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LMH6715QML Datasheet, PDF (12/20 Pages) Texas Instruments – Dual Wideband Video Op Amp
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Application Introduction
Offered in an 8-pin package for reduced space and cost, the
wideband LMH6715 dual current-feedback op amp provides
closely matched DC and AC electrical performance charac-
teristics making the part an ideal choice for wideband signal
processing. Applications such as broadcast quality video sys-
tems, IQ amplifiers, filter blocks, high speed peak detectors,
integrators and transimedance amplifiers will all find superior
performance in the LMH6715 dual op amp.
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 (RF). The Electrical Characteristics and Typical
Performance plots specify an RF of 500Ω, a gain of +2V/V and
±5V power supplies (unless otherwise specified). Generally,
lowering RF from it's recommended value will peak the fre-
quency response and extend the bandwidth while increasing
the value of RF will cause the frequency response to roll off
faster. Reducing the value of RF too far below it's recom-
mended value will cause overshoot, ringing and, eventually,
oscillation.
Frequency Response vs. RF
RF vs. Non-Inverting Gain
20151821
Both plots show the value of RF approaching a minimum value
(dashed line) at high gains. Reducing the feedback resistor
below this value will result in instability and possibly oscilla-
tion. The recommended value of RF is depicted by the solid
line, which begins to increase at higher gains. The reason that
a higher RF is required at higher gains is the need to keep
RG from decreasing too far below the output impedance of the
input buffer. For the LMH6715 the output resistance of the
input buffer is approximately 160Ω and 50Ω is a practical low-
er limit for RG. Due to the limitations on RG the LMH6715
begins to operate in a gain bandwidth limited fashion for gains
of ±5V/V or greater.
RF vs. Inverting Gain
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The plot labeled “Frequency Response vs. RF” shows the
LMH6715's frequency response as RF is varied (RL = 100Ω,
AV = +2). This plot shows that an RF of 200Ω results in peaking
and marginal stability. An RF of 300Ω gives near maximal
bandwidth and gain flatness with good stability, but with very
light loads (RL > 300Ω) the device may show some peaking.
An RF of 500Ω gives excellent stability with good bandwidth
and is the recommended value for most applications. Since
all applications are slightly different it is worth some experi-
mentation to find the optimal RF for a given circuit. For more
information see Application Note OA-13 which describes the
relationship between RF and closed-loop frequency response
for current feedback operational amplifiers.
When configuring the LMH6715 for gains other than +2V/V,
it is usually necessary to adjust the value of the feedback re-
sistor. The two plots labeled “RF vs. Non-inverting Gain” and
“RF vs. Inverting Gain” provide recommended feedback re-
sistor values for a number of gain selections.
20151822
When using the LMH6715 as a replacement for the CLC412,
identical bandwidth can be obtained by using an appropriate
value of RF . The chart “Frequency Response vs. RF” shows
that an RF of approximately 700Ω will provide bandwidth very
close to that of the CLC412. At other gains a similar increase
in RF can be used to match the new and old parts.
CIRCUIT LAYOUT
With all high frequency devices, board layouts with stray ca-
pacitances have a strong influence over AC performance.
The LMH6715 is no exception and its input and output pins
are particularly sensitive to the coupling of parasitic capaci-
tances (to AC ground) arising from traces or pads placed too
closely (<0.1”) to power or ground planes. In some cases, due
to the frequency response peaking caused by these para-
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