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

Chapter 17
Circuit Board Layout Techniques
Bruce Carter
17.1 General Considerations
Prior discussions have focused on how to design op amp circuitry, how to use ICs, and
the usage of associated passive components. There is one additional circuit component
that must be considered for the design to be a success — the printed circuit board on
which the circuit is to be located.
17.1.1 The PCB is a Component of the Op Amp Design
Op amp circuitry is analog circuitry, and is very different from digital circuitry. It must be
partitioned in its own section of the board, using special layout techniques.
Printed circuit board effects become most apparent in high-speed analog circuits, but
common mistakes described in this chapter can even affect the performance of audio circuits.
The purpose of this chapter is to discuss some of the more common mistakes made
by designers and how they degrade performance, and provide simple fixes to avoid the
problems.
In all but very rare cases, the PCB layout for analog circuitry must be designed such that
the effect of the PCB is transparent to the circuit. Any effect caused by the PCB itself
should be minimized, so that the operation of the analog circuitry in production will be the
same as the performance of the design and prototype.
17.1.2 Prototype, Prototype, PROTOTYPE!
Normal design cycles, particularly of large digital boards, dictate layout of the PCB as
soon as possible. The digital circuitry has been simulated, but in most cases, the production
PCB itself is the prototype, and may even be sold to a customer. Digital designers can
correct small mistakes by implementing cuts and jumpers, reprogramming gate arrays or
flash memories, and go on to the next project. This is not the case with analog circuitry.
Some of the common design mistakes discussed in this chapter cannot be corrected by
the cut and jumper method. They can and will render the entire PCB unusable. It is very
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