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 MUX expansion possible
by combining multiple devices.
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 75.
Cable. The output buffer is configured for a gain of 2, so
using back terminated loads will give a net gain of 1.
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.
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.
EVALUATION BOARDS
National Semiconductor provides the following evaluation
boards as a guide for high frequency layout and as an aid in
device testing and characterization. Many of the datasheet
plots were measured with these boards.
Device
Package
Evaluation Board
Part Number
LMH730151
LMH6572
TSSOP
An evaluation board is shipped when a sample request is
placed with National Semiconductor.
MULTIPLEXER EXPANSION
With the Enable or the Select pins putting the output stage
into a high impedance state, several LMH6572’s can be tied
together to form a larger input MUX. However, there is a
slight loading effect on the active output caused by the
off-channel feedback and gain set resistors, as shown in
Figure 3
below.
Figure 3
is assuming there are 4 LMH6572
outputs (2 LMH6572 devices) similar to the schematic of
Figure 4
. With the internal resistors valued at 800
, the
effect is rather slight. For the 4:1 MUX function shown in
Figure 3
, the gain error is only about -0.57 dB, or about 6%.
20109622
FIGURE 1. Typical Application
20109623
FIGURE 2. Single Supply Application
L
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