Sensors and Signal Conditioning

The measurement chain

A sensor turns a physical quantity (temperature, force, light, position) into an electrical one, usually a small, non-linear or noisy voltage, current or resistance. Signal conditioning turns that into something a microcontroller or a PLC can read accurately: it excites the sensor, amplifies and filters the signal, scales it to the ADC range, and converts and linearizes it in software. Every stage adds error, so work through the chain in order and put the effort where the biggest error is.

Stage	What it does	Typical parts
1. Sensor	Converts the quantity to a resistance, voltage or current	NTC, RTD, thermocouple, strain gauge, photodiode
2. Excitation	Supplies a current or voltage the sensor needs	Divider, constant current source, bridge supply
3. Amplification	Brings millivolts up to the ADC range	Op-amp, instrumentation amplifier
4. Filtering	Removes noise and aliasing	RC low-pass, active filter (see Capacitors)
5. Conversion	Turns the voltage into a number	ADC (see Microcontroller Electronics)
6. Software	Scales, linearizes, calibrates and averages	Beta equation, lookup table, polynomial, offset and gain

Sensor types at a glance

Quantity	Sensor	Output	Conditioning	Typical accuracy
Temperature	NTC thermistor	Resistance, 33 kΩ to 0.7 kΩ (10 kΩ NTC, 0–100 °C)	Divider and ADC; beta equation	0.5–2 °C
Temperature	RTD (Pt100, Pt1000)	Resistance, 0.385 Ω/°C (Pt100)	Constant current, 3- or 4-wire, amplifier	0.1–0.5 °C
Temperature	Thermocouple (K, J, T)	41 µV/°C (K), needs cold-junction compensation	Thermocouple amplifier chip	1–2 °C
Temperature	Digital IC (DS18B20, TMP117, LM75)	I2C or 1-Wire numbers	None	0.1–0.5 °C
Force, weight	Strain gauge, load cell	2 mV/V at full load	Bridge, instrumentation amplifier or an ADC like the HX711	0.05–0.5 percent
Pressure	Bridge pressure sensor, or a 0.5–4.5 V ratiometric one	Millivolts or volts	Amplifier or direct to the ADC	0.25–1 percent
Light	Photodiode, phototransistor, LDR	Microamperes or resistance	Transimpedance amplifier or divider	Varies
Distance	Ultrasonic, time-of-flight (laser), IR triangulation	Pulse width or digital	Timer capture or I2C	1–3 mm to 1 cm
Position, angle	Potentiometer, Hall sensor, encoder	Voltage or pulses	ADC or counter	Varies
Current	Shunt resistor, Hall sensor	Millivolts or a voltage around mid-supply	Current-sense amplifier or direct	0.5–2 percent
Industrial process variable	Any, with a transmitter	4–20 mA or 0–10 V	Burden resistor or an input card	Per transmitter

Thermistors (NTC)

An NTC thermistor is a resistor whose resistance falls steeply as temperature rises. It is cheap, sensitive and fast, but strongly non-linear. The beta equation models it between two temperatures: R = R₀ × e^{B × (1/T − 1/T₀)}, with temperatures in kelvin, R₀ the resistance at T₀ (25 °C = 298.15 K) and B (often 3 380–4 000 K) from the datasheet.

Temperature	NTC resistance	Divider voltage (10 kΩ to 3.3 V)	12-bit ADC counts
0 °C	33 621 Ω	2.54 V	3 157
25 °C	10 000 Ω	1.65 V	2 048
50 °C	3 588 Ω	0.87 V	1 082
100 °C	698 Ω	0.22 V	267

• Tolerance and beta tolerance (1 percent R, 1 percent B) limit accuracy away from 25 °C; for better, use the three-coefficient Steinhart–Hart equation, or calibrate at two points.
• Linearizing: in software use the beta equation or a lookup table with interpolation.
• Leads and mounting: thermistors are small and quick, which also makes them noisy and sensitive to how they are mounted (thermal paste, epoxy, probe housing).
• PTC thermistors (resistance rises with temperature) are used as resettable fuses and heaters, not for measurement.

RTDs (Pt100 and Pt1000)

A resistance temperature detector is a platinum element whose resistance rises almost linearly with temperature. A Pt100 is 100.00 Ω at 0 °C, with a temperature coefficient of 0.00385 per °C (IEC 60751), giving about 0.385 Ω/°C; a Pt1000 is ten times that. It is accurate, stable and standardized, but the signal is a small change in a small resistance, so the wiring and the excitation matter.

• Use a constant current source (0.25–1 mA) or a ratiometric bridge with an ADC that uses the same reference.
• Use an RTD interface chip (MAX31865, ADS1220, and similar) that does the excitation, 3/4-wire and conversion for you.
• Keep it short and shielded in noisy places.

Thermocouples

A thermocouple is two different metals joined at one end. The temperature difference between the joined end (hot junction) and the open ends (cold junction) produces a small voltage, about 41 µV per °C for a type K (nickel–chromium and nickel–aluminum). It works to high temperatures (1 000 °C and more), is rugged and cheap, but the voltage is tiny and depends on the difference, so the temperature of the connection to your circuit must be measured and added: cold-junction compensation.

