2.4 Schmitt Trigger

The Schmitt trigger is an analog comparator with hysteresis, a dead-zone where the output remains in the same state until the input exits the zone. This prevents a marginal signal from causing output oscillations. The output function for the Schmitt trigger is shown in the figure below.

Unlike other configurations, the feedback network is connected to the non-inverting input. This creates positive feedback, which helps the output latch to either V_DD or GND.

The switching points of the Schmitt trigger are set by a combination of bias voltage and feedback resistors. In the state where the output is 0V, the feedback resistors act as a voltage divider. When the divided signal is greater than the bias voltage the output flips.

V_{L O W_S W} = \frac{R_{2} + R_{1}}{R_{2}} * V_{B I A S}

In the case where the output is V_DD, the feedback resistors also act as a voltage divider (with the resistors flipped).

V_{H I G H_S W} = V_{D D} - ((\frac{R_{2} + R_{1}}{R_{1}}) * V_{B I A S})

From these two expressions, the amount of hysteresis can be calculated:

V_{H Y} = | V_{D D} - (R_{1} + R_{2}) * (\frac{1}{R_{1}} + \frac{1}{R_{2}}) * V_{B I A S} |

If the microcontroller contains an integrated resistor ladder, rather than entering absolute values into the above expression, the values of the resistors relative to a fixed (or arbitrary) value (for example, R₁ = 4R and R₂ = 6R, where R is an unknown or arbitrary number) can be entered since the resistance units cancel out.