SBAS740B October   2015  – May 2020 ADS1118-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      K-Type Thermocouple Measurement Using Integrated Temperature Sensor for Cold-Junction Compensation
  4. 4 Revision History
  5. 5 Device Comparison Table
  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 Timing Requirements: Serial Interface
    7. 7.7 Switching Characteristics: Serial Interface
    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 Analog Inputs
      3. 9.3.3 Full-Scale Range (FSR) and LSB Size
      4. 9.3.4 Voltage Reference
      5. 9.3.5 Oscillator
      6. 9.3.6 Temperature Sensor
        1. 9.3.6.1 Converting from Temperature to Digital Codes
        2. 9.3.6.2 Converting from Digital Codes to Temperature
    4. 9.4 Device Functional Modes
      1. 9.4.1 Reset and Power-Up
      2. 9.4.2 Operating Modes
        1. 9.4.2.1 Single-Shot Mode and Power-Down
        2. 9.4.2.2 Continuous-Conversion Mode
      3. 9.4.3 Duty Cycling for Low Power
    5. 9.5 Programming
      1. 9.5.1 Serial Interface
      2. 9.5.2 Chip Select (CS)
      3. 9.5.3 Serial Clock (SCLK)
      4. 9.5.4 Data Input (DIN)
      5. 9.5.5 Data Output and Data Ready (DOUT/DRDY)
      6. 9.5.6 Data Format
      7. 9.5.7 Data Retrieval
        1. 9.5.7.1 32-Bit Data Transmission Cycle
        2. 9.5.7.2 16-Bit Data Transmission Cycle
    6. 9.6 Register Maps
      1. 9.6.1 Conversion Register [reset = 0000h]
        1. Table 6. Conversion Register Field Descriptions
      2. 9.6.2 Config Register [reset = 058Bh]
        1. Table 7. Config Register Field Descriptions
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Serial Interface Connections
      2. 10.1.2 GPIO Ports for Communication
      3. 10.1.3 Analog Input Filtering
      4. 10.1.4 Single-Ended Inputs
      5. 10.1.5 Connecting Multiple Devices
      6. 10.1.6 Pseudo Code Example
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curves
  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. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Receiving Notification of Documentation Updates
    3. 13.3 Support Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

9.3.2 Analog Inputs

The ADS1118-Q1 uses a switched-capacitor input stage where capacitors are continuously charged and then discharged to measure the voltage between AINP and AINN. This frequency at which the input signal is sampled is called the sampling frequency or the modulator frequency (f(MOD)). The ADS1118-Q1 has a 1 MHz internal oscillator which is further divided by a factor of 4 to generate the modulator frequency at 250 kHz. The capacitors used in this input stage are small, and to external circuitry, the average loading appears resistive. This structure is shown in Figure 30. The resistance is set by the capacitor values and the rate at which they are switched. Figure 31 shows the setting of the switches illustrated in Figure 30. During the sampling phase, switches S1 are closed. This event charges CA1 to V(AINP), CA2 to V(AINN), and CB to (V(AINP) – V(AINN)). During the discharge phase, S1 is first opened and then S2 is closed. Both CA1 and CA2 then discharge to approximately 0.7 V and CB discharges to 0 V. This charging draws a very small transient current from the source driving the ADS1118-Q1 analog inputs. The average value of this current can be used to calculate the effective impedance (Zeff), where Zeff = VIN / IAVERAGE.

ADS1118-Q1 ai_simple_ana_in_cir_bas457_updated.gifFigure 30. Simplified Analog Input Circuit
ADS1118-Q1 ai_tim_s1s2_bas457.gifFigure 31. S1 and S2 Switch Timing

Common-mode input impedance is measured by applying a common-mode signal to the shorted AINP and AINN inputs and measuring the average current consumed by each pin. The common-mode input impedance changes depending on the full-scale range, but is approximately 6 MΩ for the default full-scale range. In Figure 30, the common-mode input impedance is ZCM.

The differential input impedance is measured by applying a differential signal to AINP and AINN inputs where one input is held at 0.7 V. The current that flows through the pin connected to 0.7 V is the differential current and scales with the full-scale range. In Figure 30, the differential input impedance is ZDIFF.

Make sure to consider the typical value of the input impedance. Unless the input source has a low impedance, the ADS1118-Q1 input impedance may affect the measurement accuracy. For sources with high output impedance, buffering may be necessary. Active buffers introduce noise, and also introduce offset and gain errors. Consider all of these factors in high-accuracy applications.

The clock oscillator frequency drifts slightly with temperature; therefore, the input impedances also drift. For most applications, this input impedance drift is negligible, and can be ignored.