mc44251 Freescale Semiconductor, Inc, mc44251 Datasheet - Page 13

• Manufacturer Part Number: mc44251
• Description: Triple 8-bit Video Three-state Outputs
• Manufacturer: Freescale Semiconductor, Inc

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 is 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.
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.
sired frequency and construct a low pass filter using one or
more appropriate capacitors in a “L”, “T”, or “PI” arrange-
ment. 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 analyzer can help determine which pins are
suspect.
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.
tal device using ferrite beads and decoupling capacitors as
shown by the example in Figure 13. The ground must be cut
so that the digital ground for the device is isolated from the
MOTOROLA
If a single unbroken plane can be devoted to the ground
Choose a ferrite bead for desired impedance at the de-
Where PC board costs constrain the number of layers
An “island” is constructed in the ground system for the digi-
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 13. 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 EMI frequency and the frequency of operation of the
device generating the EMI differ greatly. Where the EMI is
disturbing 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.
APPLICATION CIRCUIT
will produce analog–to–digital conversion of either RGB in-
formation or YUV information by setting SW1. In the YUV
mode, SW1 is set either to position ‘1’ or to the open position
depending on the desired clamp level (see Table 1). The
RGB inputs then become YUV inputs and correspondingly,
the RGB outputs are YUV. For RGB operation, SW1 is set to
the ‘0’ position.
The methods described above will work acceptably when
Figure 14 shows a typical application circuit. This circuit
MC44251
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