ISL59530 Datasheet, PDF (19/22 Pages) Intersil Corporation – 16x16 Video Crosspoint

ISL59530
For this reason, the ISL59530 must be in DC-coupled
mode (Clamp Disabled) to be compatible with s-video
and component video signals.
Bandwidth Considerations
Wide frequency response (high bandwidth) in a video
system means better video resolution. Four sets of
frequency response curves are shown in Figure 47.
Depending on the switch configurations, and the routing (the
path from the input to the output), bandwidth can vary
between 100MHz and 350MHz. A short discussion of the
trade-offs — including matrix configuration, output buffer
gain selection, channel selection, and loading — follows.
2
Mux, Av = 2
0
Mux, Av = 1
-2
Broadcast,
Av = 2
Broadcast,
-4
Av = 1
-6
-8
-10
1
10
100
Frequency [MHz]
1000
FIGURE 47. FREQUENCY RESPONSE FOR VARIOUS MODES
In multiplexer mode, one input typically drives one output
channel, while in broadcast mode, one input drives all 16
outputs. As the number of outputs driven increases, the
parasitic loading on that input increases. Broadcast Mode is
the worst-case, where the capacitance of all 16 channels
loads one input, reducing the overall bandwidth. In addition,
due to internal device compensation, an output buffer gain of
x2 has higher bandwidth than a gain of x1. Therefore, the
highest bandwidth configuration is multiplexer mode (with
each input mapped to only one output) and an output buffer
gain of x2.
The relative locations of the input and output channels also
have significant impact on the device bandwidth (due to the
layout of the ISL59530 silicon). When the input and output
channels are further away, there are additional parasitics as
a result of the additional routing, resulting in lower
bandwidth.
The bandwidth does not change significantly with resistive
loading as shown in the typical performance curves.
However several of the curves demonstrate that frequency
response is sensitive to capacitance loading. This is most
significant when laying out the PCB. If the PCB trace length
between the output of the crosspoint switch and the back-
termination resistor is not minimized, the additional parasitic
capacitance will result in some peaking and eventually a
reduction in overall bandwidth.
Linear Operating Region
In addition to bandwidth optimization, to get the best linearity
the ISL59530 should be configured to operate in its most
linear operating region. Figure 48 shows the differential gain
curve. The ISL59530 is a single supply 5V design with its
most linear region between 0.1 and 2V. This range is fine for
most video signals whose nominal signal amplitude is 1V.
The most negative input level (the sync tip for composite
video) should be maintained at 0.3V or above for best
operation.
FIGURE 48. DIFFERENTIAL GAIN RESPONSE
In a DC-coupled application, it is the system designer’s
responsibility to ensure that the video signal is always in the
optimum range.
When AC coupling, the ISL59530’s DC restore function
automatically adjusts the DC level so that the most negative
portion of the video is always equal to V_REF.
A discussion of the benefits of the DC-restored system
begins by understanding the block diagram of a typical DC-
restore circuit (Figure 49). It consists of 4 sections: an AC
coupling (DC blocking) capacitor at the input, an opamp, a
FET switch, and a current source. In the absence of an input
signal, R_TERM pulls the input node to ground. The 2µA
current source slowly drains the input capacitor of charge,
slowly lowering V_OUT. However when V_OUT goes below
V_REF, Q1 turns on, sourcing current into the capacitor until
V_OUT is equal to V_REF, at which point Q1 will turn off. So
with no V_IN signal, the voltage at the noninverting input of
the opamp will settle to approximately V_REF, with Q1
sourcing the same 2µA as the current source.
19
FN6220.1
June 12, 2006