Page 392 - Op Amps Design, Applications, and Troubleshooting

368 ARITHMETIC FUNCTION CIRCUITS

This, of course, is the result that we would expect from a circuit that is supposed to
compute the difference between two input voltages.
The input impedance for the inverting input (V B signal) can be computed
with Equation (2.7). For the circuit shown in Figure 9.5, we can compute the
inverting input resistance as

The input impedance presented to the noninverting input is essentially the value
of R 3 and R 4 in series. That is,

If resistors R 3 and R4 are quite large, then a more accurate value can be obtained by
considering that the input resistance of the op amp itself is in parallel with R 4. The
small signal bandwidth of the circuit can be computed by applying Equation
(2.22) to the (+) input. For our present circuit, we can estimate the bandwidth as

The highest practical operating frequency may be substantially lower than this
because of slew rate limiting of larger input signals. If we assume that the output
will be required to make the full output swing from +V SAT (4-13 V) to -V SAT(~13 V),
then we can apply Equation (2.11) to determine the highest input frequency that
can be applied without slew rate limiting.

9.2.3 Practical Design Techniques
Now let us design a subtracter circuit that will satisfy the following design goals:

1. Input voltages 0 to +5 volts
2. Input frequency 0 to 10 kilohertz
3. Input impedance >2.5kilohms

Compute R) to R*. The value of RI is determined by the minimum input
impedance of the circuit. Its value is found by applying Equation (2.7),

Resistors R 2 through R 4 are set equal to Rj in order to provide the correct subtrac-
ter performance. We will use standard values of 2.7 kilohms for these resistors.

Select the Op Amp. The primary considerations for op amp selection are slew
rate and bandwidth. Unless the input signals are very small, the slew rate estab-
lishes the upper frequency limit. The required slew rate to meet the design specifi-