LMH6645MF datasheet(16/19 Pages) NSC

16 / 19 page

Application Notes (Continued)

Figure 6 shows the MUX output when selecting between a

1MHz sine and a 250KHz triangular waveform.

As can be seen in

Figure 6, the output is well behaved and

there are no spikes or glitches due to the switching. Switch-

ing times are approximately around 500ns based on the time

when the output is considered “valid”.

Printed Circuit Board Layout, Component Values Selec-

tion, and Evaluation Boards:

Generally, a good high-frequency layout will keep power

supply and ground traces away from the inverting input and

output pins. Parasitic capacitances on these nodes to

ground will cause frequency response peaking and possible

circuit oscillations (see Application Note OA-15 for more

information).

Another

important

parameter

working

with

high

speed/high performance amplifiers, is the component values

selection. Choosing large valued external resistors, will ef-

fect the closed loop behavior of the stage because of the

interaction of these resistors with parasitic capacitances.

These capacitors could be inherent to the device or a

by-product of the board layout and component placement.

Either way, keeping the resistor values lower, will diminish

this interaction. On the other hand, choosing very low value

resistors could load down nodes and will contribute to higher

overall power dissipation.

National Semiconductor suggests the following evaluation

boards as a guide for high frequency layout and as an aid in

device testing and characterization:

Device

Package

Evaluation

Board PN

LMH6645MF

SOT23-5

CLC730068

LMH6645MA

8-Pin SOIC

CLC730027

LMH6646MA

8-Pin SOIC

CLC730036

LMH6646MM

8-Pin MSOP

CLC730123

LMH6647MA

8-Pin SOIC

CLC730027

LMH6647MF

SOT23-6

CLC730116

These free evaluation boards are shipped when a device

sample request is placed with National Semiconductor.

LMH6647 Evaluation:

For normal operation, tie the SD pin to V−.

20020235

FIGURE 6. 2:1 MUX Output

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