LMC662
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SNOSC51C –APRIL 1998–REVISED MARCH 2013
Using the smaller capacitors will give much higher bandwidth with little degradation of transient response. It may
be necessary in any of the above cases to use a somewhat larger feedback capacitor to allow for unexpected
stray capacitance, or to tolerate additional phase shifts in the loop, or excessive capacitive load, or to decrease
the noise or bandwidth, or simply because the particular circuit implementation needs more feedback
capacitance to be sufficiently stable. For example, a printed circuit board's stray capacitance may be larger or
smaller than the breadboard's, so the actual optimum value for CF may be different from the one estimated using
the breadboard. In most cases, the value of CF should be checked on the actual circuit, starting with the
computed value.
CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LMC662 may oscillate when its applied load appears capacitive. The threshold of
oscillation varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain
follower. See the Typical Performance Characteristics.
The load capacitance interacts with the op amp's output resistance to create an additional pole. If this pole
frequency is sufficiently low, it will degrade the op amp's phase margin so that the amplifier is no longer stable at
low gains. As shown in Figure 17, the addition of a small resistor (50Ω to 100Ω) in series with the op amp's
output, and a capacitor (5 pF to 10 pF) from inverting input to output pins, returns the phase margin to a safe
value without interfering with lower-frequency circuit operation. Thus, larger values of capacitance can be
tolerated without oscillation. Note that in all cases, the output will ring heavily when the load capacitance is near
the threshold for oscillation.
Figure 17. Rx, Cx Improve Capacitive Load Tolerance
Capacitive load driving capability is enhanced by using a pull up resistor to V+ Figure 18. Typically a pull up
resistor conducting 500 μA or more will significantly improve capacitive load responses. The value of the pull up
resistor must be determined based on the current sinking capability of the amplifier with respect to the desired
output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see Electrical
Characteristics).
Figure 18. Compensating for Large Capacitive Loads with a Pull Up Resistor
PRINTED-CIRCUIT-BOARD LAYOUT FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate with less than 1000 pA of leakage current requires
special layout of the PC board. When one wishes to take advantage of the ultra-low bias current of the LMC662,
typically less than 0.04 pA, it is essential to have an excellent layout. Fortunately, the techniques for obtaining
low leakages are quite simple. First, the user must not ignore the surface leakage of the PC board, even though
it may sometimes appear acceptably low, because under conditions of high humidity or dust or contamination,
the surface leakage will be appreciable.
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