SDAA378 August   2026 ADS125H18

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Detailed Description
    1. 2.1 Understanding ADS125H18 OWCS Operation
    2. 2.2 Using OWCS with Single-Ended Input Signals
      1. 2.2.1 Analyzing OWCS Behavior for Single-Ended Input Signals
      2. 2.2.2 Verifying OWCS Operation with a Single-Ended Input Using the ADS125H18EVM
        1. 2.2.2.1 Verifying OWCS Operation with a Single-Ended Input
        2. 2.2.2.2 Using a Fewer Number of Conversions for OWCS with a Single-Ended Input
          1. 2.2.2.2.1 Unsettled Data for Single-Ended Inputs
          2. 2.2.2.2.2 Settled Data for Single-Ended Inputs
        3. 2.2.2.3 How Input Capacitance Affects OWCS Performance for Single-Ended Inputs
        4. 2.2.2.4 Using the ADS125H18 Programmable Delay to Mitigate Settling Effects for Single-Ended Inputs
    3. 2.3 Using OWCS with Differential Input Signals
      1. 2.3.1 Analyzing OWCS Behavior for Differential Input Signals
      2. 2.3.2 Verifying OWCS Operation with a Differential Input Using the ADS125H18EVM
        1. 2.3.2.1 Verifying OWCS Operation with a Differential Input
        2. 2.3.2.2 Using a Fewer Number of Conversions for OWCS with a Differential Input
          1. 2.3.2.2.1 Unsettled Data for Differential Inputs
          2. 2.3.2.2.2 Settled Data for Differential Inputs
        3. 2.3.2.3 How Input Capacitance Affects OWCS Performance for Differential Inputs
          1. 2.3.2.3.1 Back-to-Back, Differential OWCS Measurements with Input Capacitance
        4. 2.3.2.4 Using the ADS125H18 Programmable Delay to Mitigate Settling Effects for Differential Inputs
    4. 2.4 Using the ADS125H18 Channel Sequencer to Implement an OWCS Algorithm
      1. 2.4.1 Example: Configuring the ADS125H18 Sequencer for the OWCS Algorithm
      2. 2.4.2 Understanding the OWCS Algorithm in the ADS125H18 Example Code
        1. 2.4.2.1 Initialization
          1. 2.4.2.1.1 Initialize the sequencer
          2. 2.4.2.1.2 Enter the "_step" Array Values
          3. 2.4.2.1.3 Analyze the Sequence
        2. 2.4.2.2 Starting the Sequencer and Processing Data
          1. 2.4.2.2.1 Update OWCS Step Values and Calculate "total_conversions"
          2. 2.4.2.2.2 Starting the Sequencer and Taking Data
          3. 2.4.2.2.3 Processing the OWCS Data
          4. 2.4.2.2.4 Using the OWCS Results
  6. 3Summary
  7. 4References
  8. 5Appendix A
    1. 5.1 Using OWCS with Redundant Systems
      1. 5.1.1 Dual, Redundant Systems versus Single-Device Systems
      2. 5.1.2 Using 1x OWCS versus 2x OWCS in a Dual Redundant System
      3. 5.1.3 Error Sources in Dual, Redundant Systems
        1. 5.1.3.1 RA1 and RB1 Errors
        2. 5.1.3.2 RA2 and RB2 Errors
        3. 5.1.3.3 VREF1 and VREF2 Errors
      4. 5.1.4 Measuring Differential Inputs in a Dual, Redundant System
      5. 5.1.5 Dual, Redundant System Summary

5.1.5 Dual, Redundant System Summary

The following list summarizes important conclusions about dual, redundant systems:

  • Enable both OWCS to provide the widest threshold range in a dual, redundant system. Both devices must therefore be powered and configured even if one device is not actually measuring the input
  • Use ADS125H18s with the same voltage rating. For example, use 2x ADS125H18V20 devices for a dual, redundant system. Mixing different voltage ratings complicates the analysis and can lead to narrower threshold ranges
  • Use the same reference voltage value for all devices to ensure the IOWCS current magnitudes are as close as possible. For example, set the internal reference voltage to 2.5V for both devices. If possible, use a single external voltage reference for all ADS125H18s to maintain ratiometric behavior
  • The OWCS code uses Equation 17 and Equation 18 to predict the RSOURCE_OWCS value from the measured OWCSDelta for single-ended and differential inputs, respectively. However, this section clearly shows that a dual, redundant system does not exhibit the same behavior as a system using a single ADS125H18 under any conditions. Therefore, the user must modify the OWCS code to work with redundant systems. It is recommended to generate an OWCSDelta lookup-table (LUT) based on the specific use case as well as a function to search the LUT for the corresponding RSOURCE_OWCS value. Again, this action is left to the user to complete.