LMC6084AIMX NSC [National Semiconductor], LMC6084AIMX Datasheet - Page 9

• Manufacturer Part Number: LMC6084AIMX
• Description: Precision CMOS Quad Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

Applications Hints
CAPACITIVE LOAD TOLERANCE
All rail-to-rail output swing operational amplifiers have volt-
age gain in the output stage. A compensation capacitor is
normally included in this integrator stage. The frequency
location of the dominant 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
oscillatory or underdamped pulse response. With a few ex-
ternal components, op amps can easily indirectly drive ca-
pacitive loads, as shown in Figure 2.
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 feed-
back loop.
FIGURE 2. LMC6084 Noninverting Gain of 10 Amplifier,
FIGURE 1. Cancelling the Effect of Input Capacitance
Compensated to Handle Capacitive Loads
(Continued)
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Capacitive load driving capability is enhanced by using a
pull up resistor to V
conducting 500 µA or more will significantly improve capaci-
tive 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).
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 LMC6084, typically less
than 10 fA, it is essential to have an excellent layout. Fortu-
nately, the techniques of 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 accept-
ably low, because under conditions of high humidity or dust
or contamination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LMC6084’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs, as in Fig-
ure 4. To have a significant effect, guard rings should be
placed on both the top and bottom of the PC board. This PC
foil must then be connected to a voltage which is at the same
voltage as the amplifier inputs, since no leakage current can
flow between two points at the same potential. For example,
a PC board trace-to-pad resistance of 10
mally considered a very large resistance, could leak 5 pA if
the trace were a 5V bus adjacent to the pad of the input. This
would cause a 100 times degradation from the LMC6084’s
actual performance. However, if a guard ring is held within
5 mV of the inputs, then even a resistance of 10
cause only 0.05 pA of leakage current. See Figure 5 for
typical connections of guard rings for standard op-amp con-
figurations.
FIGURE 3. Compensating for Large Capacitive Loads
with a Pull Up Resistor
+
Figure 3. Typically a pull up resistor
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Ω, which is nor-
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Ω would