Datasheet

R
F
R
3
=
0.61
3.3V ± 0.61
= 0.227
1 +
R
F
R
G
¨
¨
©
§
¨
¨
©
§
1 +
R
F
¨
¨
©
§
¨
¨
©
§
R
G
-1 = 1.5V/V
R
F
R
G
||R
3
=
2V
0.8V
+
-
V
S
= 3.3V
R
F
620:
R
G
560:
R
S
75:
R
L
75:
V
LOAD
LMH6601
U1
0V - 2V
R
3
1.3 k:
3.3V
R
1
30 k:
R
2
10 k:
R
T
75:
VIDEO IN
-0.3V to 0.75V
0.8V
PP
0.61V ± 1.41V
3.3V
+
-
COMPOSITE
VIDEO IN 0-1V
75:
V
S
= 3.3V
R
F
620:
R
G
620:
R
S
75:
R
L
75:
V
LOAD
LMH6601
U1
i
O
VIDEO DAC
CURRENT
OUTPUT
LMH6601, LMH6601-Q1
SNOSAK9E –JUNE 2006–REVISED MARCH 2013 www.ti.com
Figure 53. Single Supply Composite Video Driver for Consumer Video Outputs
If the “Video In” signal is Composite Video with negative going Sync tip, a variation of the previous configurations
should be used. This circuit produces a unipolar (above 0V) DC coupled single supply video signal as shown in
Figure 54.
Figure 54. Single Supply DC Coupled Composite Video Driver for Negative Going Sync Tip
In the circuit of Figure 54, the input is shifted positive by means of R
1
, R
2
, and R
T
in order to satisfy U1’s
Common Mode input range. The signal will loose 20% of its amplitude in the process. The closed loop gain of U1
will need to be set to make up for this 20% loss in amplitude. This gives rise to the gain expression shown below
which is based on a getting a 2 V
PP
output with a 0.8 V
PP
input:
(1)
R
3
will produce a negative shift at the output due to V
S
(3.3V in this case). R
3
will need to be set so that the
“Video In” sync tip (−0.3V at R
T
or 0.61V at U1 non-inverting input) corresponds to near 0V at the output.
(2)
Equation 1 and Equation 2 need to be solved simultaneously to arrive at the values of R
3
, R
F
, and R
G
which will
satisfy both. From the datasheet, one can set R
F
= 620Ω to be close to the recommended value for a gain of +2.
It is easier to solve for R
G
and R
3
by starting with a good estimate for one and iteratively solving Equation 1 and
Equation 2 to arrive at the results. Here is one possible iteration cycle for reference:
R
F
= 620Ω (3)
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