JAJSFR6A July   2018  – November 2018 ADS1219

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      電圧、電流、および温度監視アプリケーション
  4. 4 改訂履歴
  5. 5 概要（続き）
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 I2C Timing Requirements
    7. 7.7 I2C Switching Characteristics
    8. 7.8 Typical Characteristics
  8. 8 Parameter Measurement Information
    1. 8.1 Noise Performance
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Multiplexer
      2. 9.3.2 Rail-to-Rail Input Buffers and Programmable Gain Stage
      3. 9.3.3 Voltage Reference
      4. 9.3.4 Modulator and Internal Oscillator
      5. 9.3.5 Digital Filter
      6. 9.3.6 Conversion Times
      7. 9.3.7 Offset Calibration
    4. 9.4 Device Functional Modes
      1. 9.4.1 Power-Up and Reset
        1. 9.4.1.1 Power-On Reset
        2. 9.4.1.2 RESET Pin
        3. 9.4.1.3 Reset by Command
      2. 9.4.2 Conversion Modes
        1. 9.4.2.1 Single-Shot Conversion Mode
        2. 9.4.2.2 Continuous Conversion Mode
      3. 9.4.3 Power-Down Mode
    5. 9.5 Programming
      1. 9.5.1 I2C Interface
        1. 9.5.1.1 I2C Address
        2. 9.5.1.2 Serial Clock (SCL) and Serial Data (SDA)
        3. 9.5.1.3 Data Ready (DRDY)
        4. 9.5.1.4 Interface Speed
        5. 9.5.1.5 Data Transfer Protocol
        6. 9.5.1.6 I2C General Call (Software Reset)
        7. 9.5.1.7 Timeout
      2. 9.5.2 Data Format
      3. 9.5.3 Commands
        1. 9.5.3.1 Command Latching
        2. 9.5.3.2 RESET (0000 011x)
        3. 9.5.3.3 START/SYNC (0000 100x)
        4. 9.5.3.4 POWERDOWN (0000 001x)
        5. 9.5.3.5 RDATA (0001 xxxx)
        6. 9.5.3.6 RREG (0010 0rxx)
        7. 9.5.3.7 WREG (0100 00xx dddd dddd)
      4. 9.5.4 Reading Data and Monitoring for New Conversion Results
    6. 9.6 Register Map
      1. 9.6.1 Configuration and Status Registers
      2. 9.6.2 Register Descriptions
        1. 9.6.2.1 Configuration Register (address = 0h) [reset = 00h]
          1. Table 10. Configuration Register Field Descriptions
        2. 9.6.2.2 Status Register (address = 1h) [reset = 00h]
          1. Table 11. Status Register Field Descriptions
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Interface Connections
      2. 10.1.2 Connecting Multiple Devices on the Same I2C Bus
      3. 10.1.3 Unused Inputs and Outputs
      4. 10.1.4 Analog Input Filtering
      5. 10.1.5 External Reference and Ratiometric Measurements
      6. 10.1.6 Establishing Proper Limits on the Absolute Input Voltage
      7. 10.1.7 Pseudo Code Example
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Voltage Monitoring
        2. 10.2.2.2 High-Side Current Measurement
        3. 10.2.2.3 Thermistor Measurement
        4. 10.2.2.4 Register Settings
      3. 10.2.3 Application Curve
  11. 11Power Supply Recommendations
    1. 11.1 Power-Supply Sequencing
    2. 11.2 Power-Supply Decoupling
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13デバイスおよびドキュメントのサポート
    1. 13.1 デバイス・サポート
      1. 13.1.1 デベロッパー・ネットワークの製品に関する免責事項
    2. 13.2 ドキュメントのサポート
      1. 13.2.1 関連資料
    3. 13.3 ドキュメントの更新通知を受け取る方法
    4. 13.4 コミュニティ・リソース
    5. 13.5 商標
    6. 13.6 静電気放電に関する注意事項
    7. 13.7 Glossary
  14. 14メカニカル、パッケージ、および注文情報

10.2.2.3 Thermistor Measurement

The temperature measurement using a 10-kΩ thermistor is implemented using a ratiometric measurement approach to achieve best accuracy. The analog supply voltage, AVDD, is used as the excitation voltage for the thermistor in a resistor divider configuration, as well as the external reference voltage, VREF, for the ADS1219.

The relationship between output codes of the ADS1219 and the thermistor resistance, RThermistor, is derived using the following equations. Equation 10 expresses the input voltage at input AIN0 as the voltage across RThermistor, whereas Equation 11 shows how the ADC converts the voltage at AIN0 into corresponding digital codes.

Equation 10. VAIN0 = RThermistor / (RThermistor + RREF) · VREF
Equation 11. VAIN0 = (VREF / Gain) · (Code / 223)

Setting Equation 10 equal to Equation 11 and solving for RThermistor yields the relationship between thermistor resistance and ADC code.

Equation 12. RThermistor / (RThermistor + RREF) = Gain · (Code / 223)
Equation 13. RThermistor = RREF · Gain · (Code / 223) / [1 – Gain · (Code / 223)]

Equation 13 proves that the output code and thus the accuracy of the thermistor measurement is independent of the excitation voltage. The accuracy of the reference resistor, RREF, is typically dominating the measurement accuracy in such a ratiometric circuit implementation. A high-precision, low-drift resistor is therefore required for RREF. For best performance, the value of RREF is chosen such that the ratio between RREF and RThermistor_Max equals the ratio between RThermistor_Min and RREF. Equation 14 is therefore used to calculate RREF.

Equation 14. RREF² = RThermistor_Min · RThermistor_Max

At the two temperature measurement extremes, –40°C and +125°C, a typical 10-kΩ NTC exhibits a resistance of RThermistor_Max = 239.8 kΩ and RThermistor_Min = 425.3 Ω, respectively. Using Equation 14, RREF calculates to 10.1 kΩ. A 10-kΩ resistor is chosen for this example. Consequently, when using Equation 10, the voltage at the ADC input ranges from 0.13 V to 3.17 V. Thus, an ADC gain = 1 must be used for the measurement.

The microcontroller interfacing to the ADS1219 converts RThermistor into a corresponding thermistor temperature by either solving the Steinhart-Hart equation or leveraging a look-up table.