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

should be used. This resistor and C

delay the phase lag or increase the phase margin of the

overall system. The larger the value of R

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

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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

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