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
2.3.2.3.1 Back-to-Back, Differential OWCS Measurements with Input Capacitance

One additional consideration regarding how input capacitance affects OWCS performance for differential inputs is the order in which the OWCS measurements are made. Specifically, back-to-back measurements can result in settling behavior even when the OWCS are disabled. As an example, assume that back-to-back measurements use the following sequence:

  1. Measure AIN(n)-RESN with OWCS off
  2. Measure AIN(n)-RESN with OWCS on
  3. Measure AIN(n+1)-RESN with OWCS off
  4. Measure AIN(n+1)-RESN with OWCS on

The user might expect that steps 1 and 3 exhibit the stable behavior shown in Figure 2-27 while steps 2 and 4 exhibit the settling behavior shown in Figure 2-25. However, the differential input capacitance continues to hold charge even after step 2 completes such that step 3 also exhibits settling behavior. Figure 2-28 plots the ADC output codes during all four steps of a differential OWCS measurement between AIN12 and AIN13 where RSOURCE = 100kΩ, ODR = 12.5kSPS, and the input capacitance = 10nF:

ADC Code Behavior for Back-to-Back OWCS Measurements (Differential Input, RSOURCE = 800kΩ, ODR = 12.5kSPS, Input Capacitance = 10nF)Figure 2-28 ADC Code Behavior for Back-to-Back OWCS Measurements (Differential Input, RSOURCE = 800kΩ, ODR = 12.5kSPS, Input Capacitance = 10nF)

Figure 2-28 shows the expected behavior for steps 1, 2, and 4: stable codes during step 1 and settling behavior during steps 2 and 4. However, step 3 exhibits settling behavior even though the OWCS are disabled because the input capacitor charge accumulated during step 2 takes an equivalent amount of time to drain. The user can employ the techniques outlined in Section 2.3.2.4 to avoid or reduce this settling behavior.