LMH6609 Datasheet, PDF (11/15 Pages) National Semiconductor (TI) – 900MHz Voltage Feedback Op Amp

Application Section (Continued)
20079030
FIGURE 4. AC Coupled Non-Inverting
GAIN BANDWIDTH PRODUCT
The LMH6609 is a voltage feedback amplifier, whose
closed-loop bandwidth is approximately equal to the gain-
bandwidth product (GBP) divided by the gain (A_V). For gains
greater than 5, A_V sets the closed-loop bandwidth of the
LMH6609.
time. Refer to the Driving Capacitive Loads section for guid-
ance on selecting an output resistor for driving capacitive
loads.
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
LMH6609MA
LMH6609MF
Package
SOIC
SOT-23
Board Part #
CLC730227
CLC730216
A free evaluation board is automatically shipped when a
sample request is placed with National Semiconductor.
CIRCUIT LAYOUT CONSIDERATION
A proper printed circuit layout is essential for achieving high
frequency performance. National provides evaluation boards
for the LMH6609 as shown above. These boards were laid
out for optimum, high-speed performance. The ground plane
was removed near the input and output pins to reduce
parasitic capacitance. Also, all trace lengths were minimized
to reduce series inductances.
Supply bypassing is required for the amplifiers performance.
The bypass capacitors provide a low impedance return cur-
rent path at the supply pins. They also provide high fre-
quency filtering on the power supply traces. 10µF tantalum
and .01µF capacitors are recommended on both supplies
(from supply to ground). In addition a .1µF ceramic capacitor
can be added from V⁺ to V^− to aid in second harmonic
suppression.
20079031
For Gains less than 5, refer to the frequency response plots
to determine maximum bandwidth. For large signal band-
width the slew rate is a more accurate predictor of band-
width.
20079032
Where f_MAX = bandwidth, S_R = Slew rate and V_P = peak
amplitude.
OUTPUT DRIVE AND SETTLING TIME PERFORMANCE
The LMH6609 has large output current capability. The
100mA of output current makes the LMH6609 an excellent
choice for applications such as:
• Video Line Drivers
• Distribution Amplifiers
When driving a capacitive load or coaxial cable, include a
series resistance R_OUT to back match or improve settling
20079033
FIGURE 5. Driving Capacitive Loads with R_OUT for
Improved Stability
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R_OUT. Figure 5 shows the use
of a series output resistor, R_OUT as it might be applied when
driving an analog to digital converter. The charts "Suggested
R_O vs. Cap Load" in the Typical Performance Section give a
recommended value for mitigating capacitive loads. The val-
ues suggested in the charts are selected for .5dB or less of
peaking in the frequency response. This gives a good com-
promise between settling time and bandwidth. For applica-
tions where maximum frequency response is needed and
some peaking is tolerable, the value of R_O can be reduced
slightly from the recommended values. There will be ampli-
tude lost in the series resistor unless the gain is adjusted to
compensate; this effect is most noticeable with heavy resis-
tive loads.
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