Comparator exhibits temporary hysteresis

Victor Axenenko, CSRI, St Petersburg, Russia -- EDN, 10/14/1999

Noise and interference can be significant problems when you transform a signal from a sine wave to a square wave. When the sine wave crosses the reference level, a comparator can exhibit uncertainty, or jitter. A common method to eliminate comparator uncertainty is to introduce positive feedback by using a resistor divider at the comparator's output. The drawback of this method is that the resulting hysteresis produces an offset that's a function of the state of the comparator. With an asymmetric output, the offset of the comparator's input is also asymmetric. The result is asymmetry in the desired rectangular output signal. You can eliminate this drawback by creating a "temporary-hysteresis" characteristic. You create this characteristic by substituting a capacitor for one of the positive-feedback resistors, as in Figure 1.

When the comparator's output switches, the voltage step transfers to the noninverting input through C₁. When the comparator's output switches from a high level,V_H, to a low level, V_L, its output transistor saturates. C₁, initially charged to V_H, begins to discharge through R₁. The noninverting-input signal, V₊, rises exponentially from its initial value (V_H-V_L) to 0V with a time constant T₁=R₁*C₁ (Figure 2). When the comparator's output switches from V_L to V_H, its output transistor switches off. C₁ charges through R₁ and R₂ and creates a voltage step V₊=(V_CC-V_L)*R₁/(R₁+R₂) at the noninverting input. V₊ then decreases exponentially with the time constant T₂=(R₁+R₂)*C₁. Note that the hysteresis is present only in the time intervals defining the time constants, T₁ and T₂. Note also that upon termination of transients, the signal V₊ on the comparator's noninverting input in both cases is equal to 0V (if T₁<0.1T and T₂<0.1T, where T is the input signal's period). For maximum symmetry, you should select R₁>>R₂. (DI #2425)