LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 19

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

Application Information
With the LMH6601 input common mode range including the
V
level shifting and the input can directly drive the non-
inverting input which has the additional advantage of high
amplifier input impedance. With LMH6601’s wide rail-to-rail
output swing, as stated earlier, the video black level of 0V is
maintained at the load with minimal circuit complexity and
using no AC coupling capacitors. Without true rail-to-rail
output swing of the LMH6601, and more importantly without
the LMH6601’s ability of exceedingly close swing to V
circuit would not operate properly as shown at the expense
of more complexity. This circuit will also work for higher input
voltages. The only significant requirement is that there is at
least 1.8V from the maximum input voltage to the positive
supply (V
The Composite Video Output of some low cost consumer
video equipment consists of a current source which devel-
ops the video waveform across a load resistor (usually 75Ω),
as shown in Figure 6 below. With these applications, the
same circuit configuration just described and shown in Fig-
ure 6 will be able to buffer and drive the Composite Video
waveform which includes sync and video combined. How-
ever, with this arrangement, the LMH6601 supply voltage
needs to be at least 3.3V or higher in order to allow proper
input common mode voltage headroom because the input
can be as high as 1V peak.
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 7.
FIGURE 6. Single Supply Composite Video Driver for
−
(ground) rail, there will be no need for AC coupling or
+
).
Consumer Video Outputs
(Continued)
−
, the
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In the circuit of Figure 7, the input is shifted positive by
means of R
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 get-
ting a 2 V
R
in this case). R
tip (−0.3V at R
sponds to near 0V at the output.
Equation (1) and Equation (2) need to be solved simulta-
neously to arrive at the values of R
satisfy both. From the datasheet, one can set R
be close to the recommended value for a gain of +2. It is
easier to solve for R
mate for one and iteratively solving Equation and Equation
(2) to arrive at the results. Here is one possible iteration
cycle for reference:
3
will produce a negative shift at the output due to V
FIGURE 7. Single Supply DC Coupled Composite
Video Driver for Negative Going Sync Tip
PP
1
, R
output with a 0.8 V
3
T
2
will need to be set so that the “Video In” sync
, and R
or 0.61V at U1 non-inverting input) corre-
G
and R
T
in order to satisfy U1’s Common
3
by starting with a good esti-
PP
input:
3
, R
F
, and R
F
G
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= 620Ω to
which will
S
(3.3V
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(1)
(2)