JAJSQX5H may 1995 – august 2023 AM26LV32
PRODUCTION DATA
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The internal fail-safe circuitry was designed such that the input common-mode (VIC) and differential (VID) voltages must be observed. To ensure the outputs of unused or inactive receivers remain in a high state when the inputs are open-circuited, shorted, or terminated, extra precaution must be taken on the active signal. In applications where the drivers are placed in a high-impedance mode or are powered-down, TI recommends that for 1, 2, or 3 active receiver inputs, the low-level input voltage (VIL) must be greater than 0.4 V. As in all data transmission applications, it is necessary to provide a return ground path between the two remote grounds (driver and receiver ground references) to avoid ground differences. Table 8-2 and Figure 8-3 through Figure 8-5 are examples of active input voltages with their respective waveforms and the effect each have on unused or inactive outputs. Note that the active receivers behave as expected, regardless of the input levels.
1, 2, OR 3 ACTIVE INPUTS | SEE FIGURE | 1, 2, OR 3 ACTIVE OUTPUTS | 3, 2, OR 1 UNUSED OR INACTIVE OUTPUTS | ||
---|---|---|---|---|---|
VIL | VID | VIC | |||
900 mV | 200 mV | 1 V | Figure 8-3 | Known state | High state |
–100 mV | 200 mV | 0 V | Figure 8-4 | Known state | ? |
600 mV | 800 mV | 1 V | Figure 8-5 | Known state | High state |
0 mV | 800 mV | 400 mV | Figure 8-6 | Known state | ? |
In most applications, having a common-mode input close to ground and a differential voltage larger than 2 V is not customary. Because the common-mode input voltage is typically around 1.5 V, a 2-V VID would result in a VIL of 0.5 V, thus satisfying the recommended VIL level of greater than 0.4 V.
Figure 8-7 plots seven different input threshold curves from a variety of production lots and shows how the fail-safe circuitry behaves with the input common-mode voltage levels. These input threshold curves are representative samples of production devices. The curves specifically illustrate a typical range of input threshold variation. The AM26LV32 is specified with ±200 mV of input sensitivity to account for the variance in input threshold. Each data point represents the input’s ability to produce a known state at the output for a given VIC and VID. Applying a differential voltage at or above a certain point on a curve would produce a known state at the output. Applying a differential voltage less than a certain point on a curve would activate the fail-safe circuit and the output would be in a high state. For example, inspecting the top input threshold curve reveals that for a VIC that is approximately 1.6 V, VID yields around 87 mV. Applying 90 mV of differential voltage to this particular production lot generates a known receiver output voltage. Applying a VID of 80 mV activates the input fail-safe circuitry and the receiver output is placed in the high state. Texas Instruments specifies the input threshold at ±200 mV, because normal process variations affect this parameter. Note that at common-mode input voltages around 0.2 V, the input differential voltages are low compared to their respective data points. This phenomenon points to the fact that the inputs are very sensitive to small differential voltages around 0.2 V VIC. TI recommends that VIC levels be kept greater than 0.5 V to avoid this increased sensitivity at VIC ≈ 0.2 V. In most applications, because VIC typically is 1.5 V, the fail-safe circuitry functions properly to provide a high state at the receiver output.
Figure 8-8 represents a typical application where two receivers are not used. In this case, there is no need to worry about the output voltages of the unused receivers because these are not connected in the system architecture.
Figure 8-9 shows a common application where one or more drivers are either disabled or powered down. To ensure the inactive receiver outputs are in a high state, the active receiver inputs must have VIL > 0.4 V and
VIC > 0.5 V.
Figure 8-10 is an alternative application design to replace the application in Figure 8-9. This design uses two AM26LV32 devices instead of one. However, this design does not require the input levels be monitored to ensure the outputs are in the correct state, only that they comply to the RS-232 standard.
Figure 8-11 and Figure 8-12 show typical applications where a disconnected cable occurs. Figure 8-11 illustrates a typical application where a cable is disconnected. Similar to Figure 8-9, the active input levels must be monitored to make sure the inactive receiver outputs are in a high state. An alternative solution is shown in Figure 8-12.
Figure 8-12 is an alternative solution so the receiver inputs do not have to be monitored. This solution also requires the use of two AM26LV32 devices instead of one.
When designing a system using the AM26LV32, the device provides a robust solution where fail-safe and fault conditions are of concern. The RS422-like inputs accept common-mode input levels from −0.3 V to 5.5 V with a specified sensitivity of ±200mV. As previously shown, take care with active input levels because this can affect the outputs of unused or inactive bits. However, most applications meet or exceed the requirements to allow the device to perform properly.