LMH6601_09 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

Application Information
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, broad-
cast video specs), the LMH6601 is optimized for single supply
DC coupled output applications where the load current is re-
turned 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-op-
timized mode of operation for the LMH6601. In application
where the LMH6601 output is AC coupled or when it is pow-
ered 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 1 com-
pares the distortion results between a DC and an AC coupled
load to show the magnitude of this difference. See the DG/DP
plots in the Typical Performance Characteristics section for a
comparison between DC and AC coupling of the video load.
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FIGURE 1. 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 load-
ing into source-only. This comes at the price of higher total
power dissipation and increased output current requirement.
Figure 2 shows how to calculate the pull-down resistor value
for both the dual supply and for the AC coupled load applica-
tions.
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FIGURE 2. Output Pull-Down Value for Dual Supply & AC
Coupling
Furthermore, with a combination of low closed loop gain set-
ting (i.e. A_V = +1 for example where device bandwidth is the
highest), light output loading (R_L > 1 kΩ) , and with a signifi-
cant capacitive load (C_L > 10 pF) , the LMH6601 is most stable
if output sink current is kept to less than about 5 mA. The pull-
down method described in Figure 2 is applicable in these
cases as well where the current that would normally be sunk
by the op amp is diverted to the R_P path instead.
SHUTDOWN CAPABILITY AND TURN ON/ OFF
BEHAVIOR
With the device in shutdown mode, the output goes into high
impedance (R_OUT > 100 MΩ) mode. In this mode, the only
path between the inputs and the output pin is through the ex-
ternal components around the device. So, for applications
where there is active signal connection to the inverting input,
with the LMH6601 in shutdown, the output could show signal
swings due to current flow through these external compo-
nents. For non-inverting amplifiers in shutdown, no output
swings would occur, because of complete input-output isola-
tion, with the exception of capacitive coupling.
For maximum power saving, the LMH6601 supply current
drops to around 0.1 μA in shutdown. All significant power
consumption within the device is disabled for this purpose.
Because of this, the LMH6601 turn on time is measured in
micro-seconds whereas its turn off is fast (nano-seconds) as
would be expected from a high speed device like this.
The LMH6601 SD pin is a CMOS compatible input with a pico-
ampere range input current drive requirement. This pin needs
to be tied to a level or otherwise the device state would be
indeterminate. The device shutdown threshold is half way be-
tween the V⁺ and V^− pin potentials at any supply voltage. For
example, with V⁺ tied to 10V and V^− equal to 5V, you can
expect the threshold to be at 7.5V. The state of the device
(shutdown or normal operation) is guaranteed over tempera-
ture as longs as the SD pin is held to within 10% of the total
supply voltage.
For V⁺ = 10V, V^− = 5V, as an example:
• Shutdown Range
• Normal Operation Range
5V ≤ SD ≤ 5.5V
9.5V ≤ SD ≤ 10V
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