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MC141627 bảng dữ liệu(PDF) 11 Page - Motorola, Inc

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MC141627
11
MOTOROLA
EMI SUPPRESSION
When using ICs in or near television receiver circuits, EMI
(electromagnetic interference) and subsequent unwanted
display artifacts and distortion are probable unless adequate
EMI suppression is implemented. A common misconception
is that some offending digital device is the culprit. This is er-
roneous in that an IC itself has insufficient surface area to
produce sufficient radiation. The device, while it is the gener-
ator of interfering signals, must be coupled to an antenna be-
fore EMI is radiated. The source for the EMI is not the IC
which generates the offending signals but rather the circuitry
which is attached to the IC.
Potential EMI signals are generated by
all digital devices.
Whether they become a nuisance is dependent upon their
frequency and whether they have a sufficient antenna. The
frequency and number of these signals is affected by both
circuit design within the IC and the manufacturing process.
Device speed is also a major contributor of potential EMI. Be-
cause the design is determined by the anticipated applica-
tion, the manufacturing process is fixed and the drive for
speed ever increasing, the only effective point to implement
EMI suppression is in the PC board design. The PC board
usually is the antenna which radiates the EMI. The most effi-
cient method of minimizing EMI radiation is to minimize the
efficiency of this antenna.
The most common cause of inadequate EMI suppression
lies with the ground system of the suspected digital devices.
As pointed out previously, di/dt transitions can be significant
in digital circuits. If the di/dt transitions appear in the ground
system and the ground system is inductive, the harmonics
present in these transitions are a source of potential EMI sig-
nals. The unfortunate result of putting digital devices on a
reactive ground system is guaranteed EMI problems.
The area which should be addressed first as a potential
EMI source is the ground. Without an adequate ground sys-
tem, EMI cannot be effectively reduced by decoupling. If at
all possible, the ground should be a complete unbroken
plane. Figure 5 shows two examples of relieving ground
around device pins. When relieving vias and plated through
holes, large areas of ground loss should be avoided. When
the relief pattern is equal to half the distance between pins,
over etching and process errors may remove ground be-
tween pins. If sufficient ground around enough pins are re-
moved, the ground system can become isolated or nearly
isolated “patches” which will appear inductive. If ground,
such as the vicinity of an IC, must be removed, replace with
a cross hatch of ground lines with the mesh as small as pos-
sible.
If a single unbroken plane can be devoted to the ground
system, EMI can usually be sufficiently suppressed by using
ferrite beads on suspect EMI paths and decoupling with ade-
quate values of capacitors. The value of the decoupling
capacitor depends on the frequency and amplitude of the
offending signals. Ferrite beads are available in a wide vari-
ety of shape, size and material to fit virtually any application.
Choose a ferrite bead for desired impedance at the de-
sired frequency and construct a low pass filter using one or
more appropriate capacitors in a “L”, “T” or “PI” arrangement.
Use only capacitors of low inductive and resistive properties
such as ceramic or mica. Install filters in series with each IC
pin suspected of contributing offending EMI signals and as
close to the pin as possible. Analysis using a spectrum ana-
lyzer can help determine which pins are suspect.
Where PC board costs constrain the number of layers
available, and if the EMI frequencies are far removed from
the frequencies of operation, ferrite beads and decoupling
capacitors may still be effective in reducing EMI emissions.
Where only two (or in some cases, only one!) layer is used,
the ground system is always reactive and poses an EMI
problem. If the offending EMI and normal operating frequen-
cy differ sufficiently, filtering can still work.
An “island” is constructed in the ground system for the digi-
tal device using ferrite beads and decoupling capacitors as
shown by the example in Figure 6. The ground must be cut
so that the digital ground for the device is isolated from the
rest of the ground system. Next choose a ferrite bead of the
appropriate value. Install this bead between the isolated
ground and the ground system. Install low pass filters in all
suspect lines with the capacitor closest to the device pin con-
nected to the isolated ground in all signal lines where EMI is
suspect. Also cut the power to the device and insert a ferrite
bead as shown in Figure 6. Finally, decouple the device be-
tween the power pin(s) and isolated ground pin(s) using a
low inductive/resistive capacitor of adequate value.
The methods described above will work acceptably when
the EMI frequency and the frequency of operation of the de-
vice generating the EMI differ greatly. Where the EMI is dis-
turbing the high VHF or UHF channels and the device
generating the EMI is operating within the NTSC/PAL band-
width, the energy contained in the harmonics generating the
EMI is situated well above the operating frequency and
suppressing this type of EMI poses no great problem. How-
ever, if the EMI is present on low VHF channels and/or the
operation of the device is outside the NTSC/PAL bandwidth,
such as a 2X pixel clock or 4xFSC oscillator, compromise be-
tween video quality and suppression complexity is usually re-
quired to obtain an acceptable solution. For those cases
where the operating frequency of the device is very near the
frequency of the EMI disturbance, careful attention to PCB
layout, multiple layer PCB and even shielding may be neces-
sary to obtain an acceptable design.


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