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| LMH6601 Datasheet, PDF (22/37 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Op Amp with Shutdown | |||
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LMH6601, LMH6601-Q1
SNOSAK9E â JUNE 2006 â REVISED MARCH 2013
www.ti.com
VIDEO DAC
CURRENT
COMPOSITE VS = 3.3V
OUTPUT
VIDEO IN 0-1V
+
VLOAD
iO
LMH6601
75:
- U1
RS
75:
RL
75:
RG
620:
RF
620:
Figure 53. Single Supply Composite Video Driver for Consumer Video Outputs
If the âVideo Inâ 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.
3.3V
R1
30 k:
R2
10 k:
VIDEO IN
-0.3V to 0.75V
RT
75:
0.8VPP
0.61V ± 1.41V
3.3V
R3
1.3 k:
VS = 3.3V
+
LMH6601
- U1
0V - 2V
RS
75:
RG
560:
RF
620:
VLOAD
RL
75:
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 R1, R2, and RT in order to satisfy U1âs
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 VPP output with a 0.8 VPP input:
RF
2V
=
-1 = 1.5V/V
RG||R3 0.8V
(1)
R3 will produce a negative shift at the output due to VS (3.3V in this case). R3 will need to be set so that the
âVideo Inâ sync tip (â0.3V at RT or 0.61V at U1 non-inverting input) corresponds to near 0V at the output.
§
§
RF
R3
=
0.61
3.3V ± 0.61
¨¨1
©
+
RF
RG
=
0.227 ¨¨1
©
+
RF
RG
(2)
Equation 1 and Equation 2 need to be solved simultaneously to arrive at the values of R3, RF, and RG which will
satisfy both. From the datasheet, one can set RF = 620⦠to be close to the recommended value for a gain of +2.
It is easier to solve for RG and R3 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:
RF = 620â¦
(3)
22
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Product Folder Links: LMH6601 LMH6601-Q1
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