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AN-214
APPLICATION NOTE
ONE TECHNOLOGY WAY o P.O. BOX 9106 o NORWOOD, MASSACHUSETTS 02062-9106 0 617/329-4700
Ground Rules for High-Speed Circuits
Layout and Wiring Are Critical in Video-Converter Circuits
How to Keep Interference to a Minimum
by Don Brockman and Arnold Williams
In recent issues, Analog Dialogue has dealt extensively
with topics in shielding and grounding,'' 2 emphasizing
the techniques needed to protect the integrity and preci-
sion of analog signals in the do and audio-frequency
domain from interfering signals, whether at line fre-
quency or at much higher frequencies. To complement
those articles, we suggest here the elements of good
practice for high-resolution "video speed" converters,
i.e., converters of 10-bit or greater resolution, operating
at word rates above 1 MHz.
Electronics may be frustrating for designers who cross
the threshold from low-resolution-low-speed to high-
resolution-high-speed designs, or from digital to analog-
signal-conditioning circuits. For them, it often seems the
"ground rules" have changed.
Experienced designers can readily attest to the difficulty
of obtaining consistent grounds. They can relate stories
about the ground that wasn't where they thought it was,
or the ground that wasn't there at all, despite a convic-
tion that "it has to be." On printed-circuit (p-c) boards,
wires and/or runs that seemed to be perfectly good
grounds are transformed into inductors or worse in
high-speed or high-frequency circuits.
At ADI's Computer Labs Division, where high-speed cie
cuits are its bread and butter, applications engineers
have found that grounding is the focus of a large per-
centage of questions from designers making their initial
foray into high-speed circuits. In most cases, the design-
ers encountered difficulties as the result of being un-
aware of-or ignoring-certain basic ground rules.
BASIC PC-CARD RULES
Knowledgeable high-speed circuit designers have
learned that every square inch of a printed-circuit board
which doesn't contain circuits or conducting runs should
be ground plane. Violating that simple rule invites disas-
ter. But sometimes, strict adherence to the rule is still no
guarantee of success if circuit density is too high; then
'Alan Rich, "Understanding Interference-Type Noise," Analog Dia-
logue 16-3, 1982, pages 16-19.
2Alan Rich, "Shielding and Guarding," Analog Dialogue 17-1, 1983,
pages 8-13.
one must reduce the density and create more "real es-
tate" for the ground plane.
Our applications engineers strongly recommend that all
bread-board designs be done on double-sided copper-
clad boards. Although this is not a sure cure for ground
problems, it improves the designer's chances.
Another basic rule for working with high-speed and/or
high-frequency printed-circuit-board designs is to con-
nect analog ground and digital ground together within
the PC board. Connecting the two grounds enhances the
performance of the converters when they are operated
either by themselves or as tightly knit subsystems. How-
ever, it can raise some system-level problems, to be
discussed below.
Another rule for printed-circuit-board designs containing
analog and digital circuitry is to use every available
spare pin for making ground connections, and to use
those pins to separate the analog and digital signals
entering or leaving the board.
Avoid using purely insulating (e.g., "Vector") bread-
boards and small-diameter hookup wire (e.g., #24) for
connections, including supply voltages and grounds.
The approach will create ground and noise problems if
the circuit is intended to operate at 1 MHz or more (it will
probably lead to problems at even slower speeds).
To summarize: Use double-sided copper-clad boards
with maximum ground area and heavy, well-located
power-supply and ground-return leads. Tie rounds to-
gether locally,
GENERAL CIRCUIT PRACTICE
Any subsystem or circuit layout operating at high
speeds with both analog and digital signals needs to
have those signals physically separated as much as pos-
sible to prevent possible crosstalk between the two. Dig-
ital signals leaving or entering the layout should use
runs that have minimum length. The shorter the digital
runs, the lower the likelihooq of coupling to the analog
circuits. _
Analog signals should be routed as far from digital sig-
nals as size constraints allow; and the two, ideally
should never closely parallel one another's paths. If they
must cross, they should do so at right angles to mini-
mize interference. Coaxial cables may be necessary for
analog inputs or outputs-a demanding condition me-
chanically, but sometimes the only solution electrically.
When combining track-and-hold and a/d-converter hy-
brids or modules on the same board, keep them as close
together as is practical. All grounds need to be con-
nected to the single, low-impedance ground plane: and
the connections should be made right at the units them-
selves (another argument for having large amounts of
good, solid ground plane available all over the p-c
board).
A suggested practical approach for accomplishing this is
illustrated in Figure 1, which shows a flow-chart layout,
as the preferred method for combining high-speed ana-
log and digital circuits on a p-c board.
If one assumes a 10-volt input range on the 12-bit a/d
converter, the least-significant bit (LSB) of the ADC will
have a value of 2.5 mV (10 V/4,096). Assume that a single
pin of the p-c connector, which is used for ground, has a
resistance of 0.05 ohm-and that the p-c card draws a
total of 1.5 amperes.
The voltage drop at the ground pin could be 75 millivolts
in these circumstances. If only digital logic were used,
this voltage drop would be minuscule, hardly worth con-
sidering. However, the hypothetical real-world situation
being considered here is a mixture of both analog and
digital circuits, and the 75 mV can have a significant
impact on the subsystem's performance.
