9.2.2 Detailed Design Procedure

A small change to the comparator circuit can be made to add hysteresis. Hysteresis uses two different threshold voltages to avoid the multiple transitions introduced in the previous circuit. The input signal must exceed the upper threshold (V_H) to transition low, or below the lower threshold (V_L) to transition high.

Figure 66 illustrates hysteresis on a comparator. Resistor R_H sets the hysteresis level.

When the output is at a logic high (5 V), R_H is in parallel with R_X. This configuration drives more current into Ry, and raises the threshold voltage (VH) to 2.7 V. The input signal must drive above VH = 2.7 V to cause the output to transition to logic low (0 V).

When the output is at logic low (0 V), Rh is in parallel with Ry. This configuration reduces the current into Ry, and reduces the threshold voltage to 2.3 V. The input signal must drive below VL = 2.3 V to cause the output to transition to logic high (5 V).

For more details on this design, refer to Precision Design TIPD144, Comparator with Hysteresis Reference Design.