Datasheet

+
-
R
IN
51:
R
F
-
+
R
OUT
51:
R
G
C
L
10pF
R
L
1k:
LMH6609
SNOSA84F –AUGUST 2003–REVISED MARCH 2013 www.ti.com
When driving a capacitive load or coaxial cable, include a series resistance R
OUT
to back match or improve
settling time. Refer to the Driving Capacitive Loads section for guidance on selecting an output resistor for driving
capacitive loads.
EVALUATION BOARDS
TI offers the following evaluation boards as a guide for high frequency layout and as an aid in device testing and
characterization. Many of the data sheet plots were measured with these boards.
Device Package Board Part #
LMH6609MA SOIC LMH730227
LMH6609MF SOT-23 LMH730216
CIRCUIT LAYOUT CONSIDERATION
A proper printed circuit layout is essential for achieving high frequency performance. TI 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 current path at the supply pins. They also provide high frequency filtering on the power supply traces.
10μF tantalum and .01μF capacitors are recommended on both supplies (from supply to ground). In addition, a
0.1μF ceramic capacitor can be added from V
+
to V
−
to aid in second harmonic suppression.
Figure 36. 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 36 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 values suggested in the charts are selected for .5dB or less of peaking in the
frequency response. This gives a good compromise between settling time and bandwidth. For applications 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 amplitude lost in the series resistor unless the gain is adjusted to
compensate; this effect is most noticeable with heavy resistive loads.
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