LM6142/44 Application Ideas
(Continued)
DS012057-6
FIGURE 2.
These features allow the LM6142 to drive capacitive loads
as large as 1000 pF at unity gain and not oscillate. The
scope photos (Figure
3
and
Figure 4)
above show the
LM6142 driving a l000 pF load. In
Figure 3,
the upper trace
is with no capacitive load and the lower trace is with a 1000
pF load. Here we are operating on
±
12V supplies with a 20
Vp-p pulse. Excellent response is obtained with a C
f
of l0 pF.
In
Figure 4,
the supplies have been reduced to
±
2.5V, the
pulse is 4 Vp-p and C
f
is 39 pF. The best value for the com-
pensation capacitor is best established after the board layout
is finished because the value is dependent on board stray
capacity, the value of the feedback resistor, the closed loop
gain and, to some extent, the supply voltage.
Another effect that is common to all opamps is the phase
shift caused by the feedback resistor and the input capaci-
tance. This phase shift also reduces phase margin. This ef-
fect is taken care of at the same time as the effect of the ca-
pacitive load when the capacitor is placed across the
feedback resistor.
The circuit shown in
Figure 5
was used for these scope
photos.
DS012057-9
FIGURE 4.
DS012057-10
FIGURE 5.
Typical Applications
DS012057-8
FIGURE 3.
FISH FINDER/ DEPTH SOUNDER.
The LM6142/44 is an excellent choice for battery operated
fish finders. The low supply current, high gain-bandwidth and
full rail to rail output swing of the LM6142 provides an ideal
combination for use in this and similar applications.
11
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