LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 22

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

Application Information
(Continued)
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FIGURE 11. Headroom Considerations with AC Coupled Amplifiers
If a stage has a 3 V
unclipped swing available at a given
PP
node, as shown in Figure 11, the maximum allowable ampli-
tude for an arbitrary waveform is
⁄
of 3V or 1.5 V
. This is
1
2
PP
due to the shift in the average value of the waveform as the
duty cycle varies. Figure 11 shows what would happen if a
2 V
signal were applied. A low duty cycle waveform, such
PP
as the one in Figure 11B, would have high positive excur-
sions. At low enough duty cycles, the waveform could get
clipped on the top, as shown, or a more subtle loss of
linearity could occur prior to full-blown clipping. The con-
verse of this occurs with high duty cycle waveforms and
negative clipping, as depicted in Figure 11C.
Now, let’s get back to discussing the SAG compensation
output swing subtlety hinted at earlier. For the Figure 9
circuit, with a 1 V
Composite Video input, the op amp
PP
output will swing 2 V
because the stage gain is set to 2
PP
V/V. With the output set to V
/2 (2.5V in this case), the op
CC
amp output voltage will range from 0.5V to 4.5V, assuming
video duty cycle variation of 100% to 0% respectively. In
reality, the duty cycle only approaches these extreme end
points and it never quite gets there. Figure 12 shows the
measured response of this circuit to show the worst case
swing at the op amp output pin. Note that the extra output
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drive at the op amp output for SAG compensation, which
FIGURE 12. SAG Compensation Requires Higher
shows up as a tilt in the upper video waveform, could cause
Swing at op amp Output
clipping as the output swings even closer to the rails.
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