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3-channel thermocouple temperature measurement system, the accuracy of 0.25 ℃ circuit
Dec 24, 2017

3-channel thermocouple temperature measurement system, the accuracy of 0.25 ℃ circuit

Circuit functions and advantages

The circuit in Figure 1 provides a highly accurate, multichannel thermocouple measurement solution. Accurate thermocouple measurements require the use of precision components to form a signal chain that amplifies weak thermocouple voltages, reduces noise, corrects non-linearities, and provides accurate reference junction compensation (commonly referred to as cold junction compensation). This circuit solves all of these problems in thermocouple temperature measurement and has an accuracy of ± 0.25 ° C.

The circuit in Figure 1 shows the connection of three K-type thermocouples to the AD7793 precision 24-bit sigma-delta analog-to-digital converter (ADC) to measure the thermocouple voltage. Since a thermocouple is a differential device rather than an absolute temperature measurement device, you must know the reference junction temperature to obtain accurate absolute temperature readings. This process is known as reference junction compensation and is commonly referred to as cold junction compensation. In this circuit, the ADT7320 precision 16-bit digital temperature sensor is used for cold junction reference measurement and provides the required accuracy.

This type of application is very popular for accurate temperature measurements that need to be cost-effective over a wide temperature range provided by thermocouples.

The circuit in Figure 1 shows the connection of three K-type thermocouples to the AD7793 precision 24-bit sigma-delta analog-to-digital converter (ADC) to measure the thermocouple voltage. Since a thermocouple is a differential device rather than an absolute temperature measurement device, you must know the reference junction temperature to obtain accurate absolute temperature readings. This process is known as reference junction compensation and is commonly referred to as cold junction compensation. In this circuit, the ADT7320 precision 16-bit digital temperature sensor is used for cold junction reference measurement and provides the required accuracy.

This type of application is very popular for accurate temperature measurements that need to be cost-effective over a wide temperature range provided by thermocouples.

Figure 1. Multichannel thermocouple measurement system (schematic diagram: not shown for all connections and decoupling)

Circuit description

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

Thermocouple voltage measurement

The thermocouple connector and filter are used as the interface between the thermocouple and the AD7793 ADC. Each connector (J1, J2, and J3) is directly connected to a set of differential ADC inputs. The filter on the AD7793 input reduces the noise superimposed on any thermocouple pin before the signal reaches the ADC's AIN (+) and AIN (-) inputs. The AD7793 integrates an on-chip multiplexer, a buffer, and an instrumentation amplifier to amplify small-voltage signals from thermocouple junctions.

Cold junction measurement

The ADT7320 precision 16-bit digital temperature sensor measures the reference junction temperature (cold junction) with an accuracy of ± 0.25 ° C over the -20 ° C to + 105 ° C temperature range. The ADT7320 is fully factory calibrated and requires no user calibration. It incorporates a bandgap temperature reference, a temperature sensor and a 16-bit sigma-delta ADC for temperature measurement and 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). In addition, these two devices can also be controlled by the microcontroller.

Figure 2. EVAL-CN0172-SDPZ Circuit Evaluation Board

Figure 2 shows an EVAL-CN0172-SDPZ circuit evaluation board with three K-type thermocouple connectors. The AD7793 ADC, and the ADT7320 temperature sensor are mounted between two copper contacts of an individual flexible printed circuit board (PCB) At the reference temperature.

Figure 3 is a side view of the ADT7320 mounted on a separate flexible PCB, inserted between the two copper contacts of the thermocouple connector. The flexible PCB in Figure 3 is thinner and more flexible than the smaller FR4 PCBs. It allows the ADT7320 to be cleverly mounted between the copper contacts of the thermocouple connector to minimize the temperature gradient between the reference junction and the ADT7320.

Figure 3. Side view of the ADT7320 mounted on a flexible PCB

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

Figure 4. ADT7320 Typical Thermal Response Time

The solution is more flexible, allowing the use of other types of thermocouples, such as the J-type or T-type. In this circuit note, the choice of K-type is taken into account its more popular. The actual thermocouple selected has a bare tip. The measurement junction is located outside of the probe wall and is exposed to the target medium.

The advantage of using a bare tip is that it provides the best thermal conductivity with the fastest response time, low cost and light weight. The downside is susceptibility to mechanical damage and corrosion. Therefore, not suitable for harsh environments. However, when the need for fast response time of the occasion, the exposed tip is the best choice. If an exposed tip is used in an industrial environment, the signal chain may need to be electrically isolated. Digital isolators can be used for this purpose (see www.analog.com/icoupler).

Unlike conventional thermistors or resistive temperature detectors (RTDs), the ADT7320 is a fully plug-and-play solution that eliminates the need for multi-point calibration after board assembly and no calibration or linearization Program that consumes processor or memory resources. Its typical power consumption of only 700μW when operated at 3.3 V supplies avoids the self-heating problem that degrades the accuracy of traditional resistive sensor solutions.

Precise 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 switched-mode power supply, more than 50 kHz of noise may be produced, affecting temperature accuracy. To prevent this defect, use an RC filter between the power supply and VDD. The component values used should be carefully considered to ensure that the supply noise peaks are less than 1 mV

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

Maximum Heat Conduction: The plastic package and the exposed pad on the backside (GND) are the primary thermal conduction path from the reference junction to the ADT7320. Since the copper contacts are connected to the ADC inputs, the pads on the back side can not be connected in this application because doing so can affect the ADC input bias.

Precise Voltage Measurement Guide

The following guidelines ensure that the AD7793 accurately measures the junction voltage measured by the thermocouple.

Decoupling: The AD7793 must have a decoupling capacitor placed as close to AVDD and DVDD as possible to ensure accurate voltage measurements. The AVDD should be decoupled to GND by connecting a 0.1 μF ceramic capacitor in parallel with a 10 μF tantalum capacitor. In addition, a 0.1 μF ceramic capacitor should be connected in parallel with a 10 μF tantalum capacitor to decouple the DVDD to GND. For more discussion of grounding, layout, and decoupling techniques, refer to Tutorial MT-031 and Tutorial MT-101

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

The program to solve other problems

The following summarizes how this solution addresses the other thermocouple-related challenges mentioned earlier.

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

Thermocouple Nonlinearity Correction: The AD7793 offers excellent linearity over a wide temperature range (-40 ° C to + 105 ° C) without user calibration or calibration. In order to determine the actual thermocouple temperature, you must use the formula provided by the National Institute of Standards and Technology (NIST) to convert the reference temperature measurement to an equivalent thermoelectric voltage. This voltage is added to the thermocouple voltage measured by the AD7793 and then again converted back to the thermocouple temperature using the NIST formula. Another approach involves the use of lookup tables. However, to obtain the same precision, the size of the lookup table may vary considerably, requiring the host controller to allocate additional storage resources for it. All processing is done in software via EVAL-SDP-CB1Z. EVAL-SDP-CB1Z is done in software.

Common changes

For applications requiring low accuracy, the AD7792 16-bit Σ-Δ ADC can be used instead of the AD7793 24-bit Σ-Δ ADC. For reference temperature measurements, an ADT7310 digital temperature sensor with ± 0.5 ° C accuracy can be used instead of the ± 0.25 ° C precision ADT7320 Both the AD7792 and ADT7310 integrate the SPI interface


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