Comparator exhibits temporary hysteresis
Victor Axenenko, CSRI, St Petersburg, Russia -- EDN, October 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,VH, to a low level, VL, its output transistor saturates. C1, initially charged to VH, begins to discharge through R1. The noninverting-input signal, V+, rises exponentially from its initial value (VH-VL) to 0V with a time constant T1=R1*C1 (Figure 2). When the comparator's output switches from VL to VH, its output transistor switches off. C1 charges through R1 and R2 and creates a voltage step V+=(VCC-VL)*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)
