LMC6001BIN National Semiconductor, LMC6001BIN Datasheet - Page 8

• Manufacturer Part Number: LMC6001BIN
• Description: IC, OP-AMP, 1.3MHZ, 1.5V/µs, DIP-8
• Manufacturer: National Semiconductor

Specifications of LMC6001BIN

• Op Amp Type: Low Power
• No. Of Amplifiers: 1
• Bandwidth: 1.3MHz
• Slew Rate: 1.5V/µs
• Supply Voltage Range: 4.5V To 15.5V
• Amplifier Case Style: DIP
• No. Of Pins: 8
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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AMPLIFIER TOPOLOGY
The LMC6001 incorporates a novel op-amp design topology
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 directly
from the internal integrator, which provides both low output
impedance and large gain. Special feed-forward compensa-
tion design techniques are incorporated to maintain stability
over a wider range of operating conditions than traditional op-
amps. These features make the LMC6001 both easier to
design with, and provide higher speed than products typically
found in this 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 LMC6001.
Although the LMC6001 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 with even small
values of input capacitance, due to transducers, photodiodes,
and circuit board parasitics, reduce phase margins.
When high input impedances are demanded, guarding of the
LMC6001 is suggested. Guarding input lines will not only re-
duce 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, C
Figure
Since it is often difficult to know the exact value of C
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.
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
FIGURE 1. Cancelling the Effect of Input Capacitance
1) such that:
f
, around the feedback resistors (as in
R
1
C
IN
or
≤
R
2
C
f
IN
1188705
, C
f
can
8
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 os-
cillatory or underdamped pulse response. With a few external
components, op amps can easily indirectly drive capacitive
loads, as shown in
In the circuit of
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
pullup resistor to V
ducting 500 μA or more will significantly improve capacitive
load responses. The value of the pullup 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 pullup resistor (see
Electrical Characteristics).
FIGURE 2. LMC6001 Noninverting Gain of 10 Amplifier,
FIGURE 3. Compensating for Large Capacitive Loads
Compensated to Handle Capacitive Loads
Figure
+
with a Pullup Resistor
Figure
(Figure
2, R1 and C1 serve to counteract the
2.
3). Typically a pullup resistor con-
1188707
1188706