Modern thermocouples and a high-resolution delta-sigma ADC enable high-precision temperature measurement: A reference design for a precision DAS

Joseph Shtargot, Strategic Applications Engineer, Mohammad Qazi, Applications Engineer, Maxim Integrated -October 18, 2013

Improving the Accuracy

We return now to the NIST ITS-90 database cited above. Substantial accuracy improvement (in the order of magnitude versus linearization algorithms) can be achieved with the modern NIST ITS-90 Thermocouple Database, which allows the polynomial equation to be used to convert thermocouple voltage to temperature (°C) over a wide temperature range using standardized polynomial coefficients.9 New contemporary NIST ITS-90 coefficients are provided for the temperature intervals (versus single intervals in the previous standards), and they allow temperatures to be calculated with good accuracy (around ± 0.1°C)—more than sufficient for most industrial applications.

   

Improving Resolution

The MAX11200 used in this RD is a low-power, 24-bit, delta-sigma ADC. It has an extremely low input-referred RMS noise of 570nV at 10sps. The noise-free resolution (NFR) is around 6.6 × RMS noise and represents a value of 3.762µV. (This is called flicker-free code.) The NFR represents the minimum values that can be reliably differentiated by the ADC in voltage in the measurement range.

 

Table 1 provides more precise NFR resolution values than in Part 1 of this series10 where the assumption was that NFR = VREF/220. Therefore, a detailed calculation for a 0°C to 500°C range is:

                

Table 1 provides the calculated values of °C/NFR error for three temperature ranges. For all the temperature ranges the NFR values are below ±0.15°C, which is better than for ASICs and more than sufficient for most thermocouples in industrial and medical applications.

 

Table 1. °C/NFR Error for Defined Temp Ranges

Temperature Range (°C)

-200 to 0

0 to 500

500 to 1372

Voltage Range (VINT, mV)

-5.891

20.644

34.242

Resolution (°C/NFR)

-0.1277

0.0911

0.0871

 

 

Electronic Design

 

Interfacing a Thermocouple with the MAX11200-based DAS Board

As we noted above, our earlier article described the thermocouple interface with the MAX11200 EV kit. That design produces very good results, but includes many components, optical isolation, and features intended for general applications. Figure 2 shows a simplified schematic for the MAX11200-based DAS board, here optimized for cost-effective portable thermocouple measurements applications. Included on board is the PRTD.11

 

High resolution is achieved by using the MAX11200 fully differential capability. A ratiometric approach utilizes the ADC’s excellent common-mode rejection (100dB or better) and, therefore, allows the design to achieve a desirable signal-to-noise ratio (SNR) without optical isolation and dedicated reference

Figure 2.Block diagram of the MAX11200 DAS-based temperature measurement system. Design allows dynamic measurement of either the thermocouple or PRTD.

 

In Figure 2 the MAX11200’s GPIO is set to control the precision multiplexer, the MAX4782, which selects either the thermocouple or the PRTD. This approach allows dynamic measurement of either the thermocouple or PRTD using a single MAX11200; it improves system precision and reduces the requirements for calibration. The PRTD is a PT1000 (PTS 1206, 1000Ω) used for temperature measurement of the cold junction. The MAX8511 precision LDO powers the system and provides a reference voltage for the MAX11200.

           

The Algorithms

The algorithms selected for this design are similar to those presented in Part 1 of this article and can be used for any type of the thermocouple. The NIST ITS-90 provides different sets of the coefficients for specific temperature intervals: -200°C to 0°C, 0°C to +500°C, and +500°C to +1372°C for K-type thermocouples. Each temperature interval must be preselected for use in Equation 3.

 

T = d0 + d1× E1 + d2 × E² + ... dN × EN                                                                                                           (Eq. 3)

 

Where:

T is the temperature in °C;

E is the VOUT thermocouple output in mV;

dN is the polynomial coefficients unique to each thermocouple;

N is the maximum order of the polynomial.

 

The simplified linearization algorithm in equation 3 allows the designer to select temperature intervals based on Table 1 with sufficient accuracy.

           

Processing the Data

The firmware in the MAXQ622 microcontroller (Figure 2) provides data-reading capability to the software through USB. The software implements algorithms based on Equations 2 and 3. Raw measurement data is processed inside the PC. The software manages the following major functions which are charted in Figure 3:

 

  • Initializes the MAX11200 ADC
  • Collects and processes the ADC's output data
  • Calculates the temperature using Equations 2 and 3
 

During initialization, the MAX11200 ADC goes through the self-calibration process, enables the input signal buffers, and disables both system gain calibration and system offset calibration. This DAS allows reasonably fast data acquisition with excellent (100dB or better) normal-mode powerline 50Hz/60Hz rejection. Selection of the sample rates is very important for reducing noise and interference in the industrial measurements.

 

The recommended external clock for 60Hz line-frequency rejection is 2.4576MHz, which is effective for data rates of 1, 2, 5, 10, and 15sps. For 50Hz line-frequency rejection, the recommended external clock is 2.048MHz, which is effective for data rates of 0.83, 2.08, 4.17, 8.33, and 12.5sps.                                   

 

Input signal buffers increase the input impedance to the high megohms range. This improves measurement precision because it practically eliminates the shunting effect of the input dynamic current.

  
 

Figure 3. Chart outlines the top-level actions of the DAS firmware and software.

 

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