LMH6715_17 Datasheet, PDF (12/21 Pages) Texas Instruments – Dual Wideband Video Op Amp

LMH6715
SNOSA10C – MAY 2002 – REVISED APRIL 2013
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MATCHING PERFORMANCE
With proper board layout, the AC performance match between 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 inherently 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 achieving 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 capacitance will also affect settling time.
Placing a series resistor (R_s) at the output pin is recommended for optimal settling time performance when
driving a capacitive load. The Typical Performance plot labeled “R_S and Settling Time vs. Capacitive Load”
provides a means for selecting 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:
(2)
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-07 (Literature Number SNOA365)
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-referred noise and external resistor noise contribution can be found in OA-12 (Literature Number
SNOA375).
DIFFERENTIAL GAIN & PHASE
The LMH6715 can drive multiple video loads with very low differential gain and phase errors. Figure 19 and
Figure 20 show performance for loads from 1 to 4. The Electrical Characteristics table also specifies performance
for one 150Ω load at 4.43MHz. For NTSC video, the performance specifications also apply. Application note OA-
24 (Literature Number SNOA370) “Measuring and Improving Differential Gain & Differential 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 common-mode input and output voltage swings while at the same time increase the internal
junction temperature.
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