LMH6715QML Datasheet, PDF (13/20 Pages) Texas Instruments – Dual Wideband Video Op Amp

sitics, 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 feed-
back resistor to an absolute minimum.
The performance plots in the data sheet can be reproduced
using the evaluation boards available from National. The
CLC730036 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 tanta-
lum and ceramic) must be chosen with good high frequency
characteristics to decouple the power supplies and the phys-
ical 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 maxi-
mum possible distance is recommended. The input (R_IN) and
gain setting resistors (R_F) are the most critical. It is also rec-
ommended 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 dissi-
pation 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 dissi-
pate at a given temperature can be derived with the following
equation:
Pmax = (150º - Tamb)/ θ_JA, where Tamb = Ambient temper-
ature (°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.
MATCHING PERFORMANCE
With proper board layout, the AC performance match be-
tween the two LMH6715's amplifiers can be tightly controlled
as shown in Typical Performance plot labeled “Small-Signal
Channel Matching”.
The measurements were performed with SMT components
using a feedback resistor of 300Ω at a gain of +2V/V.
The LMH6715's amplifiers, built on the same die, provide the
advantage of having tightly matched DC characteristics.
SLEW RATE AND SETTLING TIME
One of the advantages of current-feedback topology is an in-
herently high slew rate which produces a wider full power
bandwidth. The LMH6715 has a typical slew rate of 1300V/
µs. The required slew rate for a design can be calculated by
the following equation: SR = 2πfV_pk.
Careful attention to parasitic capacitances is critical to achiev-
ing the best settling time performance. The LMH6715 has a
typical short term settling time to 0.05% of 12ns for a 2V step.
Also, the amplifier is virtually free of any long term thermal tail
effects at low gains.
When measuring settling time, a solid ground plane should
be used in order to reduce ground inductance which can
cause common-ground-impedance coupling. Power supply
and ground trace parasitic capacitances and the load capac-
itance will also affect settling time.
Placing a series resistor (R_s) at the output pin is recommend-
ed for optimal settling time performance when driving a ca-
pacitive load. The Typical Performance plot labeled “R_S and
Settling Time vs. Capacitive Load” provides a means for se-
lecting a value of R_s for a given capacitive load.
DC & NOISE PERFORMANCE
A current-feedback amplifier's input stage does not have
equal nor correlated bias currents, therefore they cannot be
canceled and each contributes to the total DC offset voltage
at the output by the following equation:
The input resistance is the resistance looking from the non-
inverting input back toward the source. For inverting DC-
offset calculations, the source resistance seen by the input
resistor R_g must be included in the output offset calculation
as a part of the non-inverting gain equation. Application note
OA-7 gives several circuits for DC offset correction. The noise
currents for the inverting and non-inverting inputs are graphed
in the Typical Performance plot labeled “Equivalent Input
Noise”. A more complete discussion of amplifier input-re-
ferred noise and external resistor noise contribution can be
found in OA-12.
DIFFERENTIAL GAIN & PHASE
The LMH6715 can drive multiple video loads with very low
differential gain and phase errors. The Typical Performance
plots labeled “Differential Gain vs. Frequency” and “Differen-
tial Phase vs. Frequency” show performance for loads from 1
to 4. The Electrical Characteristics table also specifies per-
formance for one 150Ω load at 4.43MHz. For NTSC video,
the performance specifications also apply. Application note
OA-24 “Measuring and Improving Differential Gain & Differ-
ential Phase for Video”, describes in detail the techniques
used to measure differential gain and phase.
I/O VOLTAGE & OUTPUT CURRENT
The usable common-mode input voltage range (CMIR) of the
LMH6715 specified in the Electrical Characteristics table of
the data sheet shows a range of ±2.2 volts. Exceeding this
range will cause the input stage to saturate and clip the output
signal.
The output voltage range is determined by the load resistor
and the choice of power supplies. With ±5 volts the class A/
B output driver will typically drive ±3.9V into a load resistance
of 100Ω. Increasing the supply voltages will change the com-
mon-mode input and output voltage swings while at the same
time increase the internal junction temperature.
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