A zero-crossing detector is sometimes called a zero-level detector or a Schmitt trigger. In operation, a zero-crossing detector determines if an input voltage to the comparator is greater or less than zero. In response to this determination, the output voltage of the comparator can assume only two possible states. The output state may be high or low depending upon which comparator input (plus or minus) is used to detect the incoming signal.
A single comparator may be used as a simple crossover detector, but this can allow several sources of error. These errors may be caused by the input bias and offset currents of the comparator. Temperature may also affect the zero-crossing voltage points. This basic zero-crossing detector also will have another drawback called chatter which is due to noise on the input signal. Chatter can be reduced by adding hysteresis or positive feedback. These provide noise immunity and prevent the output from “chattering” between states as the input voltage passes through zero.
An improved circuit is illustrated in Figure 3-51. This zero-crossing detector of this type uses a dual LM393 comparator, and easily controls hysteresis by the reference levels which are set on the comparator inputs.
The circuit illustrated is powered by ± 10-V power supplies. The input signal can be an ac signal level up to +8V. The output will be a positive going pulse of about 4.4V at the zero-crossover point. These parameters are compatible with TTL logic levels.
The input signal is simultaneously applied to the noninverting input of comparator A and the inverting input of comparator B. The inverting input of comparator A has a + 10-mV reference with respect to ground, while the noninverting input of comparator B has a -10-mV reference with respect to ground. As the input signal swings positive (greater than + 10 mV), the output of comparator “A” will be low while comparator “S” will have a high output. When the input signal swings negative (less than – 10 mV), the reverse is true. The result of the combined outputs will be low in either case. On the other hand, when the input signal is between the threshold points (± 10 mV around zero crossover), the output of both comparators will be high. In this state the output voltage will be one-half the 10V (VCC+) less the 0.6- V diode drop at the junction of the two 10-kΩ resistors (approximately +4.4 V). This circuit is very stable and immune to noise. If more hysteresis is needed, the ± 10-mV window may be made wider by increasing the reference voltages. The 1N914 diode in series with the outputs allows a positive going pulse at the crossover point. This circuit “squares” the input signal into positive rectangular output pulses whose pulse width corresponds to the input zero crossings.