Page 176 - Op Amps Design, Applications, and Troubleshooting

Window Voltage Comparator 159

We choose the next higher standard value of 270 ohms. Note that in this particular
case, both zeners will be connected across the positive supply voltage because we
require both references to be positive.

Select ftp. The correct value of R P is determined by two primary considerations:

1. Current capability of the driving op amps
2. Type of circuit or device being driven

Since we have no information regarding the driven circuit, let us select R P
such that the op amp output current is limited to 5 milliamps. This calculation,
from Equation (3.21), is based on Ohm's Law as follows:

Let us select the next higher standard value of 5.1 kilohms.

Select the Isolation Diodes. The requirements for the isolation diodes are
not stringent. The two primary parameters that need to be considered are

1. Reverse voltage breakdown
2. Maximum forward current

The maximum current is the same as the value of the op amp output current. In
our case, we have designed this to be 5 milliamps. The maximum reverse voltage
(ignoring diode drops) is approximately equal to the difference between +V SAT of
one op amp and -V SAT of the other. That is,

More specifically,