LMC6064EP Datasheet, PDF (11/18 Pages) Texas Instruments – Precision CMOS Quad Micropower Operational Amplifier

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AMPLIFIER TOPOLOGY
The LMC6064EP 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 LM-
C6064EP both easier to design with, and provide higher
speed than products typically found in this ultra-low power
class.
COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resistance
for amplifiers with ultra-low input current, like the LM-
C6064EP.
Although the LMC6064EP is highly stable over a wide range
of operating conditions, certain precautions must be met to
achieve the desired pulse response when a large feedback
resistor is used. Large feedback resistors and even small val-
ues of input capacitance, due to transducers, photodiodes,
and circuit board parasitics, reduce phase margins.
When high input impedances are demanded, guarding of the
LMC6064EP 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).
The effect of input capacitance can be compensated for by
adding a capacitor. Place a capacitor, C_f, around the feed-
back resistor (as in Figure 1 ) such that:
CAPACITIVE LOAD TOLERANCE
All rail-to-rail output swing operational amplifiers have voltage
gain in the output stage. A compensation capacitor is normally
included in this integrator stage. The frequency location of the
dominate pole is affected by the resistive load on the amplifier.
Capacitive load driving capability can be optimized by using
an appropriate resistive load in parallel with the capacitive
load (see typical curves).
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 external
components, op amps can easily indirectly drive capacitive
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 the LMC662
for a more detailed discussion on compensating for input ca-
pacitance.
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FIGURE 2. LMC6064EP 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-
nent of the output signal back to the amplifier's inverting input,
thereby preserving phase margin in the overall feedback loop.
Capacitive load driving capability is enhanced by using a pull
up resistor to V⁺ (Figure 3). Typically a pull up resistor con-
ducting 10 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 am-
plifier 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).
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FIGURE 1. Canceling the Effect of Input Capacitance
20114406
FIGURE 3. Compensating for Large Capacitive Loads
with a Pull Up Resistor
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201144 Version 2 Revision 3 Print Date/Time: 2011/08/30 23:14:26