AN504 Datasheet, PDF (1/12 Pages) Vishay Siliconix – Video Crosspoint Switch Simplifies Large Matrix Designs

AN504
Vishay Siliconix
Video Crosspoint Switch Simplifies Large Matrix Designs
The DG884 is a digitally selectable eight-input to four-output
monolithic bidirectional crosspoint IC suitable for wideband
analog and digital signal routing. Wideband here is used to
define signals with bandwidths in the range of 60 to 200 MHz.
Fabricated with the Vishay Siliconix D/CMOS technology, the
DG884 incorporates n-channel DMOS switching FETs with
low-power CMOS control logic, drivers, and latches. The
low-capacitance DMOS FETs are connected as low
on-resistance T-switches to achieve high levels of off-isolation
and low levels of crosstalk. On-chip TTL-compatible address
latches and data readback are included to simplify
microprocessor interfacing. Double buffering of the switch
addressing allows update information to be loaded without
affecting the existing state of the crosspoint. A single SALVO
command is all that is required to transfer previously set
information to the current event latches. This single command
significantly speeds up the setup time, especially in large
matrices requiring more than one DG884.
The Signal Path
The internal function blocks of the DG884 are shown in
Figure 1, and the signal path from one input to one output is
shown in Figure 2. The signal path is composed of a T-switch
at the matrix and an additional low-resistance switch in series
with each output. The T-switch connection maximizes the
off-isolation by shunting to ground any signal that feeds
through the small series capacitances of the off series
switches. The series output switches offer a number of
advantages, namely:
D They provide an extra stage of isolation at the output
D They reduce the off-state output capacitance, which is
necessary when other DG884 outputs are connected in
parallel.
D They reduce the off-output leakage.
IN₁ IN₂ IN₃ IN₄ IN₅ IN₆ IN₇ IN₈
8 x 4 Switch Matrix
Decode Logic, Switch Drivers
WR
CS
I/O Control
B₁
Logic
B₀
Current Event Latches
Next Event Latches
I/O A₃ A₂ A₁ A₀
FIGURE 1. DG884 Block Diagram
Document Number: 70610
05-Aug-99
OUT₁
OUT₂
OUT₃
OUT₄
4 Disable Outputs
(Quad Package
Only)
RS
SALVO
Signal Path Input to Output
SW1 SW2
Input
1 of 8
SW3
SW4
Output
1 of 4
From Input 2 of 8
From Input 8 of 8
MUX Common
Typical on-resistances at 25_C:
SW1 = SW2 = 20 W
SW3 = 200 W
SW4 = 10 W
FIGURE 2. DG884 Signal Path
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