motor PWM noise (20–50 kHz) from aliasing into the measurement band

• ADC selection: 24-bit ΔΣ ADC (e.g., ADS1256, 30 kSPS) for F/T sensors. 16-bit SAR for general purpose. Simultaneous sampling if phase coherence between sensors is required

Noise Sources and Mitigation

• Motor PWM noise: 20–50 kHz PWM with 50 V/ns edges radiating broadband EMI. The sensor PCB must have a solid ground plane (L2) as a shield. Metal enclosure with gasketed seams. No gaps in the ground plane under analog sections

• Ground bounce: The motor drivers' return currents cause ground potential differences. The sensor PCB must use a star ground topology — analog ground, digital ground, and power ground connected at a single point (the ADC). No digital return current should flow through the analog ground plane

• Power supply noise: All analog supplies derived from LDOs (not switching regulators). Ferrite bead + capacitor Pi-filter (10 μF + 0.1 μF + ferrite) on each analog supply line. PSRR of the in-amp is finite — 10 mV of supply ripple can appear as 1 μV at the output (100 dB PSRR)

PCB Layout Rules

• Guard rings: The in-amp input pins are surrounded by a guard trace driven by the input common-mode voltage — cancels leakage currents and reduces parasitic capacitance

• Kelvin connections: F/T sensor excitation uses 4-wire (force/sense) connections. The sense wires measure voltage at the bridge, compensating for cable IR drop

• Shielding: Analog input cables are shielded twisted pairs with shield grounded at the PCB end only (prevents ground loops). Coaxial for high-impedance tactile sensor connections

• Layer stack: 6 layers. L1: analog components, L2: analog GND (solid), L3: digital signals, L4: digital GND, L5: PWR, L6: digital components