LMC660EP Datasheet, PDF (8/15 Pages) National Semiconductor (TI) – CMOS Quad Operational Amplifier

Application Hints (Continued)
As a result of these demands, the integrator is a compound
affair with an embedded gain stage that is doubly fed forward
(via C_f and Cff) by a dedicated unity-gain compensation
driver. In addition, the output portion of the integrator is a
push-pull configuration for delivering heavy loads. While
sinking current the whole amplifier path consists of three
gain stages with one stage fed forward, whereas while
sourcing the path contains four gain stages with two fed
forward.
However, if the feedback pole is less than approximately 6 to
10 times the “ideal” −3 dB frequency, a feedback capacitor,
C_F, should be connected between the output and the invert-
ing input of the op amp. This condition can also be stated in
terms of the amplifier’s low-frequency noise gain: To main-
tain stability a feedback capacitor will probably be needed if
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
20114504
FIGURE 1. LMC660EP Circuit Topology (Each
Amplifier)
The large signal voltage gain while sourcing is comparable
to traditional bipolar op amps, even with a 600Ω load. The
gain while sinking is higher than most CMOS op amps, due
to the additional gain stage; however, under heavy load
(600Ω) the gain will be reduced as indicated in the Electrical
Characteristics.
COMPENSATING INPUT CAPACITANCE
The high input resistance of the LMC660EP op amps allows
the use of large feedback and source resistor values without
losing gain accuracy due to loading. However, the circuit will
be especially sensitive to its layout when these large-value
resistors are used.
Every amplifier has some capacitance between each input
and AC ground, and also some differential capacitance be-
tween the inputs. When the feedback network around an
amplifier is resistive, this input capacitance (along with any
additional capacitance due to circuit board traces, the
socket, etc.) and the feedback resistors create a pole in the
feedback path. In the following General Operational Amplifier
circuit, the frequency of this pole is Figure 2
regardless of whether the amplifier is being used in inverting
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
the feedback capacitor should be:
where C_S is the total capacitance at the inverting input,
including amplifier input capcitance and any stray capaci-
tance from the IC socket (if one is used), circuit board traces,
etc., and R_P is the parallel combination of R_F and R_IN. This
formula, as well as all formulae derived below, apply to
inverting and non-inverting op-amp configurations.
When the feedback resistors are smaller than a few kΩ, the
frequency of the feedback pole will be quite high, since C_S is
generally less than 10 pF. If the frequency of the feedback
pole is much higher than the “ideal” closed-loop bandwidth
(the nominal closed-loop bandwidth in the absence of C_S),
the pole will have a negligible effect on stability, as it will add
only a small amount of phase shift.
Note that these capacitor values are usually significant
smaller than those given by the older, more conservative
formula:
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