LMC6572 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – Low Voltage (2.7V and 3V) Operational Amplifier

Typical Performance Characteristics V_S = +3V, T_A = 25˚C, Unless otherwise specified (Continued)
Bandwidth vs
Capacitive Load
Capacitive Load
vs Phase Margin
Capacitive Load
vs Gain Margin
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Applications Hints
1.0 LOW VOLTAGE AMPLIFIER TOPOLOGY
The LMC6574/2 incorporates a novel op-amp design topol-
ogy that enables it to maintain rail-to-rail output swing even
when driving a large load. Instead of relying on a push-pull
unity gain output buffer stage, the output stage is taken di-
rectly from the internal integrator, which provides both low
output impedance and large gain. Special feed-forward com-
pensation design techniques are incorporated to maintain
stability over a wider range of operating conditions than tra-
ditional micropower op-amps. These features make the
LMC6574/2 both easier to design with, and provide higher
speed than products typically found in this ultra-low power
class.
2.0 COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resis-
tance for amplifiers with ultra-low input current, like the
LMC6574/2.
Although the LMC6574/2 is highly stable over a wide range
of operating conditions, a large feedback resistor will react
even with small values of capacitance at the input of the
op-amp to reduce phase margin. The capacitance at the in-
put of the op-amp comes from transducers, photodiodes and
circuit board parasitics.
The effect of input capacitance can be compensated for by
adding a capacitor, C_f, around the feedback resistors (as in
Figure 1) such that:
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FIGURE 1. Cancelling the Effect of Input Capacitance
3.0 CAPACITIVE LOAD TOLERANCE
Direct capacitive loading will reduce the phase margin of
many op-amps. A pole in the feedback loop is created by the
combination of the op-amp’s output impedance and the ca-
pacitive load. This pole induces phase lag at the unity-gain
crossover frequency of the amplifier resulting in either an os-
cillatory or underdamped pulse response. With a few exter-
nal components, op amps can easily indirectly drive capaci-
tive loads, as shown in Figure 2.
or
R₁ C_IN ≤ R₂ C_f
Since it is often difficult to know the exact value of C_IN, C_f can
be experimentally adjusted so that the desired pulse re-
sponse is achieved. Refer to the LMC660 and LMC662 for a
more detailed discussion on compensating for input capaci-
tance.
When high input impedances are demanded, guarding of the
LMC6574/2 is suggested. Guarding input lines will not only
reduce leakage, but lowers stray input capacitance as well.
(See Printed-Circuit-Board Layout for High Impedance
Work).
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FIGURE 2. LMC6574/2 Noninverting Gain of 10
Amplifier, Compensated to Handle Capacitive Loads
In the circuit of Figure 2, R1 and C1 serve to counteract the
loss of phase margin by feeding the high frequency compo-
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