Analog designers use as many tools as possible to ensure that the final system design performs correctly.
The first step is the intelligent use of IC macromodels, if available, to simulate the circuit. The second
step is the construction of a prototype board to further verify the design. The final pc-board layout
should as closely as possible duplicate the prototype layout.
Unfortunately, system designers are under increasing pressure to verify their designs, sometimes
exclusively, with computer simulations before committing to board layouts and hardware. Simulating
complex digital designs is beneficial, because such simulations often let you eliminate the prototype
phase. Bypassing the prototype phase in high-speed, high-performance analog or mixed-signal circuit
designs can be risky for many reasons, however.
The models available to system designers are only gross approximations of the analog components they
emulate (see box, "The limitations of analog-circuit simulation"). Even if semiconductor manufacturers
made more detailed models available, simulation times would be impractically long, and the simulations
might fail to converge. Thus, designers of analog circuits must become proficient at prototyping to
experimentally verify their analog circuit's performance. (An analog circuit is one that uses ICs, such as
op amps, instrumentation amps, programmable-gain amps, voltage-controlled amps, log amps, mixers,
and analog multipliers. A mixed-signal circuit is an ADC, DAC, or combinations of these ICs with some
digital signal processing, which may exist on the same IC.)
The basic principle of a breadboard or prototype is that it is a temporary structure to test the performance
of a circuit or system and must, therefore, be easy to modify. Many commercial prototyping systems
exist, but almost all facilitate the prototyping of digital systems, in which noise immunities are hundreds
of millivolts or more. Non-copper-clad matrix board, non-copper-clad Vectorboard (Vector Electronic
Company, Sylmar, CA), wire-wrap, and plug-in breadboard systems are, without exception, unsuitable
for high-performance or high-frequency analog prototyping. The resistance, inductance, and capacitance
of these breadboards are too high. Even the use of standard IC sockets is inadvisable in many
prototyping applications.
An important consideration in selecting a prototyping method is the requirement for a large-area ground
plane. A large ground plane is necessary for high-frequency circuits and low-speed precision circuits,
especially when those circuits include ADCs or DACs. The differentiation between high-speed and
high-precision mixed-signal circuits is difficult to make. For example, 16+-bit ADCs and DACs may
operate on high-speed clocks greater than 10 MHz with rise and fall times of less than a few
nanoseconds, even though the effective throughput rate of the converters may be less than 100k
samples/sec. Successful prototyping of these circuits requires that you pay equal attention to good
high-speed and high-precision circuit techniques.
The simplest technique for analog prototyping uses a solid copper-clad board as a ground plane
(References 1 and 2). You solder the ground pins of the ICs directly to the plane and wire together the
other components above the plane. This arrangement allows high-frequency decoupling paths to be
short. All lead lengths should be as short as possible, and signal routing should separate high- and
low-level signals. You should locate all connection wires close to the surface of the board to minimize
the possibility of stray inductive coupling. You should not bundle parallel runs because of possible
coupling. Ideally, the layout (at least the relative placement of the components on the board) should be
similar to the layout of the final pc board. This approach is often called "dead-bug" prototyping, because
the ICs mount upside down with their leads up in the air (with the exception of the ground pins, which
are bent and soldered directly to the ground plane). The upside-down ICs look liked dead bugs; hence,
the name.
Figure 1 shows a hand-wired breadboard of two high-speed op amps, which gives
excellent performance despite its lack of aesthetic appeal. The IC op amps mount upside
down on the copper board with the leads bent. Short point-to-point wiring connects the
signals. The characteristic impedance of a wire over a ground plane is about 120 Ohms, although this
number can vary as much as 140%, depending on the distance from the plane. The decoupling capacitors
connect directly from the op amps' power pins to the copper-clad ground plane. When you are working
at frequencies of several hundred megahertz, use only one side of the board for ground. Many people
drill holes in the board and connect both sides with short pieces of wire soldered to both sides of the
board. If you're not careful, however, this connection can result in unexpected ground loops between the
two sides of the board, especially at radio frequencies.
You can solder pieces of copper-clad board at right angles to the main ground plane to provide screening,
or you can construct circuitry on both sides of the board with connections through holes, and the board
itself provides screening. If the board provides screening, the board needs standoffs at the corners to
protect the omponents on the underside from being crushed.
When you construct a breadboard of this type using point-to-point wiring in the air, sometimes called "bird's nest" construction, you risk crushing the circuit and causing short circuits (Reference 2). Also, if
the circuitry rises high above the ground plane, the screening effect of the ground plane decreases, and
interaction between parts of the circuit is more likely. Nevertheless, the technique is practical and
popular, because it makes the circuit easy to modify--assuming that the person doing the modifications is
adept at using a soldering iron, a solder wick, and a solder sucker. |