• Use a thermocouple amplifier chip (MAX31855, MAX31856, AD8495): cold-junction compensation, linearization and fault detection are built in.
• Use thermocouple wire and connectors of the right type all the way to the cold junction. A copper wire joined to the thermocouple adds new junctions and errors.
• Grounded and ungrounded probes. A grounded tip is fast but can pick up ground noise; an ungrounded one is isolated and slower.
• Open-circuit detection: a broken thermocouple reads an arbitrary value; amplifier chips flag it.

Strain gauges, load cells and bridges

A strain gauge is a thin metal foil that stretches with the surface it is bonded to, changing resistance by a tiny amount. Its sensitivity is the gauge factor GF (about 2): ΔR ÷ R = GF × ε, where ε is the strain. To measure such small changes it is connected in a Wheatstone bridge, which gives zero output when balanced and a small differential voltage when a gauge changes. A load cell is a metal block with gauges already bonded and wired as a bridge, rated in mV per volt of excitation at full load.

• Use an instrumentation amplifier (INA125, AD620) or a bridge ADC chip (HX711, ADS1232, NAU7802) with a high gain, high common-mode rejection and low offset drift.
• Excitation stability matters: any change in the bridge supply changes the output. Use a ratiometric measurement (ADC reference = bridge supply) or a precision reference.
• Temperature: gauges drift; use a half or full bridge, matched gauges on the same material, and a dummy gauge for compensation.
• Shielded, twisted cable and a 6-wire connection (with sense lines) for long cables.
• Zero (tare) and span: calibrate with known weights.

4–20 mA loops and 0–10 V signals

Industrial transmitters send the measurement as a 4–20 mA current (or sometimes 0–10 V). A current loop is immune to wire resistance and voltage drop, so it works over long cables, and the 4 mA “live zero” tells a broken wire (0 mA) from a zero reading. To read it with an ADC, pass it through a precision burden resistor: V = I × R.

• Two-wire loop-powered transmitters take their power from the loop (12–30 V supply); a 250 Ω burden adds 5 V of drop at 20 mA: check the loop voltage budget.
• Protect the input with a TVS diode and a series resistor; an input card or an isolated converter adds isolation.
• Scale in software: engineering value = low + (mA − 4) ÷ 16 × span. See the Analog Scaling Tool and I/O Wiring and Signals.
• HART signals ride on the same loop: see Fieldbuses and Device Networks.

Photodiodes, light sensors and ultrasonics

Sensor	Signal	Reading circuit
Photodiode	Current proportional to light, nanoamperes to microamperes	A transimpedance amplifier: an op-amp with a feedback resistor turns current into voltage (V = I × Rf)
Phototransistor	Larger current, slower and less linear	A pull-up resistor to a digital input or an ADC
Light-dependent resistor (LDR)	Resistance falls with light, slow	A divider into an ADC; coarse and slow
Ambient light ICs (BH1750, VEML7700)	Digital lux values	I2C; no analog design needed
Ultrasonic distance (HC-SR04)	An echo pulse whose width is the time of flight	Timer capture; measures a pulse in microseconds
Laser time-of-flight (VL53L1X)	Digital distance	I2C

Digital sensors

Many modern sensors contain the conditioning and the converter and speak a digital bus (I2C, SPI, 1-Wire, UART): temperature, humidity, pressure, accelerometers, gyros, magnetometers, light and color sensors. They are the easiest to use and a good first choice when accuracy and range are adequate. Check the address (I2C), the supply voltage and logic level, the update rate and the pull-up resistors (see Microcontroller Electronics). Their limits are the range, the speed and the fixed accuracy: a custom analog front end can still be better for special cases.

• Hall-effect current sensors (ACS712, ACS758) give an output of about mid-supply plus a few tens of millivolts per ampere: with a sensitivity of 100.0 mV/A, 10 A gives 2.5 + 0.1 × 10 = 3.5 V.
• Accelerometers and gyros need filtering and calibration of offsets; vibration and mounting dominate accuracy.
• Humidity and gas sensors drift: recalibrate and consider warm-up time.
• Encoders and quadrature sensors produce pulses: use a timer or interrupt and, on long cables, differential line drivers (see Sensors).

Amplifying small signals

Millivolt signals need an amplifier with low offset, low noise and, for bridges and thermocouples, a differential input. An instrumentation amplifier (three op-amps in one package) has a very high input resistance, a high common-mode rejection ratio (CMRR) and a gain set by one resistor: G = 1 + 49.4 kΩ ÷ R_G for the AD620 family.

• Common-mode rejection matters because long sensor leads pick up the same noise on both wires: a good instrumentation amplifier cancels it. Keep the two inputs balanced.
• Chopper and zero-drift amplifiers have microvolt offsets with no drift: ideal for thermocouples and bridges.
• Protect the inputs with series resistors and diodes where cables run outdoors or near motors.
• Amplifier circuits are described in Op-amps and Integrated Circuits.

