Page 236 - Op Amps Design, Applications, and Troubleshooting

218 ACTIVE FILTERS

We will use a standard value of 22 kilohms for R l and R 2-

Determine the Value of R^. The value for resistor R 3 is calculated in the same
way as for a simple voltage follower. That is, we want equal DC resistances
between each op amp input and ground. For the circuit in Figure 5.2, we can com-
pute R 3 with Equation (5.5).

We will select the nearest standard value of 47 kilohms for R 3.

Select the Op Amp. There are three op amp parameters that we should eval-
uate before specifying a particular op amp for our low-pass filter:

1. Bandwidth
2. Slew rate
3. Op amp corner frequency

Since our op amp is operated as a voltage follower, the required bandwidth of the
amplifier is essentially the same as the cutoff frequency. That is,

In the case of our present circuit, our op amp must have a bandwidth of greater
than 1.5 kilohertz. In many cases, including this one, the bandwidth will not be a
limiting factor because the op amp is operated at unity gain.
The minimum required slew rate for the op amp can be estimated with
Equation (5.7),

where f c is the filter cutoff frequency and v o is the highest expected peak-to-
peak output swing. If the application clearly has externally imposed limits on
the maximum output amplitude, then use them. If the maximum output ampli-
tude is not specifically known, as in the present case, then design for worst case
and assume that the signal will swing between the ±V SAT levels. In the present