In this example, the digital circuits are TTL. Since TTL is
a saturated logic, ground currents vary widely, and vary-
ing current flowing through the ground often produces
noise signals which modulate the ground plane. This
noise, created by digital switching, can couple into the
analog portion of the circuit and have an important ef-
fect on performance, even at low digital levels. For ex-
ample, if only 10% of the 75-millivolt l-R drop cited here
couples into the analog signal, that would represent 3
LSBs. . .
The result The circuit intended for operation as a 12-bit
system is now reduced to a system of 10 to 11 bits,
because of noise masking the 2.5-millivolt level of the
desired 12-bit LSB. The recommended solution Assign
multiple pins for ground connections, to reduce the total
contact resistance. As Figure 1 shows, those pins are
also used to separate the analog and digital signals.
"rg" DATA Minoan SVSYEM
CONVERTER Immune
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CONDITIONING
INFORMATION
GROUNDS
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INTERFACE
DIGITAL
Figure 1. "Flow Chart" Layout for Logical Separation of
Functions.
This design approach may seem unnecessarily rigorous
and time-consuming but can prove rewarding when the
p-c board is installed in its final system location.
Locate the timing circuits near the center of the board
(Figure 1) because the timing is at the heart of the circuit,
being connected to all of the major circuit components
of the board. A central location helps assure minimum
paths for the digital signals.
Variations of this theme may not use the exact same
components or functions, but the same basic techniques
should be applied in any design containing analog and
digital circuits. For cards with all connections at one end,
avoid configurations which have analog circuits near the
p-c connector, and digital signals at the opposite end of
the card-or vice versa; either situation will cause ana-
log and digital paths to pass in close proximity to one
another.
SYSTEM GROUNDING
Although local ties for analog and digital grounds help
the performance of a card, they can cause problems for
the system designer using ADCs and DACs. In systems,
data converters should be considered as analog (not
digital) components; the system design must be as-
signed to experienced and capable analog engineers,
who are used to defending millivolt signals against
interference.
Place ADCs and DACs (like other analog devices) near
other parts of the analog section, because: (1) reflections
make it hard to transmit analog signals more than a
short distance without loss of bandwidth and amplitude;
and (2) noise generated by the digital section can couple
into the analog through the ground plane or power sup-
plies, or radiate to nearby analog components.
Each card in the system should be returned directly to
the power supply common, using heavy wire. Where it
is mandatory that a card's analog and digital grounds be
separated, each should be separately returned to the
power supply; don't connect the two grounds and return
a single ground line to the power supply.
POWER SUPPLIES
Besides ground rules, designers of high-speed circuits
must also consider the rules about power supplies to
obtain best results.
Every power-supply line leading into a high-speed p-c
card or data-acquisition circuit must be carefully
bypassed to its ground return to prevent noise from
entering the card. Ceramic capacitors, ranging in value
from 0.01 to 0.1 WF, should be used generously in the
layout, mounted as closely as possible to the device or
circuit being bypassed; and at least one good-quality
tantalum capacitor of 3 to 20 wF should be assigned to
each power-supply voltage, mounted as near as possi-
ble to the incoming power pins to keep potentially high
levels of low-frequency ripple off the card.
To some extent, the p-c's power-supply’oonnector pins
can introduce noise problems. If their contact resistance
is sufficiently high, and a varying current is flowing, the
varying IR drop which results is noise and can be cou-
pled into parts of the card. This caution applies espe-
cially to +5-volt supplies used to power TTL systems,
but the problem can be alleviated with a variation of the
rule about multiple pins for making ground connections.
Parallel the l-R drops by also using multiple pins for
power connections.
Low-noise, Iow-ripple temperature-stable linear power
supplies are the preferred choices for high-speed cir-
cuits. Switching power supplies often seem to meet
those criteria, including ripple specifications. But ripple
specs are generally expressed in terms of rms-and the
spikes generated in switchers may often produce hard-
to-filter, uncontrollable noise peaks with amplitudes of
several hundred millivolts. Their high-frequency compo-
nents may be extremely difficult to keep out of the
ground system.
If switchers cannot be avoided for high-speed designs,
they should be carefully shielded and located as far
away from the "action" as possible, and their outputs
should be filtered heavily.
ABOUT IC DESIGNS
There is often a difference in implementing designs
using high-precision IC circuits vis-ty-vis p-c card designs
using modules or hybrids. Some ICs are specifically de-
signed to keep analog and digital grounds separated
within the device, because they would be unable to per-
form their functions properly without the separation.
Recognizing this, IC manufacturers are generally very
careful in detailing how to obtain optimum performance
from their devices. Those details of the application notes
frequently instruct the user to connect analog and digital
grounds for the device together externally; when they
do, the connection needs to be made as closely as pos-
sible to the device. In other, much rarer, instances, the
characteristics of an individual device-or system- may
require some remote connection of the grounds.
The best approach for getting optimum performance
from any device is to follow diligently the recommenda-
tions of the manufacturer. If the recommendations are
missing or vague, ask for them.
Logical signal flow generates logical treatment of
ground paths and ground connections- a logical way to
prevent potential problems.
