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參數資料
| 型號： | AD8176 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 475 MHz, Triple 16 】 9 Video Crosspoint Switch |
| 中文描述： | 475兆赫，三16】9視頻交叉點開關 |
| 文件頁數： | 30/32頁 |
| 文件大小： | 489K |
| 代理商： | AD8176 |
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AD8176
Preliminary Technical Data
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 termination 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
where:
R
S
is the source resistance.
C
M
is the mutual capacitance between the test signal circuit and
the selected circuit.
s
is the Laplace transform variable.
From Equation 9, it can be observed that this crosstalk
mechanism has a high-pass nature; it can also be minimized by
reducing the coupling capacitance 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 AD8176 is specified with excellent
settling time when driving a properly terminated CAT5, 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 bond wires of the
AD8176.
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
Rev. PrA | Page 30 of 32
×
=
(9)
×
=
L
XY
R
s
M
XT
10
log
20
(10)
where:
M
XY
is the mutual inductance of output X to output Y.
R
L
is the load resistance on the measured output.
This crosstalk mechanism can be minimized by keeping the
mutual inductance low and increasing R
L
. 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 AD8176 is designed to help keep the
crosstalk to a minimum. On the BGA substrate, each pair is
carefully routed to predominately couple to each other, with
shielding traces separating adjacent signal pairs. The ball grid
array is arranged such that similar board routing can be achieved.
Input and output differential 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 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 AD8176 is large enough that using high
impedance 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 applica-
tion and properly terminate the input and output signals of the
AD8176. 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- cabling is usually driven as
differential pairs of 100 Ω differential impedance.
For flexibility, the AD8176 does not contain on-chip
termination resistors. This flexibility in application comes with
some board layout challenges. The distance between the
termination of the input transmission line and the AD8176 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 on the propagation speed (V
P
) of the
signal in the chosen board material and the distance (d)
between the termination resistor and the AD8176. If the
distance is great enough, these peaks can occur in-band. In fact,
practical experience shows that these peaks are not high-Q, and
should be pushed out to three or four times the desired
bandwidth in order to not have an effect on the signal. For a
board designer using FR4 (V
P
= 144 × 10
6
m/s), this means the
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| AD8176ABPZ | 功能描述:IC VIDEO CROSSPOINT SWIT 676BGA RoHS:是 類別:集成電路 (IC) >> 接口 - 模擬開關，多路復用器，多路分解器 系列:- 其它有關文件:STG4159 View All Specifications 標準包裝:5,000 系列:- 功能:開關 電路:1 x SPDT 導通狀態電阻:300 毫歐 電壓電源:雙電源 電壓 - 電源，單路/雙路(±):±1.65 V ~ 4.8 V 電流 - 電源:50nA 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:7-WFBGA，FCBGA 供應商設備封裝:7-覆晶 包裝:帶卷 (TR) |
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