Application Notes

ADVANTAGES OF THE LMV791

Wide Bandwidth at Low Supply Current

The LMV791 is a high performance op amp that provides a

unity gain bandwidth of 17 MHz while drawing a low supply

current of merely 1.15 mA. This makes it ideal for providing

wideband amplification in portable applications. The enable

and shutdown feature can also be used to design more

power efficient systems and obtain wider bandwidth and

better performance while using less power.

Low Input Referred Noise and Low Input Bias Current

The LMV791 has 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, hence, the input

referred current noise is very low (0.01 pA/

). This is

very helpful in maintaining signal fidelity, and makes the

LMV791 ideal for audio and sensor based applications.

Low Supply Voltage

LMV791 is guaranteed to perform at 2.5V and 5V supply.

The LMV791 is guaranteed to be operational at all supply

voltages between 2V and 5.5V, for ambient temperatures

ranging from −40˚C to 125˚C, thus utilizing the entire battery

lifetime. The LMV791 is also guaranteed to be operational at

1.8V supply voltage, for temperatures between 0˚C and

125˚C. This makes the LMV791 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 to

source more than 40 mA of current at 1.8V supply. This also

limits the performance of the LMV791 as a comparator, and

hence the usage of LMV791 in an open-loop configuration is

not recommended. The input common-mode range includes

the negative supply rail which allows direct sensing at

ground in single supply operation.

Enable and Shutdown Features

The LMV791 is ideal for battery powered systems. With a

low supply current of 1.15 mA and a shutdown current

typically less than 1 µA, it allows the designer to maximize

battery life. The enable pin of the LMV791 allows the op amp

to be turned off and reduce its supply current to less than 1

uA. To power on the op amp the enable pin should be higher

than V

op amp, the enable pin should be lesser than V

where V

Small Size

The small footprint of the LMV791 package saves space on

printed circuit boards, and enables the design of smaller

electronic products, such as cellular phones, pagers, or

other portable systems. