lmh6601mgx National Semiconductor Corporation, lmh6601mgx Datasheet - Page 22

• Manufacturer Part Number: lmh6601mgx
• Description: 250 Mhz, 2.4v Cmos Op Amp With Shutdown
• Manufacturer: National Semiconductor Corporation

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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 ½ of 3V or 1.5 V
. This is
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 converse 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 out-
put swing subtlety hinted at earlier. For the Figure 9 circuit,
with a 1 V
Composite Video input, the op amp output will
PP
swing 2 V
because the stage gain is set to 2 V/V. With the
PP
output set to V
/2 (2.5V in this case), the op amp output
CC
voltage will range from 0.5V to 4.5V, assuming video duty cy-
cle 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
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pin. Note that the extra output drive at the op amp output for
FIGURE 12. SAG Compensation Requires Higher Swing
SAG compensation, which shows up as a tilt in the upper
at op amp Output
video waveform, could cause clipping as the output swings
even closer to the rails.
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