7.4.4.1 Zero Crossing Detector With Hysteresis

To improve switching times and centering the input threshold to ground a small amount of positive feedback is added to the circuit. Voltage divider R₄ and R₅ establishes a reference voltage, V₁, at the positive input. By making the series resistance, R₁ plus R₂ equal to R₅, the switching condition, V₁ = V₂, will be satisfied when V_IN = 0.

The positive feedback resistor, R₆, is made very large with respect to R₅ || R₆ = 2000 R₅). The resultant hysteresis established by this network is very small (ΔV₁< 10 mV) but it is sufficient to insure rapid output voltage transitions.

Diode D₁ is used to ensure that the inverting input terminal of the comparator never goes below approximately −100 mV. As the input terminal goes negative, D₁ will forward bias, clamping the node between R₁ and R₂ to approximately −700 mV. This sets up a voltage divider with R₂ and R₃ preventing V₂ from going below ground. The maximum negative input overdrive is limited by the current handling ability of D₁.

Figure 23. Zero Crossing Detector With Hysteresis