LM6142, LM6144
SNOS726D –JUNE 2000–REVISED MARCH 2013
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Figure 37. Slew Rate vs. Δ VIN
VS = ±5V
This effect is most noticeable at higher supply voltages and lower gains where incoming signals are likely to be
large.
This new input circuit also eliminates the phase reversal seen in many op amps when they are overdriven.
This speed-up action adds stability to the system when driving large capacitive loads.
DRIVING CAPACITIVE LOADS
Capacitive loads decrease the phase margin of all op amps. This is caused by the output resistance of the
amplifier and the load capacitance forming an R-C phase lag network. This can lead to overshoot, ringing and
oscillation. Slew rate limiting can also cause additional lag. Most op amps with a fixed maximum slew-rate will lag
further and further behind when driving capacitive loads even though the differential input voltage raises. With the
LM6142, the lag causes the slew rate to raise. The increased slew-rate keeps the output following the input
much better. This effectively reduces phase lag. After the output has caught up with the input, the differential
input voltage drops down and the amplifier settles rapidly.
These features allow the LM6142 to drive capacitive loads as large as 1000pF at unity gain and not oscillate.
The scope photos (Figure 38 and Figure 39) above show the LM6142 driving a l000pF load. In Figure 38, the
upper trace is with no capacitive load and the lower trace is with a 1000pF load. Here we are operating on ±12V
supplies with a 20 VPP pulse. Excellent response is obtained with a Cf of l0pF. In Figure 39, the supplies have
been reduced to ±2.5V, the pulse is 4 VPP and Cf is 39pF. The best value for the compensation 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 op amps is the phase shift caused by the feedback resistor and the input
capacitance. This phase shift also reduces phase margin. This effect is taken care of at the same time as the
effect of the capacitive load when the capacitor is placed across the feedback resistor.
The circuit shown in Figure 40 was used for these scope photos.
Figure 38.
14
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