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

Packages
SINGLE
DUAL
SHORT TRACES
LONG
TRACES
Figure 17–18.
Trace Length for an Inverting Op Amp Stage
It is popular to use dual op amps for stereophonic circuits, and quad op amps for filter
stages with many sections. There is a penalty for doing so, however. Although modern
processing techniques provide high levels of isolation between amplifiers on the same
piece of silicon, there will be some crosstalk. If isolation between amplifiers is important,
then single packages should be considered. Crosstalk problems are not limited to the IC
– the dual and quad packages place a high density of passive components in close proximity
to each other. This proximity will lead to some crosstalk.
Dual and quad op amp packages offer some additional benefits beyond density. The amplifier
stages tend to be mirror images of each other. If similar stages are to be laid out
on the PCB, the layout only needs to be done once, then it can be mirror-imaged to form
the other stage. Figure 17–19 illustrates this effect for four inverting op amp stages implemented
in a quad package:
QUAD
Figure 17–19.
Mirror-Image Layout for Quad Op Amp Package
These illustrations, however, do not show all connections required for operation, in particular,
the half-supply generator for single-supply operation. Modifying the diagram of Figure
17–19 to use the fourth op amp as a half supply generator is shown in Figure 17–20.
Circuit Board Layout Techniques
17-25