The circuit in Figure 1 functionally provides a high-precision, multi-channel thermocouple measurement solution. Accurate thermocouple measurements require precision components to form a signal chain that can amplify weak thermocouple voltages, reduce noise, correct for nonlinearity, and provide accurate reference junction compensation (often called cold junction compensation). This circuit solves all these challenges of thermocouple temperature measurement with an accuracy of ±0.25°C or better.

The circuit in Figure 1 shows connecting three Type K thermocouples to an AD7793 precision 24-bit Σ-Δ analog-to-digital converter (ADC) to measure the thermocouple voltage. Because a thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to obtain an accurate absolute temperature reading. This process is called reference junction compensation, often called cold junction compensation. The ADT7320 precision 16-bit digital temperature sensor in this circuit is used for cold junction reference measurements and provides the required accuracy.

This type of application is very popular for applications that require cost-effective and accurate temperature measurement over the wide temperature range provided by thermocouples.

The circuit in Figure 1 is designed for simultaneous measurement of three K-type thermocouples using the ADT7320 , a ±0.25°C accurate, 16-bit digital SPI temperature sensor.

Thermocouple voltage measurement

A thermocouple connector and filter are used as the interface between the thermocouple and the AD7793 ADC. Each connector (J1, J2, and J3) connects directly to a set of differential ADC inputs. Filters at the inputs of the AD7793 reduce any noise superimposed on the thermocouple pins before the signal reaches the AIN(+) and AIN(−) inputs of the ADC. The AD7793 integrates on-chip multiplexers, buffers, and instrumentation amplifiers to amplify small voltage signals from the thermocouple measurement junction.

Cold junction measurement

The ADT7320 precision 16-bit digital temperature sensor measures reference junction (cold junction) temperature with an accuracy of ±0.25°C over the −20°C to +105°C temperature range. The ADT7320 is fully factory calibrated and does not require user calibration. It has a built-in bandgap temperature reference source, a temperature sensor and a 16-bit Σ-Δ ADC to measure temperature and perform digital conversion with a resolution of 0.0078°C.

Both the AD7793 and ADT7320 are controlled by the SPI interface using the system demonstration platform ( EVAL-SDP-CB1Z ). Additionally, both devices can also be controlled by a microcontroller.

Figure 2 shows the EVAL-CN0172-SDPZ circuit evaluation board with three K-type thermocouple connectors , the AD7793 ADC, and the ADT7320 temperature sensor mounted between two copper contacts on a separate flexible printed circuit board (PCB), using Measured at reference temperature.

Figure 3 is a side view of the ADT7320 mounted on a separate flex PCB, with the device inserted between the two copper contacts of the thermocouple connector. The flexible PCB in Figure 3 is thinner and more flexible, which has advantages over small FR4 type PCBs. It allows the ADT7320 to be strategically mounted between the copper contacts of the thermocouple connector to minimize the temperature gradient between the reference junction and the ADT7320.

The small, thin flexible PCB also allows the ADT7320 to respond quickly to temperature changes at the reference junction. Figure 4 shows the typical thermal response time of the ADT7320.

This solution is flexible and allows the use of other types of thermocouples, such as Type J or Type T. In this circuit note, type K was chosen due to its greater popularity. The actual thermocouple selected has an exposed tip. The measuring junction is located outside the probe wall and is exposed to the target medium.

The advantages of using an exposed tip are that it provides the best thermal conductivity, has the fastest response time, and is low cost and lightweight. The disadvantage is that it is susceptible to mechanical damage and corrosion. Therefore, it is not suitable for use in harsh environments. But where fast response times are required, exposed tips are the best option. If exposed tips are used in an industrial environment, the signal chain may need to be electrically isolated. A digital isolator can be used for this purpose (see www.analog.com/icoupler ).

