Page 168 - Op Amps Design, Applications, and Troubleshooting

Voltage Comparator with Hysteresis 151

old voltages are both above 0 volts but equally spaced on either side of the refer-
ence voltage. The thresholds in some circuits are not equally spaced around the
reference,

Hysteresis. The hysteresis of the circuit shown in Figure 3.11 is simply the dif-
ference, as given by Equation (3.6), between the two threshold voltages. That is,

Recall that the value of hysteresis primarily determines the 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 3.3 volts before the output will respond.
The hysteresis in this circuit may also be computed directly with Equation
(3.7),

Maximum Frequency of Operation. The upper frequency of operation is
limited in the same manner as the zero-crossing detector discussed previously.
This frequency is estimated with Equation (3.10):

where t s is computed with Equation (3.8) as shown:

Or, in the usual case of symmetrical power supplies, we can simply use Equation
(3.9) as follows:

In our present case, let us determine the minimum switching time with Equation
(3,9):