LMH6639_14 Datasheet, PDF (18/26 Pages) National Semiconductor (TI) – LMH6639 190MHz Rail-to-Rail Output Amplifier with Disable

LMH6639
SNOS989G – JANUARY 2002 – REVISED MARCH 2013
SD
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V⁺
+
-
+ _V+
-
V^{- SD}
LMH6639
Figure 50. Shutdown Interface (Single Supply, Open Collector Logic)
The LMH6639 has an internal pull-up resistor on SD such that if left un-connected, the device will be in normal
operation. Therefore, no pull-up resistor is needed on this pin. Another common application is where the
transistor in Figure 50 above, would be internal to an open collector (open drain) logic gate; the basic
connections will remain the same as shown.
PCB LAYOUT CONSIDERATION AND COMPONENTS SELECTION
Care should be taken while placing components on a PCB. All standard rules should be followed especially the
ones for high frequency and/ or high gain designs. Input and output pins should be separated to reduce cross-
talk, especially under high gain conditions. A groundplane will be helpful to avoid oscillations. In addition, a
ground plane can be used to create micro-strip transmission lines for matching purposes. Power supply, as well
as shutdown pin de-coupling will reduce cross-talk and chances of oscillations.
Another important parameter in working with high speed amplifiers is the component values selection. Choosing
high value resistances reduces the cut-off frequency because of the influence of parasitic capacitances. On the
other hand choosing the resistor values too low could "load down" the nodes and will contribute to higher overall
power dissipation. Keeping resistor values at several hundreds of ohms up to several kΩ will offer good
performance.
Texas Instruments suggests the following evaluation boards as a guide for high frequency layout and as an aid in
device testing and characterization:
Device
LMH6639MA
LMH6639MF
Package
8-Pin SOIC
SOT-23-6
Evaluation Board PN
CLC730027
CLC730116
18
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