LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 21

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

Application Information
In this circuit, the output coupling capacitor value and size is
greatly reduced at the expense of slightly higher op amp
output voltage drive. Note that C
compensation, but it also sets the amplifier’s DC gain to 0 dB
so that the output is set to mid-rail for linearity purposes. Also
note that exceptionally high values are chosen for the R
R
low input bias current which allows this selection thereby
reducing the C
be a non-polar capacitors which will reduce cost.
At high enough frequencies where both C
considered to be shorted out, R
loop gain is determined by:
At intermediate frequencies, where the C
experiences low frequency gain loss, the R
provides feedback from the load side of C
side gain reduced at these lower frequencies, the feedback
to the op amp inverting node reduces, causing an increase
at the op amp’s output as a response.
For NTSC video, low values of C
black level shift occurs during the vertical blanking interval
(∼1.5 ms) which has no video activity and thus is sensitive to
C
output SAG. An especially tough pattern is the NTSC pattern
called “Pulse & Bar.” With this pattern the entire top and
bottom portion of the field is black level video where for
about 8.5 ms C
video activity to replenish that charge.
Figure 10 shows the output of the Figure 9 circuit with the
scope’s cursors pointing to the SAG.
2
O
’s charge dissipation through the load which could cause
biasing resistors (510 kΩ). The LMH6601 has extremely
Closed_loop_Gain (V/V)= V
IN
O
value in this circuit such that C
is discharging through the load with no
[R
L
/(R
FIGURE 9. AC Coupled Video Amplifier/Driver with SAG Compensation
L
+R
O
)]= 1V/V
L
1
O
3
/V
is not only part of the SAG
shunts R
influence how much video
IN
= (1+ (R
(Continued)
O
4
O
O
. With the load
and C
and the closed
, R
3
3
, R
||R
IN
O
5
4
, R
, C
)/ R
can even
1
can be
L
1
5
1
)
path
path
and
(4)
21
There is a subtlety with the additional output drive in the SAG
correction circuit, compared to a circuit with no SAG com-
pensation, especially when using lower power supply volt-
ages. This will be explained later after a brief introduction,
below, on the effects of AC coupling on output swing.
With the circuit of Figure 9 and any other AC coupled pulse
amplifier, the waveform duty cycle variations exert additional
restrictions on voltage swing at any node. This is illustrated
in the waveforms shown in Figure 11.
FIGURE 10. Figure 9 Scope Photo Showing Video SAG
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