LMC6032 Datasheet, PDF (7/13 Pages) National Semiconductor (TI) – CMOS Dual Operational Amplifier

Application Hints (Continued)
where
is the amplifier’s low-frequency noise gain and GBW is the
amplifier’s gain bandwidth product. An amplifier’s
low-frequency noise gain is represented by the formula
regardless of whether the amplifier is being used in an invert-
ing or non-inverting mode. Note that a feedback capacitor is
more likely to be needed when the noise gain is low and/or
the feedback resistor is large.
If the above condition is met (indicating a feedback capacitor
will probably be needed), and the noise gain is large enough
that:
the following value of feedback capacitor is recommended:
If
larger feedback capacitor to allow for unexpected stray ca-
pacitance, or to tolerate additional phase shifts in the loop, or
excessive capacitive load, or to decrease the noise or band-
width, or simply because the particular circuit implementa-
tion needs more feedback capacitance to be sufficiently
stable. For example, a printed circuit board’s stray capaci-
tance may be larger or smaller than the breadboard’s, so the
actual optimum value for C_F may be different from the one
estimated using the breadboard. In most cases, the value of
C_F should be checked on the actual circuit, starting with the
computed value.
CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LMC6032 may oscillate when
its applied load appears capacitive. The threshold of oscilla-
tion 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 re-
sistance 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 3, 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 ca-
pacitance 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.
the feedback capacitor should be:
Note that these capacitor values are usually significantly
smaller than those given by the older, more conservative for-
mula:
DS011135-5
FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance
Capacitive load driving capability is enhanced by using a pull
up resistor to V⁺ (Figure 4). Typically a pull up resistor con-
ducting 500 µA or more will significantly improve capacitive
load responses. The value of the pull up resistor must be de-
termined based on the current sinking capability of the ampli-
fier 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).
DS011135-4
C_S consists of the amplifier’s input capacitance plus any stray capacitance
from the circuit board and socket. C_F compensates for the pole caused by
C_S and the feedback resistor.
FIGURE 2. General Operational Amplifier Circuit
Using the smaller capacitors will give much higher band-
width with little degradation of transient response. It may be
necessary in any of the above cases to use a somewhat
DS011135-22
FIGURE 4. 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 LMC6032, typically less
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