SensorFebruary 27, 202613 min read

Sensor Signal Conditioning: From Raw Measurement to Accurate Reading

How to design the amplifier, filter, and ADC interface between a sensor and a microcontroller. Covers RTDs, thermocouples, load cells, current shunts, and 4-20mA loops.

What Is Signal Conditioning?

Sensors produce small, noisy, or non-linear signals that are incompatible with microcontroller ADC inputs. Signal conditioning is the analog front-end that transforms these signals into clean, scaled voltages ready to digitise.

The signal chain: Sensor → Excitation → Amplification → Filtering → ADC

Each stage introduces error. Use the Sensor Accuracy Budget calculator to track total system accuracy through the chain.

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RTD Signal Conditioning (PT100 / PT1000)

An RTD (Resistance Temperature Detector) changes resistance with temperature. The PT100 follows the Callendar-Van Dusen equation:

R(T) = R_0 \left(1 + AT + BT^2\right) \quad \text{for } T \geq 0°C

where $A = 3.9083 \times 10^{-3}$ /°C, $B = -5.775 \times 10^{-7}$ /°C².

Use the PT100 Resistance calculator to find R at any temperature.

Measurement Circuit

The classic approach is a constant-current source through the RTD, measuring voltage:

V_{RTD} = I_{excitation} \times R(T)

3-wire connection eliminates lead resistance errors by measuring the voltage drop across the RTD separately from the current-carrying lead resistance. 4-wire (Kelvin) connection eliminates all lead resistance, achieving 0.01°C accuracy with a precision current source.

Key Considerations

Self-heating: too much excitation current heats the RTD. Keep $I^2 R < 1$ mW.
Use a low-noise instrumentation amplifier (INA128, AD8221)
PT1000 is preferred for battery-powered designs (higher resistance = smaller excitation current)

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Thermocouple Signal Conditioning

Thermocouples generate a very small EMF (microvolts to millivolts) proportional to the temperature difference between the hot junction and the cold junction:

E = S \times (T_{hot} - T_{cold})

where $S$ is the Seebeck coefficient (Type K: ~41 μV/°C).

Use the Thermocouple Voltage calculator to find the expected EMF.

Cold Junction Compensation

The cold junction is where the thermocouple wire connects to your PCB. Its temperature must be measured (usually with an NTC or RTD on the PCB) and added to the reading.

Integrated ICs like the MAX31855 (Type K) or LTC2986 handle amplification, cold junction compensation, and linearisation internally. Use these unless you have a reason not to. Discrete design requires: 1. ~10 mV/°C gain through a precision instrumentation amplifier 2. A separate temperature sensor for cold junction compensation 3. A linearisation lookup table or polynomial in firmware

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Load Cell / Strain Gauge Signal Conditioning

A load cell is a Wheatstone bridge of strain gauges. Full-scale output is typically 1–3 mV/V of excitation:

V_{out} = V_{excitation} \times S \times \frac{F}{F_{FS}}

With 5V excitation and 2 mV/V sensitivity, full-scale = 10 mV — tiny, and buried in noise.

Use the Load Cell Amplifier calculator to find the required gain.

Amplifier Selection

INA125P / INA128 are classic choices. The INA125 includes a precision voltage reference for excitation:

Set gain with one external resistor: $G = 4 + 60\,\text{k}\Omega / R_G$
RTI noise: ~8 nV/√Hz typical — adequate for 24-bit ADC

HX711 is a purpose-built 24-bit ADC for load cell and bridge sensor interfaces. Used in virtually all low-cost scale designs.

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Current Sensing with Shunt Resistors

Current is measured by the voltage drop across a low-value shunt resistor:

V_{shunt} = I \times R_{shunt}

A 10 mΩ shunt at 10A gives 100 mV — a reasonable signal for a difference amplifier.

Use the Current Shunt calculator to verify shunt voltage, power dissipation, and ADC resolution.

High-Side vs Low-Side Sensing

Low-side (shunt between load and GND): Simplest. Single-ended amplifier works. Problem: load is not at true ground. High-side (shunt between supply and load): Load stays at true ground, no load ground shift. Requires a difference amplifier or bidirectional current sense IC (INA219, INA240).

Selecting Shunt Resistance

R_{shunt} = \frac{V_{full-scale}}{I_{max} \times G_{amp}}

Target $V_{shunt}$ = 50–100 mV at full scale. Too small → poor SNR. Too large → excessive power dissipation ( $P = I^2 R$ ).

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Photodiode / Optical Sensor Signal Conditioning

Photodiodes produce current proportional to incident light. A transimpedance amplifier (TIA) converts this to voltage:

V_{out} = I_{photo} \times R_f

A 10 μA photocurrent with $R_f$ = 100 kΩ gives 1V output.

Use the Photodiode TIA calculator to find bandwidth and noise.

Stability

The TIA can oscillate without a feedback capacitor. Add $C_f$ across $R_f$ to stabilise:

f_{-3dB} = \frac{1}{2\pi R_f C_f}

Typically $C_f$ = 1–10 pF provides stability while maintaining adequate bandwidth.

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4–20 mA Current Loop

Industrial sensors use a 4–20 mA current loop over long cable runs (up to 1 km). The current encoding is immune to cable resistance:

4 mA = 0% of range (also provides power to the transmitter)
20 mA = 100% of range

Use the 4–20 mA Transmitter calculator to find sensor value from loop current and verify voltage budget.

Receiving the Signal

At the receiver, a 250Ω precision resistor converts 4–20 mA to 1–5V (for a 0–5V ADC):

V_{sense} = I_{loop} \times 250\,\Omega

This 1–5V range conveniently indicates cable breaks (0V) and sensor faults (<1V).

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Building an Accuracy Budget

Every component in the signal chain contributes error:

Source	Typical Error
Sensor nonlinearity	0.1–0.5% FS
Amplifier offset	0.02–0.2% FS
ADC quantisation	LSB/2
Temperature drift	50–500 ppm/°C
Reference voltage	0.05–0.5%

The total system accuracy (RSS method):

e_{total} = \sqrt{e_1^2 + e_2^2 + \cdots + e_n^2}

Use the Sensor Accuracy Budget calculator to model your system's accuracy across temperature.

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Summary

Sensor Type	Typical Output	Recommended IC
PT100 RTD	100–400 Ω	INA128 + constant-current source
Thermocouple	1–50 mV	MAX31855
Load cell	1–10 mV	HX711 or INA125
Photodiode	1 nA–100 μA	TIA with OPA2134
Current shunt	10–100 mV	INA219 or INA240
4–20 mA loop	1–5 V (via 250 Ω)	ADC direct

The most common mistakes: under-amplifying bridge sensors (too little gain), ignoring cold junction compensation in thermocouple designs, and forgetting the feedback capacitor in TIA circuits.

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