Thermocouple Design

Objective:

Design a 3V temperature measurement system using a K type TC at 125°C with an accuracy of 2.5°C

Solution:

• Understand there are multiple contributing components to the accuracy of a TC measurement system. This includes the accuracy of the TC itself, the accuracy of the temp sensor used for measuring reference junction temperature and the accuracy/resolution of the analog to digital converter.
• ADS1118IDGST is the ideal Low power Solution for Thermocouples with reference junction compensation. It has an internal Temperature Sensor whose absolute accuracy is 0.5C at 25C. The integrated Programmable Gain Amplifier improves resolution.

Thermocouple measurement: Estimate of Accuracy/Resolution

• Estimate of Accuracy using the Total Unadjusted Error Calculation Approach:
• Example using K-type TC at 150°C Using FS +/- 0.256V;

Thermocouple Voltage at 150°C = 6.138mV

• Estimate of ADC voltage conversion absolute accuracy at 150°C :

Total Error = [(offset)2 + (gain error)2 + (INL)2]1/2

• Offset Error = +/- (2 LSB)*(0.256V*2) /( 216 -1)= +/-15.63µV
• INL Error = (1 LSB)*(0.256V*2) / (216-1) =+/-7.81µV
• Gain Error = +/-0.15%*6.138 =+/-9.207µV
• This yields an accuracy error = +/-19.75µV or +/- (19.75µV/ 40µV/°C) = +/-0.494°C

Total Accuracy Error= [(IC Temp Sensor)2 + (ADC Error)2 +(Thermocouple Error)2]1/2

Total Accuracy Error= [(0.5 °C)2 + (0.494°C)2 +(2.2°C)2]1/2

Total Accuracy Error =  +/-2.3095°C @ 150°C

• Estimate of Accuracy using the Total Unadjusted Error Calculation Approach:
• Example using K-type TC at -40°C Using FS +/- 0.256V;

Thermocouple Voltage at -40°C = -1.527mV

• Estimate of ADC voltage conversion absolute accuracy at 150°C :

Total Error = [(offset)2 + (gain error)2 + (INL)2 ]1/2

• Offset Error = +/- (2 LSB)*(0.256V*2) /( 216 -1)= +/-15.63µV
• INL Error = (1 LSB)*(0.256V*2) / (216-1) =+/-7.81µV
• Gain Error = +/-0.15%*1.527mV =+/-2.29µV
• This yields an accuracy error = +/-17.62µV or +/- (17.62µV/ 40µV/°C) = +/-0.441°C

Total Accuracy Error= [(IC Temp Sensor)2 + (ADC Error)2 +(Thermocouple Error)2]1/2

Total Accuracy Error= [(0.5 °C)2 + (0.441°C)2 +(2.2°C)2]1/2

Total Accuracy Error =  +/-2.298°C @ -40°C

• Example using K-type TC

Using FS +/- 0.256V