LMH6723MF National Semiconductor, LMH6723MF Datasheet - Page 14

• Manufacturer Part Number: LMH6723MF
• Description: OP AMP, 1MA WIDEBAND CFB, POWERWISE
• Manufacturer: National Semiconductor

Specifications of LMH6723MF

• Op Amp Type: Wideband
• No. Of Amplifiers: 1
• Bandwidth: 370MHz
• Slew Rate: 600V/µs
• Supply Voltage Range: 4.5V To 12V
• Amplifier Case Style: SOT-23
• No. Of Pins: 5
• Operating Temperature Range: -40°C To
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Application Section
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R
of a series output resistor, R
output under capacitive loading. The charts "Suggested
R
a series output resistor for mitigating capacitive loads. The
values suggested in the charts are selected for .5 dB or less
of peaking in the frequency response. This gives a good
compromise between settling time and bandwidth. For appli-
cations where maximum frequency response is needed and
some peaking is tolerable, the value of R
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 loads (R
An alternative approach is to place Rout inside the feedback
loop as shown in Figure 6. This will preserve gain accuracy,
but will still limit maximum output voltage swing.
FIGURE 6. Series Output Resistor inside feedback loop
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was
not intentionally added. It comes about from electrical inter-
action between conductors. Parasitic capacitance can be
reduced but never entirely eliminated. Most parasitic capaci-
tances that cause problems are related to board layout or
lack of termination on transmission lines. Please see the
section on Layout Considerations for hints on reducing prob-
lems due to parasitic capacitances on board traces. Trans-
mission lines should be terminated in their characteristic
impedance at both ends.
High speed amplifiers are sensitive to capacitance between
the inverting input and ground or power supplies. This shows
up as gain peaking at high frequency. The capacitor raises
device gain at high frequencies by making R
smaller. Capacitive output loading will exaggerate this effect.
One possible remedy for this effect is to slightly increase the
value of the feedback (and gain set) resistor. This will tend to
offset the high frequency gain peaking while leaving other
parameters relatively unchanged. If the device has a capaci-
tive load as well as inverting input capacitance using a series
output resistor as described in the section on "Driving Ca-
pacitive Loads" will help.
OUT
vs. Cap Load" give a recommended value for selecting
L
<
150Ω).
OUT
OUT
, to stabilize the amplifier
. Figure 5 shows the use
(Continued)
OUT
can be reduced
G
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When higher currents are required than a single amplifier
can provide, the circuit of Figure 7 can be used. Although the
example circuit was intended for the LMH6725 quad op amp,
higher thermal efficiency can be obtained by using four
separate SOIC op amps. Careful attention to a few key
components will help get good performance from this circuit.
The first thing to note is that the buffers need slightly higher
value feedback resistors than if the amplifiers were individu-
ally configured. As well, R
frequency compensation to further improve stability. The
composite amplifier has approximately twice the phase delay
of a single circuit. The larger values of R
as the high frequency attenuation provided by C
ensure that the circuit does not oscillate.
Resistors R
current distribution between the amplifiers. Since they are
inside the feedback loop they have no effect on the gain of
the circuit. The circuit shown has a gain of 5. The frequency
response is shown in Figure 8.
FIGURE 8. Composite Amplifier Frequency Response
FIGURE 7. High Output Current Composite Amplifier
4
, R
5
, R
6
and R
7
11
are necessary to ensure even
and C
1
8
provide mid circuit
, R
9
and R
1
10
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and R
as well
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