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

the mid supply point.

GAIN ACCURACY

(0.3% typical) and stable over temperature. The internal gain

setting resistors, R

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

LMH6572

TSSOP

LMH730151

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

Ω, 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

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