Page 47 - Op Amps Design, Applications, and Troubleshooting

3© BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

mizes the resistance of the ground path and provides a shield against external
interference. Additionally, when the insulated ±V CC wires are routed against the
ground plane, the inductance of the power distribution system is greatly reduced.
Radiation into neighboring circuits is also minimized. It should be noted that mul-
tilayer printed circuit boards use one or more entire layers for the ground plane.
This practice minimizes the ground impedance and dramatically reduces both
emissions from the circuit and susceptibility to externally generated noise.
Protoboard provides one of the fastest methods for prototyping circuits.
Additionally, the metal base plate (on some models) provides a ground plane. The
ground currents, however, do not flow through this ground plane. The ground
and V cc distribution buses are physically close and parallel, which lowers the
inductance and radiation. One potential problem that you must be aware of with
this type of board is that there is a significant amount of capacitance between the
various connections on the board. The problems caused by this capacitance
increase as the frequency increases and/or the signal amplitudes decrease. Boards
of this type are generally best suited for low-frequency circuits. It should be noted,
however, that most of the circuits presented in this text were constructed and tested
on such a system with excellent results.
The term quiet ground is most often used with reference to digital systems or,
more commonly, systems that have a combination of analog and digital devices.
Digital circuits can generate high levels of transient currents in the ground net-
work. These currents can interfere with the proper operation of low-level analog
circuits. The degree of interference is generally worsened for faster rise times and
increased current drive in the digital gates. To minimize the ground noise problem
(sometimes called ground bounce), the circuit should be constructed such that the
analog devices are connected directly to the main ground (power input) connec-
tion. That is, the ground currents for the digital devices should not be allowed to
flow through the ground wires of the analog devices. Figure 1.21 clarifies this con-
cept. In Figure 1.21 (a), the return, or ground, current for the digital circuitry flows
through wires (generally printed circuit traces) that are common to the analog cir-
cuitry. This will likely cause interference or noise in the analog circuitry. By con-
trast, Figure 1.21(b) shows a similar circuit that utilizes a separate ground path for
the digital and analog circuitry. In practice, we can often achieve this result through
proper positioning of the components on the circuit board.

1.5.7 Actual Circuit Performance
This section is included as a final effort to make a believer of you. Figure 1.22
shows two sets of oscilloscope waveforms. These waveforms are real and were
obtained from the plotter output on a digitizing oscilloscope; they are not theo-
retical drawings. Each set of waveforms illustrates the effect of one of the circuit
construction rules presented in preceding sections. The first oscilloscope plot in
each set illustrates circuit performance with a construction rule violated. The
companion waveform shows the exact circuit with that one rule implemented
properly. The results clearly indicate the change in performance. Figure 1.22(a)
shows an effect of improper component placement. Figure 1.22(b) illustrates how
the circuit performance can deteriorate when excessive lead lengths are used to
construct the circuit.