The signals from temperature sensors are often not directly suitable for digitization and processing. Thermocouple outputs are in the millivolt range with non-linear characteristics. RTD signals require precise resistance measurement with lead wire compensation. Converting these raw sensor signals into accurate digital temperature readings requires careful signal conditioning and data acquisition (DAQ) system design.
This guide covers the key design considerations for temperature sensor signal conditioning and DAQ systems.
• Amplification：Millivolt-level signals require gains of 100-1000×
• CJC：Cold junction compensation at the input terminals
• Linearization：Non-linear voltage-temperature relationship requires digital correction
• Open-circuit detection：Detect broken thermocouple wires
• Input protection：Overvoltage and reverse-polarity protection
• Excitation current：Precise constant current source (typically 0.1-1mA)
• Lead wire compensation：3-wire or 4-wire measurement technique
• Self-heating management：Minimize excitation current to reduce self-heating error
• Resistance-to-voltage conversion：Wheatstone bridge or current-source method
The front-end amplifier for temperature sensor signals must provide: high input impedance (especially for RTDs); high common-mode rejection (CMRR > 80dB); low offset voltage and drift; programmable gain for different sensor types.
Temperature signals are inherently slow-moving, so aggressive low-pass filtering can remove most noise without affecting signal integrity.
• RC filter：Simple first-order filter; corner frequency typically 1-10 Hz
• Active filter：Higher-order response for sharper rolloff
• Digital filter：Implemented in firmware after ADC; most flexible
For the lowest offset and drift specifications, chopper-stabilized (auto-zero) amplifiers continuously correct their own offset errors. Essential for high-precision RTD measurements.
Parameter | Thermocouple | RTD |
Resolution | 16-24 bits | 16-24 bits |
Sample Rate | 1-100 SPS | 1-10 SPS |
Input Range | ±50mV to ±100mV | 0-400Ω |
Noise-Free Counts | >20 bits | >18 bits |
The preferred ADC architecture for temperature measurement. Provides high resolution (up to 24 bits) at low sample rates with excellent noise rejection. Built-in programmable gain amplifiers and reference inputs simplify circuit design.
For multi-channel systems, analog multiplexers switch between sensor inputs before the ADC. Key considerations: channel-to-channel crosstalk; on-resistance and variation; charge injection from switching; settling time after channel change.
Thermocouple voltage-temperature relationships are defined by polynomials in IEC 60584. The DAQ system must implement these polynomials (or equivalent look-up tables) to convert the measured voltage to temperature.
• Moving average：Simple and effective for steady-state signals
• FIR filter：Linear phase response, no distortion
• IIR filter：More efficient than FIR for same performance
• Median filter：Effective at removing impulse noise (spikes)
Apply sensor-specific calibration coefficients stored in non-volatile memory. Correction may include: offset correction (additive); gain correction (multiplicative); higher-order polynomial correction for non-linearity.
• Use differential measurement (not single-ended)
• Maintain high CMRR in the instrumentation amplifier
• Guard-driven input cables for RTD measurements
• Star grounding at the ADC input
• Low-pass filtering (analog and/or digital)
• Integration-type ADC (rejects specific frequencies)
• Synchronous detection (lock-in amplifier technique)
• Proper cable shielding and routing
Ground loops occur when sensor and instrument are at different ground potentials, causing current to flow through the sensor wiring.
• Use isolated inputs (optical or transformer isolation)
• Single-point grounding
• Isolated DC-DC converters for sensor excitation
Best for one or two high-accuracy measurement points. Dedicated ADC per channel, no multiplexing artifacts.
Cost-effective for many channels. One ADC shared across channels via multiplexer. Watch for channel-to-channel crosstalk and settling time.
Smart sensor nodes with local digitization, communicating via digital bus (Modbus, SPI, I2C). Eliminates analog signal transmission issues.
BANBEKE offers products that simplify signal conditioning and DAQ system design:
• Temperature transmitter modules：Complete signal conditioning in a DIN-rail package; thermocouple and RTD input options; 4-20mA, HART, and Modbus outputs
• USB DAQ modules：Multi-channel temperature input for lab and test applications; plug-and-play with included software
• Industrial I/O modules：Rack-mount DAQ systems for permanent installations; redundant power and communication
• Custom ASIC solutions：For high-volume OEM applications with specific requirements
✉ BANBEKE signal conditioning products — from raw sensor signal to actionable data, made simple.