LMH6715_16 Datasheet, PDF (11/21 Pages) Texas Instruments – Dual Wideband Video Op Amp

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R_F vs. Inverting Gain
LMH6715
SNOSA10C – MAY 2002 – REVISED APRIL 2013
Figure 31.
When using the LMH6715 as a replacement for the CLC412, identical bandwidth can be obtained by using an
appropriate value of R_F . The chart “Frequency Response vs. R_F” (see Figure 29) shows that an R_F of
approximately 700Ω will provide bandwidth very close to that of the CLC412. At other gains a similar increase in
R_F can be used to match the new and old parts.
CIRCUIT LAYOUT
With all high frequency devices, board layouts with stray capacitances 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 capacitances (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 parasitics, a small
adjustment of the feedback resistor value will serve to compensate the frequency response. Also, it is very
important to keep the parasitic capacitance across the feedback resistor to an absolute minimum.
The performance plots in the data sheet can be reproduced using the evaluation boards available from Texas
Instruments. The LMH730036 board uses all SMT parts for the evaluation of the LMH6715. The board can serve
as an example layout for the final production printed circuit board.
Care must also be taken with the LMH6715's layout in order to achieve the best circuit performance, particularly
channel-to-channel isolation. The decoupling capacitors (both tantalum and ceramic) must be chosen with good
high frequency characteristics to decouple the power supplies and the physical placement of the LMH6715's
external components is critical. Grouping each amplifier's external components with their own ground connection
and separating them from the external components of the opposing channel with the maximum possible distance
is recommended. The input (R_IN) and gain setting resistors (R_F) are the most critical. It is also recommended that
the ceramic decoupling capacitor (0.1μF chip or radial-leaded with low ESR) should be placed as closely to the
power pins as possible.
POWER DISSIPATION
Follow these steps to determine the Maximum power dissipation for the LMH6715:
1. Calculate the quiescent (no-load) power: P_AMP = I_CC (V_CC - V_EE)
2. Calculate the RMS power at the output stage: P_O = (V_CC -V_LOAD)(I_LOAD), where V_LOAD and I_LOAD are the voltage
and current across the external load.
3. Calculate the total RMS power: Pt = P_AMP + P_O
The maximum power that the LMH6715, package can dissipate at a given temperature can be derived with the
following equation:
Pmax = (150º - Tamb)/ θ_JA, where Tamb = Ambient temperature (°C) and θ_JA = Thermal resistance, from junction to
ambient, for a given package (°C/W). For the SOIC package θ_JA is 145°C/W.
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