Page 342 - Op Amps Design, Applications, and Troubleshooting

320 SIGNAL PROCESSING CIRCUITS

For design purposes, we will independently compute the required values for
R 5 and Q to satisfy the ripple and the response time criteria. We will then make a
judgment as to the optimum choice of values.
The minimum required RC time constant based on the ripple specification
can be found by applying Equation (7.12). (We could use Equation (7.13) if the rip-
ple goal were 1 percent.)

Any RC time constant that is longer than this minimum value will satisfy the rip-
ple goal.
The maximum R 5Q time constant based on the response time design goal can
be found by applying Equation (7.14).

— JLX.-X.W/ //**7
Any RC time constant that is less than this value will satisfy the response time
requirement.
At this point, we must choose values for R 5 and Q such that the RC time con-
stant falls within the above window (i.e., 109.4 ms £ R 5Ci < 124.3 ms). Addition-
ally, we don't want to use resistor values smaller than a few kilohms or larger than
the low megohms. In the present case, let us select a standard value of 1.0 micro-
farad for Cj. The limits for R 5 can then be computed as