This section analyzes the behavior of the OWCS when a single-ended input with source impedance R_SOURCE is applied to the ADS125H18. Figure 2-2 shows the circuit used to analyze a single-ended input with the OWCS disabled and a single-ended R_SOURCE impedance:

Figure 2-2 Analyzing OWCS Behavior for Single-Ended Input Signals

The ADC measures the single-ended voltage difference between nodes V_AIN0 and V_RESN in Figure 2-2 such that Equation 1 can be expanded to Equation 3:

Use superposition to analyze the voltage contribution from each source in the circuit by shorting voltage sources or opening current sources. For example, Figure 2-3 shows that calculating the total voltage at AIN0 with the OWCS enabled, V(AIN0)_{OWCS_SE}, requires three different superposition configurations:

• V(AIN0)_{AIN0_AVDD_SE} = AVDD (AIN0) connected, all other sources removed (Figure 2-3, left)
• V(AIN0)_{RESN_AVDD_SE} = AVDD (RESN) connected, all other sources removed (Figure 2-3, middle)
• V(AIN0)_{AIN0_OWCS_SE} = I_OWCS connected, all other sources removed (Figure 2-3, right)

Figure 2-3 Example Superposition Networks for a Single-Ended Input

Table 2-2 describes each nodal voltage after applying this same analysis to every term in Equation 3:

Importantly, Table 2-2 reveals that virtually all nodal voltages are identical such that Equation 3 can be reduced to Equation 4:

Figure 2-4 shows the superposition configurations for the terms in Equation 4:

• V(AIN0)_{OWCS_SE} = I_OWCS connected, all other sources removed (Figure 2-4, left)
• V(RESN)_{OWCS_SE} = I_OWCS connected, all other sources removed (Figure 2-4, right)

Figure 2-4 Superposition Networks for V(AIN0)_{OWCS_SE} and V(RESN)_{OWCS_SE}

Figure 2-4 also shows that the voltage at RESN with OWCS enabled, V(RESN)_{OWCS_SE}, is 0V because the RESN pin is grounded. Therefore, Equation 4 reduces to Equation 5, which is represented by Figure 2-4 (left):

Equation 6 rewrites V(AIN0)_{OWCS_SE} in Figure 2-4 (left) in terms of resistances RA, RB, and R_SOURCE as well as current I_OWCS:

Recall from Equation 2 that the actual OWCS current, I_OWCS, is relative to the reference voltage, VREF. Therefore, the I_OWCS term in Equation 6 can be rewritten in terms of the nominal OWCS current, I_{OWCS_NOM}, and simplified as shown in Equation 7:

Equation 7 can also be rewritten to predict the single-ended R_{SOURCE_OWCS} value from the measured OWCS_Delta threshold, as shown in Equation 8:

Table 2-1 includes RB and I_{OWCS_NOM} values for all ADS125H18 versions. Figure 2-5 applies all possible parameters from Table 2-1 to Equation 7 and sweeps R_SOURCE from 1kΩ to 1GΩ to generate the ideal OWCS_{Delta_SE} curves:

Figure 2-5 OWCS_Delta Behavior for all ADS125H18 Variants Measuring a Single-Ended Input (R_SOURCE = 1kΩ to 1GΩ)

Figure 2-5 reveals two important takeaways for single-ended OWCS measurements:

1. The significant change in OWCS_{Delta_SE} occurs in the range from 100kΩ to 10MΩ. This range indicates where a change in input impedance is most detectable
2. Each OWCS_{Delta_SE} curve has a theoretical minimum and maximum at R_SOURCE = 0Ω and R_SOURCE = ∞, respectively. The difference between the minimum and maximum decreases as the attenuation factor increases, resulting in a narrower margin to indicate an OWCS fault. Table 2-3 summarizes these results.