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

2.3.2.1 Verifying OWCS Operation with a Differential Input

Figure 2-21 shows the simple OWCS verification setup using a differential input on the ADS125H18 EVM. No passive components were installed between the terminal block and the ADC input pins. Various 0.1% RSOURCE resistors were connected between the positive and negative inputs on the EVM terminal blocks – for example, between AIN8 and AIN9 – to verify performance across a wide range of input impedances. This configuration results in a 0V input signal to the ADC to simplify the analysis.

OWCS Verification Setup for a Differential InputFigure 2-21 OWCS Verification Setup for a Differential Input

Recall from Section 2.1 that detecting an open wire for a differential input requires four measurements: one baseline and one detection measurement per wire. First, the positive input channel was measured by taking 512 conversions with the OWCS disabled followed by 512 conversions with the OWCS enabled. Next, the negative input channel was measured by taking 512 conversions with the OWCS disabled followed by 512 conversions with the OWCS enabled. The 1024 conversions with OWCS disabled were averaged and the 1024 conversions with OWCS enabled were averaged. Two OWCSDelta_Diff values were then calculated using Equation 1 and converted to a predicted RSOURCE_OWCS value by rearranging the terms in Equation 13. Table 2-11 shows the measured, averaged results across 3 different ODRs and 6 different RSOURCE values:

Table 2-11 Measured OWCSDelta and Predicted RSOURCE_OWCS Values Using Different ODRs and RSOURCE Inputs (Differential Input)

RSOURCE

(kΩ)

ODR = 1.6kSPSODR = 12.5kSPSODR = 50kSPS
OWCSDelta_DiffRSOURCE_OWCS (kΩ)OWCSDelta_DiffRSOURCE_OWCS (kΩ)OWCSDelta_DiffRSOURCE_OWCS (kΩ)
25.523.76%29.3323.76%29.6823.76%26.18
99.923.80%102.8023.80%105.2423.80%99.04
499.923.96%502.8923.96%504.6623.96%492.91
1000.023.41%999.9124.11%1003.1324.01%938.98
1999.924.31%1994.7724.30%1991.7624.30%1970.80
∞ (Open)25.01%>1G25.01%>1G25.00%>1G

Table 2-11 reveals that the OWCS correctly determined the applied differential source impedance across different ODRs for virtually all RSOURCE values. However, Table 2-11 also shows an approximate 4kΩ offset between the predicted RSOURCE_OWCS values and the actual RSOURCE values. Fortunately, this seemingly large error in kΩ translates to a very small 0.05% average input code variation that can be attributed to ADS125H18 resistor divider matching. The end result is that the OWCS fault detection behaves as intended: this feature accurately identified an open circuit as well as significant changes in the source impedance.