8.3.2 Threshold

The LVDS Standard (ANSI/TIA/EIA-644-A) specifies a maximum threshold of ±100 mV for the LVDS receiver. The DS90LT012AH supports an enhanced threshold region of −100 mV to 0 V. This is useful for fail-safe biasing. The threshold region is shown in the Voltage Transfer Curve (VTC) in Figure 11. The typical DS90LT012AH LVDS receiver switches at about −30 mV. Note that with V_ID = 0 V, the output will be in a HIGH state. With an external fail-safe bias of +25 mV applied, the typical differential noise margin is now the difference from the switch point to the bias point. In the example shown in Figure 11, this would be 55 mV of Differential Noise Margin (DNM) (+25 mV − (−30 mV)). With the enhanced threshold region of −100 mV to 0 V, this small external fail-safe biasing of +25 mV (with respect to 0 V) gives a DNM of a comfortable 55 mV. With the standard threshold region of ±100 mV, the external fail-safe biasing must be +25 mV with respect to +100 mV or +125 mV, giving a DNM of 155 mV that is a stronger fail-safe biasing than necessary for the DS90LT012AH. If more DNM is required, then a stronger fail-safe bias point can be set by changing resistor values.

Figure 11. VTC of the DS90LT012AH LVDS Receiver