LMH6720MA/NOPB National Semiconductor, LMH6720MA/NOPB Datasheet - Page 9

• Manufacturer Part Number: LMH6720MA/NOPB
• Description: IC OPAMP SNGL 400MHZ W/SD 8SOIC
• Manufacturer: National Semiconductor
• Series: PowerWise®, VIP10™r

Specifications of LMH6720MA/NOPB

• Applications: Current Feedback
• Number Of Circuits: 1
• -3db Bandwidth: 400MHz
• Slew Rate: 1800 V/µs
• Current - Supply: 5.6mA
• Current - Output / Channel: 70mA
• Voltage - Supply, Single/dual (±): 8 V ~ 12.5 V, ±4 V ~ 6.25 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• For Use With: LMH730227 - BOARD EVALUATION FOR SOIC PKGLMH730165 - BOARD EVALUATIONLMH730216 - BOARD EVAL HS MONO AMP SOT23
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LMH6720MA/NOPB

Application Section
FEEDBACK RESISTOR SELECTION
One of the key benefits of a current feedback operational am-
plifier is the ability to maintain optimum frequency response
independent of gain by using appropriate values for the feed-
back resistor (R
Performance plots specify an R
±5V power supplies (unless otherwise specified). Generally,
lowering R
quency response and extend the bandwidth while increasing
the value of R
faster. Reducing the value of R
mended value will cause overshoot, ringing and, eventually,
oscillation.
The plot labeled "Frequency Response vs. R
LMH6714/LMH6720/LMH6722's frequency response as R
varied (R
147Ω results in peaking. An R
bandwidth and gain flatness with good stability. An R
400Ω gives excellent stability with only a small bandwidth
penalty. Since all applications are slightly different it is worth
some experimentation to find the optimal R
Note that it is not possible to use a current feedback amplifier
with the output shorted directly to the inverting input. The
buffer configuration of the LMH6714/LMH6720/LMH6722 re-
quires a 600Ω feedback resistor for stable operation.
For more information see Application Note OA-13 which de-
scribes the relationship between R
cy response for current feedback operational amplifiers. The
value for the inverting input impedance for the LMH6714/
LMH6720/LMH6722 is approximately 180Ω. The LMH6714/
LMH6720/LMH6722 is designed for optimum performance at
gains of +1 to +6 V/V and −1 to −5V/V. When using gains of
±7V/V or more the low values of R
verting input impedances very low.
When configuring the LMH6714/LMH6720/LMH6722 for
gains other than +2V/V, it is usually necessary to adjust the
value of the feedback resistor. The two plots labeled “R
Non-inverting Gain” and “R
ommended feedback resistor values for a number of gain
selections.
FIGURE 1. Frequency Response vs. R
L
F
= 100Ω, A
from it's recommended value will peak the fre-
F
F
will cause the frequency response to roll off
). The Electrical Characteristics and Typical
V
= +2). This plot shows that an R
F
vs. Inverting Gain” provide rec-
F
F
of 300Ω gives near maximal
of 300Ω, a gain of +2V/V and
F
F
too far below it's recom-
and closed-loop frequen-
G
required will make in-
F
for a given circuit.
F
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" shows the
F
F
F
F
F
vs.
of
of
is
9
In the “R
Gain” charts the recommended value of R
solid line, which starts high, decreases to 200Ω and begins
increasing again. The reason that a higher R
higher gains is the need to keep R
below the output impedance of the input buffer. For the
LMH6714/LMH6720/LMH6722 the output resistance of the
input buffer is approximately 180Ω and 50Ω is a practical low-
er limit for R
LMH6720/LMH6722 begins to operate in a gain bandwidth
limited fashion for gains of ±5V/V or greater.
ACTIVE FILTERS
When using any current feedback Operational Amplifier as an
active filter it is important to be very careful when using reac-
tive components in the feedback loop. Anything that reduces
the impedance of the negative feedback, especially at higher
frequencies, will almost certainly cause stability problems.
Likewise capacitance on the inverting input needs to be avoid-
ed. See Application Notes OA-7 and OA-26 for more infor-
mation on Active Filter applications for Current Feedback Op
Amps.
F
FIGURE 2. R
vs. Non-Inverting Gain” and the “R
G
FIGURE 3. R
. Due to the limitations on R
F
vs. Non-Inverting Gain
F
vs. Inverting Gain
G
from decreasing too far
F
is depicted by the
G
F
the LMH6714/
F
is required at
vs. Inverting
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