Complete Thermocouple Measurement System Based on AD7793

The circuit shown in Figure 1 is a complete thermocouple system based on the AD7793 24-bit Σ-Δ ADC. The AD7793 is a low power, low noise, complete analog front end for high precision measurement applications. It has an internal PGA, reference, clock, and excitation current, which greatly simplifies the thermocouple system design. The peak-to-peak noise of the system is approximately 0.02°C.

 

The AD7793 consumes only 500 μA maximum, making it suitable for low power applications such as smart transmitters where the entire transmitter must consume less than 4 mA. The AD7793 also has a power-down option. In this mode, the entire ADC and its auxiliary functions are shut down, reducing the maximum power consumption of the device to 1 μA.

 

The AD7793 provides an integrated thermocouple solution that can interface directly to thermocouples. Cold junction compensation is provided by a thermistor and a precision resistor. The circuit requires only these external components to perform the cold junction measurement, along with some simple RC filters to meet electromagnetic compatibility (EMC) requirements.

 

Figure 1. Thermocouple Measurement System with Cold Junction Compensation (Simplified Schematic: Decoupling and All Connections Not Shown)

 

Circuit Description

This circuit uses a type T thermocouple. This thermocouple is constructed of copper and constantan and has a temperature measurement range of −200°C to +400°C and produces a temperature-dependent voltage of typically 40 μV/°C.

 

The transfer function of a thermocouple is not linear. Over the temperature range of 0°C to +60°C, its response is very close to linear. However, over a wider temperature range, a linearization procedure must be used.

 

The test circuit does not include linearization, so the useful measurement range for this circuit is 0°C to +60°C. Over this temperature range, the thermocouple produces a voltage between 0 mV and 2.4 mV. The internal 1.17 V reference is used for the thermocouple conversion. Therefore, the AD7793 is configured for a gain of 128.

 

The AD7793 operates from a single supply, and the signal generated by the thermocouple must be biased above ground to be within the range supported by the ADC. For a gain of 128, the absolute voltage at the analog input must be within the range of GND + 300 mV to AVDD – 1.1 V.

 

The AD7793's on-chip bias voltage generator biases the thermocouple signal so that its common-mode voltage is AVDD/2, ensuring that the input voltage limit requirements are met with considerable margin.

 

The thermistor has a value of 1 kΩ at +25°C, a typical value of 815 Ω at 0°C, and a typical value of 1040 Ω at +30°C. Assuming a linear transfer function from 0°C to 30°C, the relationship between the cold junction temperature and thermistor resistance, R, is:

 

Cold Junction Temperature = 30 × (R – 815)/(1040 – 815)

 

The 1 mA excitation current from the AD7793 is used to power the thermistor and the 2 kΩ precision resistor. The reference voltage is generated using this external 2 kΩ precision resistor. This architecture provides a ratiometric configuration where the excitation current is used to power the thermistor and generate the reference voltage. Therefore, deviations in the excitation current value do not change the accuracy of the system.

 

When sampling the thermistor channel, the AD7793 operates at a gain of 1. For a maximum cold junction temperature of +30°C, the maximum voltage developed across the thermistor is 1 mA × 1040 Ω = 1.04 V.

 

The thermistor is selected so that the maximum voltage generated across the thermistor multiplied by the PGA gain is less than or equal to the voltage generated across the precision resistor.

For the conversion value of ADC_CODE, the corresponding thermistor value R is equal to:

 

R = (ADC_CODE – 0x800000) × 2000/223

 

Another consideration is the output compliance voltage of the AD7793 IOUT1 pin. With a 1 mA excitation current, the output compliance voltage is equal to AVDD – 1.1 V. From the above calculations, the circuit meets this requirement because the maximum voltage at IOUT1 is equal to the voltage across the precision resistor plus the voltage across the thermistor, which equals 2 V + 1.04 V = 3.04 V.

 

The AD7793 operates at an output data rate of 16.7 Hz. For every 10 thermocouple conversions read, one thermistor conversion is read. The corresponding temperature is equal to:

 

Temperature = Thermocouple Temperature + Cold Junction Temperature

 

The conversion result of AD7793 is processed by analog microcontroller ADuC832, and the obtained temperature is displayed on LCD display.

 

This thermocouple design is powered by a 6 V (2 3 V lithium cells) battery. A diode drops the 6 V voltage to a level suitable for the AD7793 and the analog microcontroller ADuC832. There is an RC filter between the ADuC832 power supply and the AD7793 power supply to reduce the power supply digital noise entering the AD7793.

 

Figure 2 shows the voltage generated across a T-type thermocouple versus temperature. The area within the circle is from 0°C to +60°C, and the transfer function in this area is nearly linear.

 

Figure 2. Relationship between thermocouple electromotive force and temperature

 

When the system is at room temperature, the thermistor should indicate the value of room temperature. The thermistor indicates the relative temperature relative to the cold junction temperature, that is, the temperature difference between the cold junction (thermistor) and the thermocouple. Therefore, at room temperature, the thermocouple should indicate 0°C.

 

If you place the thermocouple in an ice bucket, the thermistor will still measure the ambient (cold junction) temperature. The thermocouple should indicate the negative of the thermistor value, making the total temperature equal to 0.

 

Finally, for an output data rate of 16.7 Hz and a gain of 128, the rms noise of the AD7793 is equal to 0.088 μV. The peak-to-peak noise is equal to:

6.6 × rms noise = 6.6 × 0.088 μV = 0.581 μV

 

If the thermocouple's sensitivity is exactly 40 μV/°C, the temperature measurement resolution of the thermocouple is:

0.581 μV ÷ 40 μV = 0.014°C

 

The actual test board is shown in Figure 3. The system was evaluated by measuring the thermistor temperature, thermocouple temperature, and resolution at room temperature and with the thermocouple placed in an ice bucket. The results are shown in Table 1.

 

Figure 3. Thermocouple system using AD7793.

 

 

From Table 1, we can see that the temperature reported by the thermocouple is correct, but the thermistor has an error of 0.3°C. This is the accuracy of the system without linearization. If the thermocouple and thermistor are linearized, the accuracy of the system will improve and the system will be able to measure a wider temperature range.

 

If the difference between the minimum and maximum temperature readings is calculated every 10 readings, the peak-to-peak noise expressed in temperature is 0.02°C. Therefore, the actual peak-to-peak resolution is very close to the expected value.

 

Common changes

The AD7793 is a low noise, low power ADC. Other suitable ADCs are the AD7792 and AD7785, which have the same feature set as the AD7793, but the AD7792 is a 16-bit ADC and the AD7785 is a 20-bit ADC.

 

Circuit Evaluation and Testing

The test data was acquired using the test board shown in Figure 3. Complete documentation for this system is available in the CN-0206 Design Support Package: www.analog.com/CN0206-DesignSupport