Every electronics product measures temperature somewhere — thermal foldback, battery protection, ambient compensation, process control. And every temperature measurement technology makes a different trade between range, accuracy, cost, and conditioning complexity. Here is the map.
The Four Technologies at a Glance
| Sensor | Range | Accuracy Potential | Signal Conditioning | Best For |
|---|---|---|---|---|
| Thermocouple (K, J, T...) | −270 to +1,800 °C | ±0.5–2.5 °C | µV signals, cold-junction compensation, amplifier | Extreme temperatures, industrial processes |
| RTD (PT100/PT1000) | −200 to +850 °C | ±0.1 °C achievable | Excitation current, 3/4-wire Kelvin, ADC | Precision industrial measurement |
| Thermistor (NTC) | −55 to +150 °C | ±0.1–1 °C (locally) | Divider + ADC, Steinhart-Hart linearization | Cost-sensitive narrow-range sensing |
| Digital IC sensor | −55 to +125 °C | ±0.5–2 °C | None — I2C/SMBus digital out | Board-level ambient and die temperature |
Thermocouples: µV Signals and the Cold Junction Problem
A thermocouple generates a microvolt-per-degree EMF at the junction of two dissimilar metals — but the measured voltage includes the cold junction where the thermocouple wires meet copper. Every accurate thermocouple system therefore measures its own terminal temperature (with an IC sensor) and adds the correction — cold junction compensation. Design realities:
- Signals are single-digit millivolts at best: a low-noise, high-gain amplifier with input filtering is mandatory, and thermocouple extension wire is not optional — copper junctions reintroduce unknown EMFs.
- Type K's ~41 µV/°C sensitivity means 10 µV of amplifier offset is a 0.25 °C error: chopper-stabilized amplifiers earn their keep here.
- Integrated thermocouple front ends (with built-in CJC, filtering, and linearization) have collapsed the design effort — check their accuracy specs against discrete chains.
RTDs: Precision Through Ratiometric Measurement
A PT100 is a resistor with a well-defined temperature curve — measure resistance accurately and temperature follows:
- Excite with a current, measure the voltage — and make the measurement ratiometric: use the same current for the RTD and the ADC reference so excitation drift cancels. This single technique removes the largest error source.
- Wire resistance is the enemy: a PT100 is 100 Ω; copper lead resistance adds directly. 2-wire connections are for throwaway accuracy; 3-wire cancels lead resistance if leads match; 4-wire (Kelvin) eliminates it entirely and is the standard for precision work.
- Self-heating: excitation current heats the sensor (I²R). 1 mA through a PT100 dissipates 100 µW — small, but in still air it shifts readings. Balance excitation current against resolution and duty-cycle the measurement when possible.
Thermistors and Digital ICs: The Practical Defaults
- NTC thermistors deliver big signals (kΩ changes over their range) at cents each — but strongly nonlinear. A voltage divider plus Steinhart-Hart (or lookup-table) conversion handles it; accuracy is excellent over a narrow window and degrades at the extremes.
- Digital IC sensors integrate the sensing element, converter, and interface: wire it to I2C and read degrees. Accuracy of ±0.5 °C (best-in-class) covers board-level thermal monitoring, and features like remote diode sensing measure other chips' junction temperatures through their test transistors — the standard technique for CPU/FPGA thermal management.
- Their limits: package self-heating, thermal gradient between the die and what you actually want to measure, and a range capped near the silicon's comfort zone.
Selection Logic
- Define range and required accuracy independently — the pair usually forces the technology choice by elimination.
- Extreme range → thermocouple; precision over moderate range → RTD; narrow range, low cost → thermistor; board ambient → digital IC.
- For analog chains, use ratiometric measurement and Kelvin connections wherever excitation is involved.
- Calibrate at known points when the budget demands better than the sensor class guarantees.
JTDZ Tech supplies thermocouple and RTD front-end ICs, digital temperature sensors with remote-diode inputs, precision amplifiers, and references from the major manufacturers. Tell us your range and accuracy target and we will quote the sensing chain that closes the budget.
