Signal Conditioning, Interference and Isolation for Thermocouples and RTDs

Contents

1. Signal Conditioning, Interference and Isolation
2. Series Mode Interference
3. Common Mode Interference
4. Isolation
5. Summary

Signal Conditioning, Interference and Isolation

Signal conditioning is critical in electronic temperature measurement, ensuring that the signal from a thermocouple or RTD is accurate, reliable, and robust enough for instrumentation to interpret. Since temperature sensors produce small signals—often just millivolts—these signals must be amplified, filtered, converted, and protected against interference before they’re usable.

Signal conditioning typically includes several key functions:

• Isolation
• Interference rejection
• Signal conversion (e.g. voltage-to-current, analogue-to-digital)
• Scaling and amplification
• (Optionally) Linearisation and transmission

In most temperature measurement systems, these functions are integrated into the transmitter, which acts as the signal’s interface to the broader control or monitoring system.

Modern instrumentation has made major strides in resolving issues that once made signal conditioning complex. Advanced systems now isolate and filter signals effectively, even in noisy industrial environments, allowing temperature data to be measured with excellent accuracy and repeatability.

Interference, which can distort or corrupt small sensor signals, is one of the main challenges signal conditioning must address. It generally falls into two categories: series mode and common mode interference. These are caused by external AC or DC signals—usually from mains electricity or electromagnetic coupling—and by differences in ground potential between the sensor and the measuring equipment.

Series Mode Interference

Series mode interference occurs when signal lines pick up unequal voltages from surrounding magnetic fields. In the UK, the 50Hz mains frequency is the most common source. Since thermocouple and RTD signals are often just millivolts, a nearby 240V AC line can easily introduce unwanted noise.

If this interference is DC, there is little that can be done, since the transmitter is designed to measure DC voltages. However, AC interference can be reduced or eliminated using several techniques:

• Low-pass filters can remove higher-frequency components, though they may be overkill for slow-changing temperature signals.
• Signal averaging is a more practical and cost-effective method. Because temperature changes slowly, averaging the signal over one or more interference cycles cancels out noise.
• Phase-locked sampling synchronises the analogue-to-digital conversion to the interference frequency, further reducing error.

Common Mode Interference

Common mode interference arises from uniform voltage signals that affect both signal leads equally. It is often caused by electromagnetic fields, static coupling, or differences in ground potential.

The most effective method of rejecting common mode noise is to use a differential input with high impedance. This allows the measuring circuit to "float," effectively ignoring the induced voltages.

Additional strategies include:

• Isolation amplifiers, which electrically separate the sensor from the measuring circuit.
• Guard systems, which work by referencing the guard to the same potential as the common mode voltage. This prevents current from flowing through the signal leads, diverting it safely to earth via the guard circuit.

These techniques significantly improve both series and common mode interference rejection.

Isolation

Good isolation practices are essential for maintaining signal integrity. Interference - whether series or common mode - can easily enter a system if isolation is inadequate.

Most modern transmitters offer galvanic isolation, typically rated at 500V or more. Some provide three-way isolation between input, output, and power supply. This is achieved using transformer-coupled or opto-coupled systems, with signal transfer methods including mark-space ratio, frequency modulation, or digital encoding.

High-quality systems can offer common mode rejection ratios (CMRR) up to 150dB at 50Hz, ensuring extremely effective suppression of unwanted signals and reliable temperature data in even the harshest environments.

Summary

Accurate temperature measurement with thermocouples and RTDs relies on effective signal conditioning - a set of techniques used to amplify, filter, isolate, and convert low-level sensor signals. It also includes managing interference and maintaining signal integrity from sensor to instrument.

Interference falls into two categories:

• Series mode (e.g., 50Hz mains noise) arises from uneven electromagnetic coupling between signal wires and can often be reduced using averaging, low-pass filtering, or phase-locked sampling.
• Common mode stems from static or electromagnetic fields and differences in ground potential. It's countered using differential inputs, isolation amplifiers, and guard circuits.

Isolation is critical to protect the measurement system and maintain accuracy. Modern instruments offer robust galvanic isolation (typically 500V or more) using transformer or opto-coupling, with high common mode rejection - ensuring stable performance even in electrically noisy environments.