LMH6572_05 Datasheet, PDF (8/12 Pages) National Semiconductor (TI) – Triple 2:1 High Speed Video Multiplexer

Application Notes
GENERAL INFORMATION
The LMH6572 is a high-speed triple 2:1 multiplexer, opti-
mized for very high speed and low distortion. With a fixed
gain of 2 and excellent AC performance, the LMH6572 is
ideally suited for switching high resolution, presentation
grade video signals. The LMH6572 has no internal ground
reference. Single or split supply configurations are both pos-
sible. The LMH6572 features very high speed channel
switching and disable times. When disabled the LMH6572
output is high impedance, making multiplexer expansion
possible by combining multiple devices.
SINGLE SUPPLY OPERATION
The LMH6572 uses mid-supply referenced circuits for the
select and disable pins. In order to use the LMH6572 in
single supply configuration, it is necessary to use a circuit
similar to Figure 2. In this configuration the logical inputs are
compatible with high breakdown open collector TTL, or open
drain CMOS logic. In addition, the default logic state is
reversed since there is a pull-up resistor on those pins.
Single supply operation also requires the input to be biased
to within the common mode input range of roughly ±2V from
the mid-supply point.
FIGURE 1. Typical Application
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VIDEO PERFORMANCE
The LMH6572 has been designed to provide excellent per-
formance with production quality video signals in a wide
variety of formats such as HDTV and High Resolution VGA.
Best performance will be obtained with back-terminated
loads. The back termination reduces reflections from the
transmission line and effectively masks transmission line
and other parasitic capacitances from the amplifier output
stage.Figure 1 shows a typical configuration for driving a
75Ω cable. The output buffer is configured for a gain of 2, so
using back terminated loads will give a net gain of 1.
FIGURE 2. Single Supply Application
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GAIN ACCURACY
The gain accuracy of the LMH6572 is accurate to ±0.5%
(0.3% typical) and stable over temperature. The internal gain
setting resistors, R_F and R_G, match very well; however, over
process and temperature their absolute value will change.
EXPANDING THE MULTIPLEXER
It is possible to build higher density multiplexers by parallel-
ing several LMH6572s. Figure 3 shows a 4:1 RGB MUX
using two LMH6572s:
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