Page 46 - Op Amps Design, Applications, and Troubleshooting

Circuit Construction Requirements 29

through the inductance and resistance of the power supply distribution lines.
With the decoupling capacitors in place, short-term demands for increased cur-
rent (i.e., transients or high-frequency changes) can be supplied by the decoupling
capacitor. You may view it as a filter capacitor that disallows sudden changes in
voltage across its terminals. You may also consider that the decoupling capacitor
has a low reactance to high-frequency signals and bypasses those signals around
the circuit being decoupled. In any case, the net result is that the circuits are pro-
vided with a more stable, electrically quiet source of DC power and are more
effectively isolated from each other.
In most cases, ceramic disc capacitors in the range of 0.01-0.1 microfarad are
good choices for circuit decoupling capacitors. Aluminum electrolytic capacitors
are useless for this purpose because of their high internal inductance. It should be
clear from Figure 1.20 that the decoupling capacitor must be connected physically
close to the circuit or device being decoupled in order to be effective. Additionally,
the leads of the decoupling capacitor should be kept as short as possible. Lengths
3
as small as /4 inch can nullify the effects of the decoupling capacitor in many cases.
Power-entry decoupling provides a similar function but is applied at the
point where the power supply leads attach to the circuit under test. Power-entry
decoupling consists of the following:

1. A tantalum electrolytic capacitor connected between each V cc line and
ground. The ideal value is dependent upon the circuit being tested, but
generally a value of 25 to 100 microfarad is adequate.
2. A 0.1-microfarad ceramic capacitor connected in parallel with the tantalum
decoupling capacitor.
3. An optional, but desirable, ferrite bead slipped over the ±V CC wires leading
to the power supply.
4. Twisted leads between the power supply and the power entry point.

1.5.6 Grounding Considerations
Since ground is inherently part of the power distribution system, many of the prac-
tices presented in Sections 1.5.4 and 1.5.5 apply to the ground structure as well. In
addition to these practices, though, we must take some additional precautions to
ensure reliable circuit performance. We shall examine the following techniques:

1. Use of a ground plane
2. Quiet grounds

The performance of a circuit can nearly always be improved by using a large
planar area as the ground connection. For prototyping purposes, however, it is not
always easy to get a ground plane. Perforated board is probably the most imprac-
tical method of prototyping when a ground plane is desired. Wirewrap boards, on
the other hand, are available with an integral ground plane.
Copper-clad board prototyping is probably the least professional from the
standpoint of appearance, but can provide electrical results that exceed those of
the other methods when high-frequency operation is required. In this case, the
entire surface of the copper-clad board is connected to system ground. This mini-