LMH6601_09 Datasheet, PDF (19/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

With the LMH6601 input common mode range including the
V^− (ground) rail, there will be no need for AC coupling or 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 abil-
ity of exceedingly close swing to V^−, the circuit would not
operate properly as shown at the expense of more complex-
ity. 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 develops
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 Figure 6 will
be able to buffer and drive the Composite Video waveform
which includes sync and video combined. However, 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.
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FIGURE 7. Single Supply DC Coupled Composite Video
Driver for Negative Going Sync Tip
In the circuit of Figure 7, the input is shifted positive by means
of R₁, R₂, and R_T in order to satisfy U1’s Common Mode input
range. The signal will loose 20% of its amplitude in the pro-
cess. 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)
R₃ will produce a negative shift at the output due to V_S (3.3V
in this case). R₃ 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) corre-
sponds to near 0V at the output.
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FIGURE 6. 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 7.
(2)
Equation 1 and Equation 2 need to be solved simultaneously
to arrive at the values of R₃, 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 R₃ by starting with a good estimate for one and iter-
atively solving Equation and Equation 2 to arrive at the results.
Here is one possible iteration cycle for reference:
R_F = 620Ω
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