Page 44 - Op Amps Design, Applications, and Troubleshooting

Circuit Construction Requirements 27

1. Use a wire size that is large enough to minimize impedance.
2. Run ±V CC and ground parallel and as close as possible to each other.
3. Avoid longer lead lengths than necessary.
4. Do not allow the current for digital or high-current devices to flow through
the same ground wires as small linear signals (except for the main system
groundpoint).
5. Twist the power distribution lines that run between the power supply and
the circuit under test.

For many circuits, these rules and practices can be severely abused with no
apparent reduction in circuit performance. But why take the chance? If failure to
apply these techniques is the cause of poor circuit performance, it may be very dif-
ficult to isolate, and an otherwise good design may be classed as unpredictable,
unreliable, impractical, and so on.

1.5.5 Power Supply Decoupling

Closely associated with power supply distribution is power supply decoupling.
Again, this is an area that is hard to appreciate and frequently gets slighted. To
help you understand the mechanisms involved, let us examine the problem of
power distribution more closely.
In theory, each device or circuit connected between a DC source and ground
receives the same voltage, and they are unaffected by each other (i.e., they are in
parallel). In practice, however, the wires supplying the power contain resistance
and inductance. Figure 1.19 shows a simplified representation of the problem.
As the current for circuit 1 flows through the power supply lines, the induc-
tance and resistance cause voltage to be dropped. Thus circuit 1 receives less volt-
age than expected. Circuit 2 cannot receive more voltage than circuit 1 and, in fact,
receives even less due to the voltage drop across the resistances and inductances
between circuits 1 and 2.
You may not be alarmed at this point because you know that the resistance in
copper wire is very low and so the resulting voltage drop must surely be very low.
You may be right as long as both the current and the frequency are low.
In the case of an op amp circuit, the frequency is rarely low. Even if it is your
intention to build a DC amplifier, there still will be high-frequency noise signals
in the circuit. The high frequencies, whether desired or undesired, cause high-
frequency fluctuations in the power supply current. These changes in current

FIGURE 1.19 Printed circuit traces and wire used for DC power distribution
have distributed inductance and resistance that cause voltage drops for high-
frequency currents.