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SECTION 9
HARDWARE DESIGN TECHNIQUES
Prototyping Analog Circuits
Evaluation Boards
Noise Reduction and Filtering for
Switching Power Supplies
Low Dropout References and Regulators
EMI/RFI Considerations
Sensors and Cable Shielding
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SECTION 9
HARDWARE DESIGN TECHNIQUES
Walt Kester, James Bryant, Walt Jung,
Adolfo Garcia, John McDonald
PROTOTYPING AND SIMULATING ANALOG CIRCUITS
Walt Kester, James Bryant
While there is no doubt that computer analysis is one of the most valuable tools that
the analog designer has acquired in the last decade or so, there is equally no doubt
that analog circuit models are not perfect and must be verified with hardware. If the
initial test circuit or "breadboard" is not correctly constructed, it may suffer from
malfunctions which are not the fault of the design but of the physical structure of
the breadboard itself. This section considers the art of successful breadboarding of
high performance analog circuits.
Real electronic circuits contain many "components" which were not present in the
circuit diagram, but which are there because of the physical properties of
conductors, circuit boards, IC packages, etc. These components are difficult, if not
impossible, to incorporate into computer modeling software, and yet they have
substantial effects on circuit performance at high resolutions, or high frequencies, or
both.
It is therefore inadvisable to use SPICE modeling or similar software to predict the
ultimate performance of such high performance analog circuits. After modeling is
complete, the performance must be verified by experiment.
This is not to say that SPICE modeling is valueless - far from it. Most modern high
performance analog circuits could never have been developed without the aid of
SPICE and similar programs, but it must be remembered that such simulations are
only as good as the models used, and these models are not perfect. We have seen the
effects of parasitic components arising from the conductors, insulators and
components on the PCB, but it is also necessary to appreciate that the models used
within SPICE simulations are not perfect models.
Consider an operational amplifier. It contains some 20-40 transistors, almost as
many resistors, and a few capacitors. A complete SPICE model will contain all these
components, and probably a few of the more important parasitic capacitances and
spurious diodes formed by the diffusions in the op-amp chip. This is the model that
the designer will have used to evaluate the device during his design. In simulations,
such a model will behave very like the actual op-amp, but not exactly.
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SPICE MODELING
SPICE modeling is a powerful tool for predicting the performance
of analog circuits.
Analog Devices provides macromodels for over 450 ICs
HOWEVER
Models omit real-life effects
No model can simulate all the parasitic effects of discrete
components and a PCB layout
THEREFORE
Prototypes must be built and proven before production.
Figure 9.1
However, this model is not published, as it contains too much information which
would be of use to other semiconductor companies who might wish to copy or
improve on the design. It would also take far too long for a simulation of a system
containing such models of a number of op-amps to reach a useful result. For these,
and other reasons, the SPICE models of analog circuits published by manufacturers
or software companies are "macro" models, which simulate the major features of the
component, but lack some of the fine detail. Consequently, SPICE modeling does not
always reproduce the exact performance of a circuit and should always be verified
experimentally.
The basic principle of a breadboard is that it is a temporary structure, designed to
test the performance of a circuit or system, and must therefore be easy to modify.
There are many commercial breadboarding systems, but almost all of them are
designed to facilitate the breadboarding of digital systems, where noise immunities
are hundreds of millivolts or more. (We shall discuss the exception to this generality
later.) Non copper-clad Matrix board (Vectorboard, etc.), wire-wrap, and plug-in
breadboard systems (Bimboard, etc.) are, without exception, unsuitable for high
performance or high frequency analog breadboarding. They have too high resistance,
inductance, and capacitance. Even the use of standard IC sockets is inadvisable.
PRACTICAL BREADBOARDING TECHNIQUES
The most practical technique for analog breadboarding uses a copper-clad board as a
ground plane. The ground pins of the components are soldered directly to the plane
and the other components are wired together above it. This allows HF decoupling
paths to be very short indeed. All lead lengths should be as short as possible, and
signal routing should separate high-level and low-level signals. Ideally the layout
should be similar to the layout to be used on the final PCB. This approach is often
referred to as "deadbug" because the ICs are often mounted upside down with their
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leads up in the air (with the exception of the ground pins, which are bent over and
soldered directly to the ground plane). The upside-down ICs look liked deceased
insects, hence the name.
Figure 9.2 shows a hand-wired breadboard based around two high speed op amps
which gives excellent performance in spite of its lack of esthetic appeal. The IC op
amps are mounted upside down on the copper board with the leads bent over. The
signals are connected with short point-to-point wiring. The characteristic impedance
of a wire over a ground plane is about 120ohms, although this may vary as much as
±40% depending on the distance from the plane. The decoupling capacitors are
connected directly from the op amp power pins to the copper-clad ground. When
working at frequencies of several hundred MHz, it is a good idea to use only one side
of the board for ground. Many people drill holes in the board and connect both sides
together with short pieces of wire soldered to both sides of the board. If care is not
taken, however, this may result in unexpected ground loops between the two sides of
the board, especially at RF frequencies.
"DEADBUG" PROTOTYPE TECHNIQUE
Figure 9.2
Pieces of copper-clad may be soldered at right angles to the main ground plane to
provide screening, or circuitry may be constructed on both sides of the board (with
connections through holes) with the board itself providing screening. In this case,
the board will need legs to protect the components on the underside from being
crushed.
When the components of a breadboard of this type are wired point-to-point in the air
(a type of construction strongly advocated by Robert A. Pease of National
Semiconductor (Reference 1) and sometimes known as "bird's nest" construction)
there is always the risk of the circuitry being crushed and resulting short-circuits.
Also if the circuitry rises high above the ground plane, the screening effect of the
ground plane is diminished, and interaction between different parts of the circuit is
more likely. Nevertheless the technique is very practical and widely used because
the circuit may so easily be modified.
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Another "deadbug" prototype is shown in Figure 9.3. The board is single-sided
copper clad with holes pre-drilled on 0.1" centers. Power busses are at the top and
bottom of the board. The decoupling capacitors are used on the power pins of each
IC. Because of the loss of copper area due to the pre-drilled holes, this technique
does not provide as low a ground impedance as a completely covered copper-clad
board.
"DEADBUG" PROTOTYPE USING PRE-DRILLED
SINGLE-SIDED COPPER-CLAD BOARD
Figure 9.3
In a variation of this technique, the ICs and other components are mounted on the
non-copper-clad side of the board. The holes are used as vias, and the point-to-point
wiring is done on the copper-clad side of the board. The copper surrounding each
hole used for a via must be drilled out so as to prevent shorting. This approach
requires that all IC pins be on 0.1" centers. Low profile sockets can be used for low
frequency circuits, and the socket pins allow easy point-to-point wiring.
IC sockets can degrade the performance of high speed or high precision analog ICs.
Even "low-profile" sockets often introduce enough parasitic capacitance and
inductance to degrade the performance of the circuit. If sockets must be used, an IC
socket made of individual "pin sockets" (sometimes called "cage jacks") mounted in
the ground plane board may be acceptable (clear the copper, on both sides of the
board, for about 0.5mm around each ungrounded pin socket and solder the grounded
ones to ground on both sides of the board. Both capped and uncapped versions of
these pin sockets are available (AMP part numbers 5-330808-3, and 5-330808-6,
respectively).