Filtering, noise and the ADC

• Filter before the ADC. A low-pass with a cutoff at about a third of the sampling rate (or lower) removes noise and prevents aliasing: high-frequency noise folding into the band. A simple RC works for slow signals (see the RC Time Constant and Filter Calculator).
• Average in software. Averaging N independent samples reduces random noise by √N.
• Match the resolution to the need. A 12-bit ADC with 5 mV of noise is effectively a 9–10 bit ADC; more bits will not help unless the noise falls.
• Reference and ground. Use a quiet reference, a separate analog ground return and short wires.
• Avoid ground loops between sensor and board: use differential inputs, isolation or a floating sensor.
• 50/60 Hz hum: an integration time of one or more full mains cycles (20 ms or 16.7 ms) averages it away; many converters offer a 50/60 Hz rejection mode.

Calibration

Method	What it corrects	How
One-point (offset)	A constant error	Measure at one known value (for example an ice bath at 0 °C); subtract the error
Two-point (offset and gain)	Offset and slope	Measure at two known values (0 °C and 100 °C); compute value = gain × reading + offset
Multi-point or polynomial	Non-linearity	Several points; fit a curve or a lookup table
Ratiometric measurement	Supply and reference variations	Use the same reference for the sensor and the ADC
Zero (tare) at power-up	Drift and mounting offsets	Read with no load and store the offset

Calibrate against a reference better than the accuracy you want: an ice and boiling water bath for temperature (check the boiling point against your altitude), certified weights for force, a precision voltage source or multimeter for voltages. Store the constants in non-volatile memory and note the date.

Common faults

Symptom	Likely cause	Try
Temperature reads far too high or low	Wrong resistor value, beta or divider orientation; NTC and fixed resistor swapped	Check the divider wiring and the beta equation with a known resistor in place of the sensor
Reading jumps around	Noise, no filter, a floating input, a poor ground	Add an RC filter, average, check the shield and routing
Reading drifts	Self-heating, supply or reference drift, temperature of the amplifier	Reduce the excitation, use a reference, warm up, use a zero-drift amplifier
Thermocouple reads room temperature or an offset	No cold-junction compensation, the wrong thermocouple type, extension wire not matched	Use a thermocouple amplifier; check the type and the wire
RTD reads high	Lead resistance in a 2-wire connection	Use 3- or 4-wire; subtract the lead resistance
Load cell reads wrong or drifts	Excitation not stable, a mechanical load path error, temperature, a damaged cable	Check the supply, the mounting and the zero; apply known weights
4–20 mA reads zero or off-scale	Broken loop, wrong burden, no loop power, wrong polarity	Break the loop and measure the current with a meter; check the supply
Signal saturates the amplifier	Gain too high, offset not removed, supply too low	Lower the gain, add offset correction, check the rails
Digital sensor not found	Wrong I2C address, missing pull-ups, wrong voltage level	Scan the bus; check pull-ups and the supply
Mains hum on the reading	Ground loop or poor shielding	Differential input, shielded twisted pair, a 50/60 Hz notch or averaging over a mains cycle

Related

• Analog Circuits and Sensing: the overview of this guide.
• Op-amps and Integrated Circuits: amplifier circuits and comparators.
• Microcontroller Electronics: ADC input stages and level shifting.
• Resistors and Capacitors: thermistors, dividers and filters.
• Sensors and Sensor Wiring and Installation: industrial sensors and how to wire them.
• Analog Scaling Tool.
• RC Time Constant and Filter Calculator.

FAQ

Which temperature sensor should I choose?

A digital IC (DS18B20, TMP117) is easiest for −50 to 125 °C. An NTC thermistor is cheap and sensitive over a narrow range. A Pt100 or Pt1000 is accurate and stable. A thermocouple covers very high temperatures and rugged probes. Match the range, accuracy, cost and size.

Do I need an instrumentation amplifier?

For millivolt signals from bridges, thermocouples and long cables, yes (or a dedicated converter chip). For a thermistor in a divider or a 0–10 V signal, a plain divider and the ADC are enough.

Why is my 12-bit ADC only accurate to a few counts?

Noise, reference error and source impedance reduce the usable resolution. Filter, average, use a good reference and a buffer, and calibrate.

How do I read a thermistor without self-heating it?

Use a larger fixed resistor, apply the voltage only while measuring, and use a small excitation current (below the datasheet’s limit for a 0.1 °C error).

What is ratiometric measurement?

The sensor’s output is proportional to its supply, and the ADC uses the same supply as its reference, so the supply variations cancel. Many bridge and potentiometer sensors are designed for it.

References

• IEC 60751, Industrial platinum resistance thermometers and platinum temperature sensors; ASTM E1137.
• NIST, ITS-90 Thermocouple Database (reference functions and tables).
• Vishay and TDK NTC thermistor datasheets (beta values and tolerances); Maxim, Thermocouple and RTD measurement application notes.
• Texas Instruments and Analog Devices application notes on bridge sensors, instrumentation amplifiers and transimpedance amplifiers.
• Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press: chapters on op-amps and precision circuits.

Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use.