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參數(shù)資料
| 型號： | AD8178ABPZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 29/40頁 |
| 文件大小： | 0K |
| 描述： | IC VIDEO CROSSPOINT SWIT 676BGA |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 功能： | 視頻交叉點(diǎn)開關(guān) |
| 電路： | 3 x 16:5 |
| 電壓電源： | 單/雙電源 |
| 電壓 - 電源，單路/雙路(±)： | 4.5 V ~ 5.5 V，±2.5V |
| 電流 - 電源： | 460mA |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 676-BGA |
| 供應(yīng)商設(shè)備封裝： | 676-BGA（27x27） |
| 包裝： | 管件 |
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AD8178
Rev. 0 | Page 35 of 40
is programmed to drive OUTPUT2, no signal should be present.
Any signal that is present can be attributed to the other 15 hostile
input signals because no other outputs are driven (they are all
disabled). Thus, this method measures the all hostile input
contribution to crosstalk into INPUT7. Of course, the method
can be used for other input channels and combinations of
hostile inputs.
For output crosstalk measurement, a single input channel is
driven (INPUT0, for example) and all outputs other than
a given output (OUTPUT2 in the middle) are programmed to
connect to INPUT0. OUTPUT2 is programmed to connect to
INPUT15 (far away from INPUT0), which is terminated to
ground. Thus, OUTPUT2 should not have a signal present because
it is listening to a quiet input. Any signal measured at OUTPUT2
can be attributed to the output crosstalk of the other eight hostile
outputs. Again, this method can be modified to measure other
channels and other crosspoint matrix combinations.
Effect of Impedances on Crosstalk
The input side crosstalk can be influenced by the output
impedance of the sources that drive the inputs. The lower the
impedance of the drive source, the lower the magnitude of the
crosstalk. The dominant crosstalk mechanism on the input side
is capacitive coupling. The high impedance inputs do not have
significant current flow to create magnetically induced crosstalk.
However, significant current can flow through the input termi-
nation resistors and the loops that drive them. Thus, the PC board
on the input side can contribute to magnetically coupled crosstalk.
From a circuit standpoint, the input crosstalk mechanism looks
like a capacitor coupling to a resistive load. For low frequencies,
the magnitude of the crosstalk is given by
[
s
C
R
XT
M
S
×
=
)
(
log
20
10
]
(9)
where:
RS is the source resistance.
CM is the mutual capacitance between the test signal circuit and
the selected circuit.
s is the Laplace transform variable.
Equation 9 illustrates that this crosstalk mechanism has a high-pass
nature; it can also be minimized by reducing the coupling capaci-
tance of the input circuits and lowering the output impedance
of the drivers. If the input is driven from a 75 Ω terminated cable,
the input crosstalk can be reduced by buffering this signal with
a low output impedance buffer.
On the output side, the crosstalk can be reduced by driving
a lighter load. Although the AD8178 is specified with excellent
settling time when driving a properly terminated Cat-5, the
crosstalk is higher than the minimum obtainable due to the
high output currents. These currents induce crosstalk via the
mutual inductance of the output pins and the bond wires of
the AD8178.
From a circuit standpoint, this output crosstalk mechanism
looks like a transformer with a mutual inductance between the
windings that drives a load resistor. For low frequencies, the
magnitude of the crosstalk is given by
×
=
L
XY
R
s
M
XT
10
log
20
(10)
where:
MXY is the mutual inductance of output X to output Y.
RL is the load resistance on the measured output.
This crosstalk mechanism can be minimized by keeping the
mutual inductance low and increasing RL. The mutual
inductance can be kept low by increasing the spacing of the
conductors and minimizing their parallel length.
PCB Layout
Extreme care must be exercised to minimize additional crosstalk
generated by the system circuit board(s). The areas that must be
carefully detailed are grounding, shielding, signal routing, and
supply bypassing.
The packaging of the AD8178 is designed to help keep crosstalk
to a minimum. On the BGA substrate, each pair is carefully routed
to predominately couple to each other, with shielding traces sepa-
rating adjacent signal pairs. The ball grid array is arranged such
that similar board routing can be achieved. Input and output differ-
ential pairs are grouped by channel rather than by color to allow
for easy, convenient board routing.
The input and output signals have minimum crosstalk if they
are located between ground planes on layers above and below,
and are separated by ground in between. Vias should be located
as close to the IC as possible to carry the inputs and outputs to
the inner layer. The input and output signals surface at the input
termination resistors and the output series back-termination
resistors. To the extent possible, these signals should also be
separated as soon as they emerge from the IC package.
PCB Termination Layout
As frequencies of operation increase, the importance of proper
transmission line signal routing becomes more important. The
bandwidth of the AD8178 is large enough that using high imped-
ance routing does not provide a flat in-band frequency response
for practical signal trace lengths. It is necessary for the user to
choose a characteristic impedance suitable for the application and
properly terminate the input and output signals of the AD8178.
Traditionally, video applications have used 75 Ω single-ended
environments. RF applications are generally 50 Ω single-ended
(and board manufacturers have the most experience with this
application). Cat-5 cabling is usually driven as differential pairs
of 100 Ω differential impedance.
For flexibility, the AD8178 does not contain on-chip termina-
tion resistors. This flexibility in application comes with some
board layout challenges. The distance between the termination
of the input transmission line and the AD8178 die is a high
impedance stub and causes reflections of the input signal. With
some simplification, it can be shown that these reflections cause
peaking of the input at regular intervals in frequency, dependent
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