Application Information

LMP7701/LMP7702/LMP7704

The LMP7701/LMP7702/LMP7704 are single, dual, and

quad low offset voltage, rail-to-rail input and output precision

amplifiers each with CMOS input stage and wide supply

voltage range of 2.7V to 12V. The LMP7701/LMP7702/

at room temperature.

The wide supply voltage range of 2.7V to 12V over the

extensive temperature range of −40˚C to 125˚C makes the

LMP7701/LMP7702/LMP7704 excellent choices for low volt-

age precision applications with extensive temperature re-

quirements.

cal input referred offset voltage and this offset is guaranteed

dual and quad, over temperature. This minimal offset voltage

allows more accurate signal detection and amplification in

precision applications.

input referred voltage noise of 9 nV/

give the LMP7701/

LMP7702/LMP7704 superiority for use in sensor applica-

tions. Lower levels of noise introduced by the amplifier mean

better signal fidelity and a higher signal-to-noise ratio.

National Semiconductor is heavily committed to precision

amplifiers and the market segment they serve. Technical

support and extensive characterization data is available for

sensitive applications or applications with a constrained error

budget.

The LMP7701 is offered in the space saving 5-Pin SOT23

package, the LMP7702 comes in the 8-pin MSOP, and the

LMP7704 is offered in the 14-Pin TSSOP package. These

small packages are ideal solutions for area constrained PC

boards and portable electronics.

CAPACITIVE LOAD

The LMP7701/LMP7702/LMP7704 can each be connected

as a non-inverting unity gain follower. This configuration is

the most sensitive to capacitive loading.

The combination of a capacitive load placed on the output of

an amplifier along with the amplifier’s output impedance

creates a phase lag which in turn reduces the phase margin

of the amplifier. If the phase margin is significantly reduced,

the response will be either underdamped or it will oscillate.

In order to drive heavier capacitive loads, an isolation resis-

tor, R

resistor, the capacitive load is isolated from the amplifier’s

output, and hence, the pole caused by C

feedback loop. The larger the value of R

the output voltage will be. If values of R

large, the feedback loop will be stable, independent of the

value of C

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 LMP7701/

LMP7702/LMP7704 enhance this performance by having

low input referred voltage noise of 9 nV/

. In order to

achieve this a larger input stage has been used. This larger

input stage increases the input capacitance of the LMP7701/

LMP7702/ LMP7704. The typical value of this input capaci-

tance, C

The 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 DC conditions, but will play a bigger role as the fre-

quency increases. At higher frequencies, the presence of

this pole will decrease phase margin and will also cause gain

peaking. In order to compensate for the input capacitance,

care must be taken in choosing the feedback resistors. In

addition 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 2 is simply

–R

For the time being, ignore C

Figure 2 can be calculated as follows:

20127321

FIGURE 1. Isolating Capacitive Load

20127344

FIGURE 2. Compensating for Input Capacitance

www.national.com

15