Datasheet

+
-
V
CC
_5V
R
O
75:
V
L
LMH6601
U1
C
O
68 PF
CABLE
V
CC
_5V
C
IN
0.47 PF
R
1
510 k:
R
2
510 k:
+
-
+
-
R
4
2 k:
R
3
1 k:
C
1
22 PF
R
5
680:
V
IN
V
O
R
T
75:
R
L
75:
LMH6601, LMH6601-Q1
SNOSAK9E –JUNE 2006–REVISED MARCH 2013 www.ti.com
Figure 56. AC Coupled Video Amplifier/Driver with SAG Compensation
In this circuit, the output coupling capacitor value and size is reduced at the expense of a slightly more
complicated circuitry. Note that C1 is not only part of the SAG 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 R1 and R2 biasing resistors (510 kΩ). The LMH6601 has extremely low input bias current
which allows this selection thereby reducing the C
IN
value in this circuit such that C
IN
can even be a non-polar
capacitors which will reduce cost.
At high enough frequencies where both C
O
and C
1
can be considered to be shorted out, R
3
shunts R
4
and the
closed loop gain is determined by:
Closed_loop_Gain (V/V)= V
L
/V
IN
= (1+ (R
3
||R
4
)/ R
5
)x [R
L
/(R
L
+R
O
)]= 0.99V/V (5)
At intermediate frequencies, where the C
O
, R
O
, R
L
path experiences low frequency gain loss, the R
3
, R
5
, C
1
path
provides feedback from the load side of C
O
. With the load 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
O
influence how much video black level shift occurs during the vertical blanking
interval (∼1.5 ms) which has no video activity and thus is sensitive to C
O
's charge dissipation through the load
which could cause 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 11 ms, C
O
is
discharging through the load with no video activity to replenish that charge.
Figure 57 shows the output of the Figure 56 circuit highlighting the SAG.
Figure 57. AC Coupled Video Amplifier/Driver Output Scope Photo Showing Video SAG
With the circuit of Figure 56 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 58.
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