PCB for Sensor: Analog Signal Routing Techniques

Sensor PCBs bridge the physical world and digital processing. Temperature sensors, pressure transducers, strain gauges, and gas sensors produce tiny analog signals (microvolts to millivolts) that must be amplified and digitised without corruption from noise. The PCB layout determines the achievable signal-to-noise ratio — and therefore the measurement accuracy — of your sensor system. This guide covers analog routing techniques for Indian engineers designing sensor circuits for IoT, industrial, and medical applications.

Table of Contents

• Analog Signal Challenges
• Analog Trace Routing
• Grounding for Sensors
• Input Filtering
• ADC Layout
• Shielding Techniques
• Practical Sensor Board Layout
• Frequently Asked Questions

Analog Signal Challenges

Sensor signals are vulnerable because of their small amplitude:

Analog Trace Routing

• Differential routing: Route sensor signals as differential pairs. Common-mode noise cancels when using a differential amplifier. Match trace lengths within 1mm
• Guard traces: Surround high-impedance analog traces with a guard ring connected to the signal ground or a driven guard voltage. This prevents leakage currents from corrupting the measurement
• Short traces: Keep the distance from sensor to first amplifier stage as short as possible. Below 50mm is ideal
• No routing under ICs: Analog signal traces should not pass under digital ICs on adjacent layers — the switching noise couples capacitively
• Separate analog and digital traces: If analog and digital traces must cross, do so at right angles to minimise coupling

Grounding for Sensors

• Use a continuous ground plane as the return path for all sensor signals
• Do NOT split the ground plane between analog and digital. Instead, place analog and digital components in separate physical areas over the same continuous plane
• Route the sensor ground return from the sensor through the analog section back to the ADC ground pin. Avoid routing this ground through the digital section
• For highest accuracy, use Kelvin (4-wire) connections to the sensor: separate current-carrying and voltage-sensing traces

Input Filtering

• RC low-pass: Place a simple RC filter (1kΩ + 100nF = 1.6kHz cutoff) at the ADC input to suppress high-frequency noise. Use C0G/NP0 capacitors for linearity
• Common-mode filter: For differential inputs, add matching filters to both lines. Capacitor values must match to maintain CMRR
• Anti-aliasing: The filter cutoff must be below the ADC Nyquist frequency (half the sample rate). A 2nd or 4th order filter provides adequate attenuation
• ESD protection: Add TVS diodes at the sensor input if the sensor is external (exposed to ESD). Place before the filter components

ADC Layout

• Place the ADC close to the analog signal source — short traces improve accuracy
• Use a dedicated low-noise LDO for the ADC analog power supply (AVDD). Do not share with digital power
• Connect AGND and DGND pins at the ADC chip as specified in the datasheet (usually connect at the chip)
• Place decoupling capacitors (100nF + 10µF) directly at the AVDD pin
• Route the reference voltage trace carefully — any noise on the reference appears as noise on all measurements

Shielding Techniques

• Board-level shield can: Place a metal shield over the analog front-end section. Ground it to the analog ground pour
• Via fence: Create a wall of ground vias between analog and digital sections, spaced every 2-3mm
• Shielded cables: For external sensors, use shielded cables with the shield connected to analog ground at the board end only (to prevent ground loops)
• Guard ring on PCB: For high-impedance inputs (above 10MΩ), add a guard ring around the input pin and trace. Drive the guard ring with the same potential as the signal to eliminate leakage current

Practical Sensor Board Layout

1. Place the sensor connector at one edge of the board
2. Place the analog front-end (amplifier, filter) immediately next to the connector
3. Place the ADC next to the analog front-end
4. Place the MCU in the digital section, separated from analog by a via fence
5. Place the power supply section separate from both, with LDOs feeding each section
6. Add test points on: sensor input, amplifier output, ADC reference, and power rails

Frequently Asked Questions

How do I reduce 50Hz noise from mains?

Use a low-pass filter with cutoff below 50Hz if your measurement bandwidth allows it. For faster measurements, use a notch filter (twin-T network) at 50Hz. Software digital filtering (moving average of 20ms samples) also rejects 50Hz effectively. For highest rejection, sample at a rate that is an exact multiple of 50Hz (e.g., 500 SPS) and average.

Should I use a 4-layer board for sensor PCBs?

For precision measurements (better than 12-bit accuracy), yes. The dedicated ground plane on Layer 2 provides a much better signal return path than a 2-layer board. For basic 10-bit sensor boards (Arduino ADC level), a well-designed 2-layer board with generous ground pour is adequate.

How do I handle thermocouple PCB connections?

Thermocouple junctions form wherever two dissimilar metals meet. At the PCB, the thermocouple wire connects to copper traces — this creates a parasitic junction. Keep both thermocouple terminal connections at the same temperature (close together, away from heat sources) so the parasitic voltages cancel. Use a cold-junction compensation IC (MAX31855, ADS1118) near the thermocouple terminals.

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