9.2.1.2 Detailed Design Procedure

The biasing resistors R_B1 and R_B2 are used to set the common-mode voltage of the thermocouple such that the input voltages do not exceed the absolute input voltage range of the PGA (in this example, to mid-supply AVDD / 2). If the application requires the thermocouple to be biased to GND, either a bipolar supply (for example, AVDD = 2.5 V and AVSS = –2.5 V) must be used for the device to meet the absolute input voltage requirement of the PGA, or the PGA must be bypassed. When choosing the values of the biasing resistors, care must be taken so that the biasing current does not degrade measurement accuracy. The biasing current flows through the thermocouple and can cause self-heating and additional voltage drops across the thermocouple leads. Typical values for the biasing resistors range from 1 MΩ to 50 MΩ.

In addition to biasing the thermocouple, R_B1 and R_B2 are also useful for detecting an open thermocouple lead. When one of the thermocouple leads fails open, the biasing resistors pull the analog inputs (AIN0 and AIN1) to AVDD and AVSS, respectively. The ADC consequently reads a full-scale value, which is outside the normal measurement range of the thermocouple voltage, to indicate this failure condition.

Although the device digital filter attenuates high-frequency components of noise, performance can be further improved by providing a first-order, passive RC filter at the inputs. Equation 9 calculates the cutoff frequency that is created by the differential RC filter formed by R_F1, R_F2, and the differential capacitor C_DIF.

Two common-mode filter capacitors (C_M1 and C_M2) are also added to offer attenuation of high-frequency, common-mode noise components. Choose a differential capacitor C_DIF that is at least an order of magnitude (10 times) larger than the common-mode capacitors (C_M1 and C_M2) because mismatches in the common-mode capacitors can convert common-mode noise into differential noise.

The filter resistors R_F1 and R_F2 also serve as current-limiting resistors. These resistors limit the current into the analog inputs (AIN0 and AIN1) of the device to safe levels if an overvoltage on the inputs occur. Care must be taken when choosing the filter resistor values because the input currents flowing into and out of the device cause a voltage drop across the resistors. This voltage drop shows up as an additional offset error at the ADC inputs. TI therefore recommends limiting the filter resistor values to below 1 kΩ.

The filter component values used in this design are: R_F1 = R_F2 = 1 kΩ, C_DIF = 100 nF, and C_CM1 = C_CM2 = 10 nF.

The highest measurement resolution is achieved when matching the largest potential input signal to the FSR of the ADC by choosing the highest possible gain. From the design requirement, the maximum thermocouple voltage occurs at T_(TC) = 1250°C and is V_(TC) = 50.644 mV as defined in the tables published by the National Institute of Standards and Technology (NIST) using a cold-junction temperature of T_(CJ) = 0°C. A thermocouple produces an output voltage that is proportional to the temperature difference between the thermocouple tip and the cold junction. If the cold junction is at a temperature below 0°C, the thermocouple produces a voltage larger than 50.644 mV. The isothermal block area is constrained by the operating temperature range of the device. Therefore, the isothermal block temperature is limited to –40°C. A K-type thermocouple at T_(TC) = 1250°C produces an output voltage of V_(TC) = 50.644 mV – (–1.527 mV) = 52.171 mV when referenced to a cold-junction temperature of T_(CJ) = –40°C. The maximum gain that can be applied when using the internal 2.048-V reference is then calculated as (2.048 V / 52.171 mV) = 39.3. The next smaller PGA gain setting that the device offers is 32.

The device integrates a high-precision temperature sensor that can be used to measure the temperature of the cold junction. To measure the internal temperature of the ADS122C04, the device must be set to internal temperature sensor mode by setting the TS bit to 1 in the configuration register. For best performance, careful board layout is critical to achieve good thermal conductivity between the cold junction and the device package.

However, the device does not perform automatic cold-junction compensation of the thermocouple. This compensation must be done in the microcontroller that interfaces to the device. The microcontroller requests one or multiple readings of the thermocouple voltage from the device and then sets the device to internal temperature sensor mode (TS = 1) to acquire the temperature of the cold junction. An algorithm similar to the following must be implemented on the microcontroller to compensate for the cold-junction temperature:

1. Measure the thermocouple voltage, V_(TC), between AIN0 and AIN1
2. Measure the temperature of the cold junction, T_(CJ), using the temperature sensor mode of the ADS122C04
3. Convert the cold-junction temperature into an equivalent thermoelectric voltage, V_(CJ), using the tables or equations provided by NIST
4. Add V_(TC) and V_(CJ) and translate the summation back into a thermocouple temperature using the NIST tables or equations again

In some applications, the integrated temperature sensor of the ADS122C04 cannot be used (for example, if the accuracy is not high enough or if the device cannot be placed close enough to the cold junction). The additional analog input channels of the device can be used in this case to measure the cold-junction temperature with a thermistor, RTD, or an analog temperature sensor. Figure 71 illustrates the LM94022 temperature sensor being used for cold-junction compensation.

As shown in Equation 10, the rms noise of the ADS122C04 at gain = 32 and DR = 20 SPS (0.24 µV_rms) is divided by the average sensitivity of a K-type thermocouple (41 µV/°C) to obtain an approximation of the achievable temperature resolution.

Table 25 shows the register settings for this design.