Application Information
LMP7715/LMP7716
The LMP7715/LMP7716 are single and dual, low noise, low
offset, rail-to-rail output precision amplifiers with a wide gain
bandwidth product of 17 MHz and low supply current. The
wide bandwidth makes the LMP7715/LMP7716 ideal
choices for wide-band amplification in portable applications.
The LMP7715/LMP7716 are superior for sensor applica-
tions. The very low input referred voltage noise of only 5.8
nV/ at 1 kHz and very low input referred current noise
of only 10 fA/ mean more signal fidelity and higher
signal-to-noise ratio.
The LMP7715/LMP7716 have a supply voltage range of
1.8V to 5.5V over a wide temperature range of 0˚C to 125˚C.
This is optimal for low voltage commercial applications. For
applications where the ambient temperature might be less
than 0˚C, the LMP7715/LMP7716 are fully operational at
supply voltages of 2.0V to 5.5V over the temperature range
of −40˚C to 125˚C.
The outputs of the LMP7715/LMP7716 swing within 25 mV
of either rail providing maximum dynamic range in applica-
tions requiring low supply voltage. The input common mode
range of the LMP7715/LMP7716 extends to 300 mV below
ground. This feature enables users to utilize this device in
single supply applications.
The use of a very innovative feedback topology has en-
hanced the current drive capability of the LMP7715/
LMP7716, resulting in sourcing currents of as much as 47
mA with a supply voltage of only 1.8V.
The LMP7715 is offered in the space saving SOT23 pack-
age and the LMP7716 is offered in an 8-pin MSOP. These
small packages are ideal solutions for applications requiring
minimum PC board footprint.
CAPACITIVE LOAD
The unity gain follower is the most sensitive configuration to
capacitive loading. The combination of a capacitive load
placed directly on the output of an amplifier along with the
output impedance of the amplifier creates a phase lag which
in turn reduces the phase margin of the amplifier. If phase
margin is significantly reduced, the response will be either
underdamped or the amplifier will oscillate.
The LMP7715/LMP7716 can directly drive capacitive loads
of up to 120 pF without oscillating. To drive heavier capaci-
tive loads, an isolation resistor, R
ISO
as shown in Figure 1,
should be used. This resistor and C
L
form a pole and hence
delay the phase lag or increase the phase margin of the
overall system. The larger the value of R
ISO
, the more stable
the output voltage will be. However, larger values of R
ISO
result in reduced output swing and reduced output current
drive.
INPUT CAPACITANCE
CMOS input stages inherently have low input bias current
and higher input referred voltage noise. The LMP7715/
LMP7716 enhance this performance by having the low input
bias current of only 50 fA, as well as, a very low input
referred voltage noise of 5.8 nV/ . In order to achieve
this a larger input stage has been used. This larger input
stage increases the input capacitance of the LMP7715/
LMP7716. Figure 2 shows typical input common mode ca-
pacitance of the LMP7715/LMP7716.
This input capacitance will interact with other impedances,
such as gain and feedback resistors which are seen on the
inputs of the amplifier, to form a pole. This pole will have little
or no effect on the output of the amplifier at low frequencies
and under DC conditions, but will play a bigger role as the
frequency increases. At higher frequencies, the presence of
this pole will decrease phase margin and also cause gain
peaking. In order to compensate for the input capacitance,
care must be taken in choosing feedback resistors. In addi-
tion to being selective in picking values for the feedback
resistor, a capacitor can be added to the feedback path to
increase stability.
The DC gain of the circuit shown in Figure 3 is simply
−R
2
/R
1
.
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FIGURE 1. Isolating Capacitive Load
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FIGURE 2. Input Common Mode Capacitance
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FIGURE 3. Compensating for Input Capacitance
LMP7715/LMP7716
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