LMH6601_14 Datasheet, PDF (18/37 Pages) National Semiconductor (TI) – LMH6601/LMH6601Q 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

LMH6601, LMH6601-Q1
SNOSAK9E – JUNE 2006 – REVISED MARCH 2013
APPLICATION INFORMATION
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OPTIMIZING PERFORMANCE
With many op amps, additional device non-linearity and sometimes less loop stability arises when the output has
to switch from current-source mode to current-sink mode or vice versa. When it comes to achieving the lowest
distortion and the best Differential Gain/ Differential Phase (DG/ DP, broadcast video specs), the LMH6601 is
optimized for single supply DC coupled output applications where the load current is returned to the negative rail
(V^−). That is where the output stage is most linear (lowest distortion) and which corresponds to unipolar current
flowing out of this device. To that effect, it is easy to see that the distortion specifications improve when the
output is only sourcing current which is the distortion-optimized mode of operation for the LMH6601. In
application where the LMH6601 output is AC coupled or when it is powered by separate dual supplies for V⁺ and
V^−, the output stage supplies both source and sink current to the load and results in less than optimum distortion
(and DG/DP). Figure 48 compares the distortion results between a DC and an AC coupled load to show the
magnitude of this difference. See the DG/DP plots, Figure 44 through Figure 47, in TYPICAL PERFORMANCE
CHARACTERISTICS, for a comparison between DC and AC coupling of the video load.
-20
V_S = 3.3V
-30 V_OUT = 2 V_PP
-40 ^{VOUT_DC = VS/2}
HD3, AC COUPLED
-50
-60
-70
-80
-90
-100
0.1
HD2, AC COUPLED
HD3, DC COUPLED
HD2, DC COUPLED
1
10
100
FREQUENCY (MHz)
Figure 48. Distortion Comparison between DC & AC Coupling of the Load
In certain applications, it may be possible to optimize the LMH6601 for best distortion (and DG/DP) even though
the load may require bipolar output current by adding a pull-down resistor to the output. Adding an output pull-
down resistance of appropriate value could change the LMH6601 output loading into source-only. This comes at
the price of higher total power dissipation and increased output current requirement.
Figure 49 shows how to calculate the pull-down resistor value for both the dual supply and for the AC coupled
load applications.
18
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