Op Amps for Everyone - The Repeater Builder's Technical ...

Packages
OUT1
IN1
+
IN2
IN3
OUT2
OUT3
Figure 17–20.
Quad Op Amp Package Layout with Half-Supply Generator
This example shows all of the connections actually required to produce three independent
inverting stages. Note that the half-supply connection can be made entirely under the IC,
keeping the trace length short. This example is not meant to be used as a suggested PCB
layout, it merely illustrates what can be done. The half-supply op amp, for example, could
be any one of the four. The passive components can be selected so that they span the
lead pitch. Surface-mount 0402 packages, for example, span a width equal to or less than
a standard SO package. This can keep traces lengths extremely short for high frequency
applications.
Package styles for op amps include the dual-in-line (DIP) and small-outline (SO). Lead
pitches for op amps have been continually decreasing, as has been the case for all ICs
in general. Decreasing lead pitches have been accompanied by a decrease in the size
of passive components as well. Decreasing the overall circuit dimensions reduces parasitic
inductance, which should allow higher frequency circuits, but it also increases the potential
for crosstalk by placing conductors closer to each other where capacitive effects
increase.
17.7.1 Through-Hole Considerations
The older technology for op amps and other components is through-hole. Components
are constructed with leads that insert through holes in the board — hence the name.
Through-hole components, due to their size, are more suited to applications where space
is not an issue. The components themselves are frequently lower in cost, but the PCB is
more expensive due to the fact that the PCB fabrication house has to drill holes for component
leads. PCBs are primarily a mechanical fabrication — the number of holes and number
of different drills have a big impact on the price.
The leads on a through-hole op amp are arranged on a 0.1-inch grid. Many PCB layout
people like to maintain the 0.1-inch grid for the rest of the components as well. Resistors
17-26