Integrated signal conditioning—a key factor in high-precision temperature measurement

When measuring real-world physical parameters such as temperature, pressure, flow, and attitude, data must often be extracted from many transducers with widely varying signal characteristics. The signals generated by these transducers may include high voltage, low voltage, current, frequency, or pulse data, and each signal presents a unique set of challenges for engineers to measure. Among them, temperature is by far the most commonly measured parameter, and thermocouples are the primary measurement devices in temperature measurement applications.

The physical characteristics of thermocouples present many unique challenges to the user. First, the low-level signal must be amplified, then the high-frequency components and noise must be filtered out, and at the same time, care must be taken to reduce interference from adjacent channels. Obviously, signal conditioning is a critical step to ensure accurate and repeatable thermocouple measurements.

Thermocouple transducers generate a low level voltage when two dissimilar metals come into contact with each other. This voltage is usually called the temperature difference electromotive force, and the signal strength is in the millivolt level. For example, a K-type thermocouple with a full-scale operating range of about 60mV will generate a voltage of 39uV at 1°C. The input voltage range of a typical analog-to-digital converter (ADC) is generally ±10V, so in order to obtain the best voltage resolution, the signal level must be amplified.

Figure 1: High-precision thermocouples and high-speed voltage measurements.

The purpose of the amplifier stage is to ensure that the signal is amplified to the point where even very subtle temperature changes can be resolved. With a gain of 100, a K-type thermocouple measuring 48.838 mV at 1200°C will be amplified to 4.8838 V. Without this necessary amplification step, the measurement result would have a much lower resolution and would be more susceptible to noise fluctuations.

Analog filtering

The millivolt level signal output by the thermocouple is also very susceptible to 60Hz interference, so the instrument must provide good bandwidth limitation to resist this interference. This is especially important in industrial environments, because in industrial environments, thermocouples are exposed to severe electronic interference generated by interference sources such as engines, generators, welding equipment, lighting equipment, etc.

Many thermocouple measurement devices , such as DMM -based systems, offer some degree of programmable 60Hz filtering capability, but this bandwidth limitation is achieved by setting the integration speed of the ADC. By integrating over an integer number of power line cycles ( PLC ), the performance of the 60Hz filter can be improved and the effect of 60Hz noise can be reduced, but this will severely reduce the channel sampling rate. And because the 60Hz filter setting is a global setting, even if only one channel requires 60Hz filtering, all channels in the system can only sample at a reduced rate.

Those PC-based relay multiplexers that are apparently designed to reduce cost usually do not provide any analog filtering, but rely on averaging or other software techniques to process the data. This processing method becomes problematic when very accurate and clean data is required within the measurement spectrum, and additional external filtering circuits must be added to improve signal integrity.

Rather than relying on the ADC or software oversampling and averaging techniques, truly leading instrument designers provide bandwidth limiting in the signal conditioning path of each channel so that the cutoff frequency of each channel can be set independently.

A flexible approach, such as that used in the EX1048 from VXI Technologies, allows different cutoff frequency ranges to be set for different channels, choosing between a 4 Hz bandwidth and a 1 kHz bandwidth. The 4 Hz bandwidth is suitable for most thermocouple and low voltage measurements, and it performs best with 60 Hz filtering; the 1 kHz bandwidth is suitable for high-precision thermocouple and high-speed voltage measurements (Figure 1).

Cold Junction Compensation

Filtering, noise reduction and signal amplification are only part of the equation when it comes to high-precision temperature measurement. It turns out that the cold junction compensation (CJC) circuit is the heart of high-precision thermocouples. Even an insulated module with a large thermal mass will slowly change temperature in the same direction as the surrounding environment, so if these effects are underestimated or not properly handled, measurement errors are inevitable.

PC card multiplexers and DMM-based systems typically measure with an accuracy of about 1.0° to 1.5°. This accuracy range expresses uncertainty due to a variety of causes, including low thermal mass of the insulation block, incorrect or insufficient CJC sensor placement, poor positioning of the terminal block relative to adjacent heat sources (such as power supplies), and display problems. In addition, the large measurement errors in most instruments are attributed to poor CJC sensor circuit design and poor thermal coupling mechanisms at the CJC input.

Precision temperature measurement instruments such as the EX1048 typically combine multiple high-precision CJC mechanisms, have large thermal mass, carefully placed components that produce internal temperature gradients, and have self-calibration capabilities. CJC sensors are usually high-precision thermistors, which are often placed in strategic locations on the insulation module. When the number of channels in the system is large, the number of insulation modules with thermistors will also increase to eliminate the errors caused by temperature measurements between different connection points (Figure 2). After paying attention to these details, the system-level measurement accuracy of the instrument may reach 0.2°C to 0.4°C.

Figure 2: When the number of channels in the system is large, the number of thermal isolation modules with thermistors will also increase to eliminate the error caused by temperature measurement between different connection points.

Signal multiplexing

When the signal is well filtered and amplified, and the resulting CJC signal is accurate, the ADC can still have a serious impact on the accuracy of the measurement from a signal conditioning perspective. Due to the relatively slow sampling requirements, most temperature measurement instruments do not use a separate ADC on each channel, but rather use a multiplexer configuration to share the same ADC for multiple channels, with typical channel configurations of 16, 32, 48, and 64 channels. Therefore, it is necessary to add high-speed solid-state multiplexer circuits to the instrument.

The nature of thermocouple signals, which are only millivolts in size, can cause system-level problems when hardware is poorly designed. If a high level or overload condition is generated on a channel adjacent to the thermocouple channel, an error will be generated when measuring the thermocouple channel. This can occur due to parasitic capacitance and charge on the line, which may not be known to the user. If the hardware design cannot handle this typical problem, the user is required to stay on a channel for a longer period of time, oversample and then average to get the measurement result.

High-quality thermocouple measurement instruments do not need to rely on oversampling and software averaging to obtain a measurement result with margin. The design of the EX1048 (Figure 3*) uses independent filtering and amplification for each channel to isolate the operation of the channel from the channel. In this way, the signal sent to the ADC and the signal from the multiplexer will not interfere with each other. This type of design ensures that the data converted by the ADC is valid for each channel regardless of whether the adjacent channel may have overvoltage or overload conditions.

Conclusion

Thermocouple measurements are so common that many instrument vendors often overlook the importance of basic signal conditioning. As a result, the end user is left with the burden of providing external signal conditioning and cold junction compensation equipment, often resulting in measurement systems that are too costly and complex, and that also impact long-term maintenance and system calibration.

Internal signal conditioning is a key issue in the design of precision thermocouple instruments such as the EX1048, which is closely related to features such as open thermocouple monitoring and self-calibration capabilities. A thermocouple measurement instrument with such comprehensive features can simplify setup and commissioning operations, avoid problems caused by external interconnect cables, and provide full field calibration on demand. There are many solutions to thermocouple measurement applications, but using an integrated instrument to complete the measurement is the way to reduce costs and simplify implementation.