LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 17

• Manufacturer Part Number: LMH6601MG
• Description: 250 MHz, 2.4V CMOS Op Amp with Shutdown
• Manufacturer: NSC [National Semiconductor]

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,
broadcast video specs), the LMH6601 is optimized for single
supply DC coupled output applications where the load cur-
rent is returned to the negative rail (V
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
the output stage supplies both source and sink current to the
load and results in less than optimum distortion (and DG/
DP). Figure 1 compares 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.
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 2 shows how to calculate the pull-down resistor value
for both the dual supply and for the AC coupled load appli-
cations.
FIGURE 1. Distortion Comparison between DC & AC
Coupling of the Load
−
). That is where the
20136406
+
and V
−
,
17
Furthermore, with a combination of low closed loop gain
setting (i.e. A
the highest), light output loading (R
significant capacitive load (C
most stable if output sink current is kept to less than about 5
mA. The pull-down method described in Figure 2 is appli-
cable in these cases as well where the current that would
normally be sunk by the op amp is diverted to the R
instead.
SHUTDOWN CAPABILITY AND TURN ON/ OFF
BEHAVIOR
With the device in shutdown mode, the output goes into high
impedance (R
path between the inputs and the output pin is through the
external 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 iso-
lation, 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 between the V
voltage. For example, with V
you can expect the threshold to be at 7.5V. The state of the
device (shutdown or normal operation) is guaranteed over
temperature as longs as the SD pin is held to within 10% of
the total supply voltage.
For V
• Shutdown Range
• Normal Operation Range
FIGURE 2. Output Pull-Down Value for Dual Supply &
+
= 10V, V
V
OUT
= +1 for example where device bandwidth is
−
= 5V, as an example:
>
100 MΩ) mode. In this mode, the only
AC Coupling
+
and V
+
L
tied to 10V and V
−
>
pin potentials at any supply
10 pF) , the LMH6601 is
L
>
1 kΩ) , and with a
9.5V ≤ SD ≤ 10V
5V ≤ SD ≤ 5.5V
−
equal to 5V,
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