Page 161 - Op Amps Design, Applications, and Troubleshooting

144 VOLTAGE COMPARATORS

In the present case, we have

V H = +0.91 V- (-0.91 V) = 1.82 V

The higher the value of hysteresis, the more noise immunity offered by the
circuit. In the present case, once the input voltage has crossed one of the threshold
levels, it will take a noise pulse of the opposite polarity and with a magnitude of at
least 1.82 volts before the output will respond.
In the case of equal ±^7- voltages, the hysteresis may be computed directly
with the following equation:

Maximum Frequency of Operation. Since the output voltage of the zero-
crossing detector switches between two extreme voltages, the upper frequency
limit is more appropriately determined by considering the effects of slew rate
rather than the falling amplification. You will recall that the slew rate of an op amp
limits the rate of change of output voltage. For purposes of this calculation, we
will determine the highest operating frequency that allows the output to switch
fully between ±V SAT. If we exceed this frequency, the output amplitude will begin
to diminish. The reduced output voltage will produce a similar reduction in
threshold voltages and the hysteresis voltage.
Appendix 1 lists the slew rate of a 741 op amp as 0.5 volts per microsecond.
The output must change from one saturation level to the other during the time for
half of the input period (assuming a symmetrical output signal). For purposes of
worst-case design, let us assume that the saturation voltages are at their highest
magnitudes (listed as ±14 volts in Appendix 1). The minimum time required to
switch between these two limits is computed as shown:

In the case of equal magnitudes of ±V SAT voltages, this can be expressed as

In our present case, the minimum switching time is determined as shown: