LMV792 datasheet(14/21 Pages) NSC

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Application Notes

ADVANTAGES OF THE LMV791/LMV792

Wide Bandwidth at Low Supply Current

The LMV791 and LMV792 are high performance op amps

that provide a unity gain bandwidth of 17 MHz while drawing

a low supply current of 1.15 mA. This makes them ideal for

providing wideband amplification in portable applications.

The enable and shutdown feature can also be used to

design more power efficient systems that offer wide band-

width and high performance while consuming less average

power.

Low Input Referred Noise and Low Input Bias Current

The LMV791/LMV792 have a very low input referred voltage

noise density (5.8 nV/

at 1 kHz). A CMOS input stage

ensures a small input bias current (100 fA) and low input

referred current noise (0.01 pA/

). This is very helpful in

maintaining signal fidelity, and makes the LMV791 and

LMV792 ideal for audio and sensor based applications.

Low Supply Voltage

The LMV791 and the LMV792 have performance guaran-

teed at 2.5V and 5V supply. The LMV791 family is guaran-

teed to be operational at all supply voltages between 2.0V

and 5.5V, for ambient temperatures ranging from −40˚C to

125˚C, thus utilizing the entire battery lifetime. The LMV791

and LMV792 are also guaranteed to be operational at 1.8V

supply voltage, for temperatures between 0˚C and 125˚C.

This makes the LMV791 family ideal for usage in low-voltage

commercial applications.

RRO and Ground Sensing

Rail-to-rail output swing provides maximum possible dy-

namic range at the output. This is particularly important

when operating at low supply voltages. An innovative posi-

tive feedback scheme is used to boost the current drive

capability of the output stage. This allows the LMV791 and

the LMV792 to source more than 40 mA of current at 1.8V

supply. This also limits the performance of the LMV791

family as comparators, and hence the usage of the LMV791

and the LMV792 in an open-loop configuration is not recom-

mended. The input common-mode range includes the nega-

tive supply rail which allows direct sensing at ground in

single supply operation.

Enable and Shutdown Features

The LMV791 family is ideal for battery powered systems.

With a low supply current of 1.15 mA and a shutdown current

of 140 nA typically, the LMV791 and LMV792 allow the

designer to maximize battery life. The enable pin of the

LMV791 and the LMV792 allows the op amp to be turned off

and reduce its supply current to less than 1 µA. To power on

the op amp the enable pin should be higher than V

+ - 0.5V,

where V

+ is the positive supply. To disable the op amp, the

enable pin voltage should be less than V

− + 0.5V, where V−

is the negative supply.

Small Size

The small footprint of the LMV791 and the LMV792 package

saves space on printed circuit boards, and enables the

design of smaller electronic products, such as cellular

phones, pagers, or other portable systems. Long traces

between the signal source and the opamp make the signal

path susceptible to noise. By using a physically smaller

LMV791 and LMV792 package, the opamp can be placed

closer to the signal source, reducing noise pickup and in-

creasing signal integrity.

CAPACITIVE LOAD TOLERANCE

The LMV791 and LMV792 can directly drive 120 pF in

unity-gain without oscillation. The unity-gain follower is the

most sensitive configuration to capacitive loading. Direct

capacitive loading reduces the phase margin of amplifiers.

The combination of the amplifier’s output impedance and the

capacitive load induces phase lag. This results in either an

underdamped pulse response or oscillation. To drive a

heavier capacitive load, the circuit in Figure 1 can be used.

In Figure 1, the isolation resistor R

ISO and the load capacitor

L form a pole to increase stability by adding more phase

margin to the overall system. The desired performance de-

pends on the value of R

ISO. The bigger the RISO resistor

value, the more stable V

OUT will be. Increased RISO would,

however, result in a reduced output swing and short circuit

current.

INPUT CAPACITANCE AND FEEDBACK CIRCUIT

ELEMENTS

The LMV791 family has a very low input bias current (100 fA)

and a low 1/f noise corner frequency (400 Hz), which makes

it ideal for sensor applications. However, to obtain this per-

formance a large CMOS input stage is used, which adds to

the input capacitance of the op-amp, C

IN. Though this does

not affect the DC and low frequency performance, at higher

frequencies the input capacitance interacts with the input

and the feedback impedances to create a pole, which results

in lower phase margin and gain peaking. This can be con-

trolled by being selective in the use of feedback resistors, as

well as by using a feedback capacitance, C

F. For example,

in the inverting amplifier shown in Figure 2,ifC

IN and CF are

ignored and the open loop gain of the op amp is considered

infinite then the gain of the circuit is −R

2/R1. An op amp,

however, usually has a dominant pole, which causes its gain

to drop with frequency. Hence, this gain is only valid for DC

and low frequency. To understand the effect of the input

capacitance coupled with the non-ideal gain of the op amp,

the circuit needs to be analyzed in the frequency domain

using a Laplace transform.

20116861

FIGURE 1. Isolation of C

L to Improve Stability

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