LMH6723MA National Semiconductor, LMH6723MA Datasheet - Page 13

• Manufacturer Part Number: LMH6723MA
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMH6723MA

• No. Of Amplifiers: 1
• Slew Rate: 600V/µs
• No. Of Pins: 8
• Peak Reflow Compatible (260 C): No
• Input Bias Current: 5000µA
• Supply Voltage Max: 12V
• Leaded Process Compatible: No
• No. Of Channels: 1
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Section
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor as shown in Figure 5. The
charts "Suggested R
value for selecting 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 re-
sponse. This gives a good compromise between settling
time and bandwidth. For applications where maximum fre-
quency response is needed and some peaking is tolerable,
the value of R
mended 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 R
loop as shown in Figure 6. This will preserve gain accuracy,
but will still limit maximum output voltage swing.
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was
not intentionally added. It is produced through electrical
interaction between conductors and can be reduced but
never entirely eliminated. Most parasitic capacitances that
cause problems are related to board layout or lack of termi-
nation on transmission lines. Please see the section on
Layout Considerations for hints on reducing problems due to
parasitic capacitances on board traces. Transmission 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.
FIGURE 6. Series Output Resistor inside feedback loop
FIGURE 5. Decoupling Capacitive Loads
OUT
can be reduced slightly from the recom-
L
OUT
<
150Ω).
vs. Cap Load" give a recommended
(Continued)
OUT
inside the feedback
G
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appear
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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
Capacitive Loads" will help.
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 optimize performance from this circuit. The
first thing to note is that the buffers need slightly higher value
feedback resistors than if the amplifiers were individually
configured. As well, R
compensation to further improve stability. The composite
amplifier has approximately twice the phase delay of a single
circuit. The larger values of R
high frequency attenuation provided by C
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 in Figure 7 has a gain of 5. The
frequency response of this circuit is shown in Figure 8.
FIGURE 7. High Output Current Composite Amplifier
4
, R
5
, R
6
, and R
11
and C
7
are necessary to ensure even
1
8
, R
provide mid circuit frequency
9
and R
1
10
and R
, as well as the
www.national.com
11
, ensure
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