SBAS751B October   2017  – October 2018 ADS122C04

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      K-Type Thermocouple Measurement
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 I2C Timing Requirements
    7. 6.7 I2C Switching Characteristics
    8. 6.8 Typical Characteristics
  7. 7 Parameter Measurement Information
    1. 7.1 Noise Performance
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Multiplexer
      2. 8.3.2  Low-Noise Programmable Gain Stage
        1. 8.3.2.1 PGA Input Voltage Requirements
        2. 8.3.2.2 Bypassing the PGA
      3. 8.3.3  Voltage Reference
      4. 8.3.4  Modulator and Internal Oscillator
      5. 8.3.5  Digital Filter
      6. 8.3.6  Conversion Times
      7. 8.3.7  Excitation Current Sources
      8. 8.3.8  Sensor Detection
      9. 8.3.9  System Monitor
      10. 8.3.10 Temperature Sensor
        1. 8.3.10.1 Converting From Temperature to Digital Codes
          1. 8.3.10.1.1 For Positive Temperatures (For Example, 50°C):
          2. 8.3.10.1.2 For Negative Temperatures (For Example, –25°C):
        2. 8.3.10.2 Converting From Digital Codes to Temperature
      11. 8.3.11 Offset Calibration
      12. 8.3.12 Conversion Data Counter
      13. 8.3.13 Data Integrity Features
    4. 8.4 Device Functional Modes
      1. 8.4.1 Power-Up and Reset
        1. 8.4.1.1 Power-On Reset
        2. 8.4.1.2 RESET Pin
        3. 8.4.1.3 Reset by Command
      2. 8.4.2 Conversion Modes
        1. 8.4.2.1 Single-Shot Conversion Mode
        2. 8.4.2.2 Continuous Conversion Mode
      3. 8.4.3 Operating Modes
        1. 8.4.3.1 Normal Mode
        2. 8.4.3.2 Turbo Mode
        3. 8.4.3.3 Power-Down Mode
    5. 8.5 Programming
      1. 8.5.1 I2C Interface
        1. 8.5.1.1 I2C Address
        2. 8.5.1.2 Serial Clock (SCL) and Serial Data (SDA)
        3. 8.5.1.3 Data Ready (DRDY)
        4. 8.5.1.4 Interface Speed
        5. 8.5.1.5 Data Transfer Protocol
        6. 8.5.1.6 I2C General Call (Software Reset)
        7. 8.5.1.7 Timeout
      2. 8.5.2 Data Format
      3. 8.5.3 Commands
        1. 8.5.3.1 Command Latching
        2. 8.5.3.2 RESET (0000 011x)
        3. 8.5.3.3 START/SYNC (0000 100x)
        4. 8.5.3.4 POWERDOWN (0000 001x)
        5. 8.5.3.5 RDATA (0001 xxxx)
        6. 8.5.3.6 RREG (0010 rrxx)
        7. 8.5.3.7 WREG (0100 rrxx dddd dddd)
      4. 8.5.4 Reading Data and Monitoring for New Conversion Results
      5. 8.5.5 Data Integrity
    6. 8.6 Register Map
      1. 8.6.1 Configuration Registers
      2. 8.6.2 Register Descriptions
        1. 8.6.2.1 Configuration Register 0 (address = 00h) [reset = 00h]
          1. Table 19. Configuration Register 0 Field Descriptions
        2. 8.6.2.2 Configuration Register 1 (address = 01h) [reset = 00h]
          1. Table 20. Configuration Register 1 Field Descriptions
        3. 8.6.2.3 Configuration Register 2 (address = 02h) [reset = 00h]
          1. Table 22. Configuration Register 2 Field Descriptions
        4. 8.6.2.4 Configuration Register 3 (address = 03h) [reset = 00h]
          1. Table 23. Configuration Register 3 Field Descriptions
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Interface Connections
      2. 9.1.2 Connecting Multiple Devices on the Same I2C Bus
      3. 9.1.3 Unused Inputs and Outputs
      4. 9.1.4 Analog Input Filtering
      5. 9.1.5 External Reference and Ratiometric Measurements
      6. 9.1.6 Establishing Proper Limits on the Absolute Input Voltage
      7. 9.1.7 Pseudo Code Example
    2. 9.2 Typical Applications
      1. 9.2.1 K-Type Thermocouple Measurement (–200°C to +1250°C)
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curves
      2. 9.2.2 3-Wire RTD Measurement (–200°C to +850°C)
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1 Design Variations for 2-Wire and 4-Wire RTD Measurements
        3. 9.2.2.3 Application Curves
      3. 9.2.3 Resistive Bridge Measurement
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
  10. 10Power Supply Recommendations
    1. 10.1 Power-Supply Sequencing
    2. 10.2 Power-Supply Decoupling
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9.2.1.2 Detailed Design Procedure

The biasing resistors RB1 and RB2 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, RB1 and RB2 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 RF1, RF2, and the differential capacitor CDIF.

Equation 9. fC = 1 / [2π · (RF1 + RF2) · CDIF]

Two common-mode filter capacitors (CM1 and CM2) are also added to offer attenuation of high-frequency, common-mode noise components. Choose a differential capacitor CDIF that is at least an order of magnitude (10 times) larger than the common-mode capacitors (CM1 and CM2) because mismatches in the common-mode capacitors can convert common-mode noise into differential noise.

The filter resistors RF1 and RF2 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: RF1 = RF2 = 1 kΩ, CDIF = 100 nF, and CCM1 = CCM2 = 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 µVrms) is divided by the average sensitivity of a K-type thermocouple (41 µV/°C) to obtain an approximation of the achievable temperature resolution.

Equation 10. Temperature Resolution = 0.24 µV / 41 µV/°C = 0.006°C

Table 25 shows the register settings for this design.

Table 25. Register Settings

REGISTER SETTING DESCRIPTION
00h 0Ah AINP = AIN0, AINN = AIN1, gain = 32, PGA enabled(1)
01h 08h DR = 20 SPS, normal mode, continuous conversion mode, internal reference
02h 00h Conversion data counter disabled, data integrity disabled, burnout current sources disabled, IDACs off
03h 00h No IDACs used
(1) To measure the cold junction temperature using the LM90422, change register 00h to B1h (AINP = AIN3, AINN = AVSS, gain = 1, PGA disabled).