Unlike traditional thermistors or resistance temperature detectors (RTDs), the ADT7320 is a completely plug-and-play solution that does not require multi-point calibration after board assembly and does not require calibration coefficients or linearization. The program consumes processor or memory resources. It consumes only 700μW typical power when operating from a 3.3 V supply, avoiding self-heating issues that can reduce the accuracy of traditional resistive sensor solutions.

Precision Temperature Measurement Guide

The following guidelines ensure that the ADT7320 accurately measures the reference junction temperature.

Power Supply : If the ADT7320 is powered from a switching power supply, noise above 50 kHz may be generated, affecting temperature accuracy. To prevent this defect, an RC filter should be used between the supply and V DD . Component values ​​used should be carefully considered to ensure peak power supply noise is less than 1 mV

Decoupling : The ADT7320 must have decoupling capacitors installed as close as possible to V DD to ensure accurate temperature measurements. It is recommended to use a decoupling capacitor such as a 0.1μF high-frequency ceramic type. In addition, a low-frequency decoupling capacitor, such as a 10 μF to 50 μF tantalum capacitor, should be used in parallel with the high-frequency ceramic capacitor.

Maximum Thermal Conduction : The plastic package and the exposed pad (GND) on the back side are the primary thermal conduction paths from the reference junction to the ADT7320. Since the copper contacts are connected to the ADC inputs, the pads on the back cannot be connected in this application as doing so would affect the bias of the ADC inputs.

Precision Voltage Measurement Guide

The following guidelines ensure that the AD7793 accurately measures thermocouple measurement junction voltages.

Decoupling : The AD7793 must have decoupling capacitors installed as close as possible to AV DD and DV DD to ensure voltage measurement accuracy. Decouple AVDD to GND with a 0.1 μF ceramic capacitor in parallel with a 10 μF tantalum capacitor. Additionally, DVDD should be decoupled to GND with a 0.1 μF ceramic capacitor in parallel with a 10 μF tantalum capacitor. For more discussion of grounding, layout, and decoupling techniques, please refer to Tutorial MT-031 and Tutorial MT-101

Filtering : The differential inputs of the AD7793 are used to eliminate most of the common-mode noise on the thermocouple lines. For example, placing R1, R2, and C3, which form a differential low-pass filter, on the front end of the AD7793 can eliminate possible superimposed noise on the thermocouple pins. Capacitors C1 and C2 provide additional common-mode filtering. Since the AIN(+) and AIN(−) inputs to the ADC are analog differential inputs, most of the voltages in the analog modulator are common-mode voltages. The AD7793's excellent common-mode rejection (100dB minimum) further eliminates common-mode noise from these input signals.

Other problems solved by this solution

Below is a summary of how this solution solves the other thermocouple-related challenges mentioned earlier.

Thermocouple voltage amplification : The thermocouple output voltage changes with temperature only by a few μV per degree. The common K-type thermocouple used in this example has a variation of 41μV/°C. This weak signal requires a higher gain stage before conversion by the ADC. The AD7793 internal programmable gain amplifier (PGA) can provide a maximum gain of 128. The gain of 16 in this solution allows the AD7793 to run the internal full-scale calibration function from the internal voltage reference.

Nonlinear Correction of Thermocouples : The AD7793 has excellent linearity over a wide temperature range (–40°C to +105°C) and requires no user correction or calibration. In order to determine the actual thermocouple temperature, the reference temperature measurement must be converted to an equivalent thermoelectric voltage using the formula provided by the National Institute of Standards and Technology (NIST). This voltage is added to the thermocouple voltage measured by the AD7793, and the sum of the two is converted back to the thermocouple temperature again using the NIST formula. Another approach involves the use of lookup tables. However, to achieve the same accuracy, the size of the lookup table can be significantly different, requiring the host controller to allocate additional storage resources to it. All processing is done in software via EVAL-SDP-CB1Z. EVAL-SDP-CB1Z is done in software.

To view the complete schematic and layout of the EVAL-CN0172-SDPZ , see the CN-0172 Design Support Package: www.analog.com/CN0172-DesignSupport .