LMH6583_14 Datasheet, PDF (13/24 Pages) Texas Instruments – LMH6583 16x8 550 MHz Analog Crosspoint Switch, Gain of 2

LMH6583
www.ti.com
SNOSAP5E – APRIL 2006 – REVISED MARCH 2013
In the example in Figure 38, the resistor R_L is required to provide a load for the crosspoint output buffer. Without
R_Lexcessive frequency response peaking is likely and settling times of transient signals will be poor. As the value
of R_L is reduced the bandwidth will also go down. The amplifier shown in the example is an LMH6703 this
amplifier offers high speed and flat bandwidth. Another suitable amplifiers is the LMH6702. The LMH6702 is a
faster amplifier that can be used to generate high frequency peaking in order to equalize longer cable lengths. If
board space is at a premium the LMH6739 or the LMH6734 are triple, selectable gain buffers which require no
external resistors.
CROSSTALK
When designing a large system such as a video router crosstalk can be a very serious problem. Extensive
testing in our lab has shown that most crosstalk is related to board layout rather than occurring in the crosspoint
switch. There are many ways to reduce board related crosstalk. Using controlled impedance lines is an important
step. Using well decoupled power and ground planes will help as well. When crosstalk does occur within the
crosspoint switch itself it is often due to signals coupling into the power supply pins. Using appropriate supply
bypassing will help to reduce this mode of coupling. Another suggestion is to place as much grounded copper as
possible between input and output signal traces. Care must be taken, though, not to influence the signal trace
impedances by placing shielding copper too closely. One other caveat to consider is that as shielding materials
come closer to the signal trace the trace needs to be smaller to keep the impedance from falling too low. Using
thin signal traces will result in unacceptable losses due to trace resistance. This effect becomes even more
pronounced at higher frequencies due to the skin effect. The skin effect reduces the effective thickness of the
trace as frequency increases. Resistive losses make crosstalk worse because as the desired signal is attenuated
with higher frequencies crosstalk increases at higher frequencies.
DIGITAL CONTROL
SWITCH
MATRIX
CFG
BCST
DATA IN
CS
CLK
MODE
136
CONFIGURATION
REGISTER
40
LOAD
REGISTER
RST
DATA OUT
Figure 39. Block Diagram
The LMH6583 has internal control registers that store the programming states of the crosspoint switch. The logic
is two staged to allow for maximum programming flexibility. The first stage of the control logic is tied directly to
the crosspoint switching matrix. This logic consists of one register for each output that stores the on/off state and
the address of which input to connect to. These registers are not directly accessible by the user. The second
level of logic is another bank of registers identical to the first, but set up as shift registers. These registers are
accessed by the user via the serial input bus. As described further below, there are two modes for programing
the LMH6582, SERIAL PROGRAMMING MODE and ADDRESSED PROGRAMMING MODE.
The LMH6583 is programmed via a serial input bus with the support of 4 other digital control pins. The Serial bus
consists of a clock pin (CLK), a serial data in pin (DIN), and a serial data out pin (D_OUT). The serial bus is gated
by a chip select pin (CS). The chip select pin is active low. While the chip select pin is high all data on the serial
input pin and clock pins is ignored. When the chip select pin is brought low the internal logic is set to begin
receiving data by the first positive transition (0 to 1) of the clock signal. The chip select pin must be brought low
at least 5 ns before the first rising edge of the clock signal. The first data bit is clocked in on the next negative
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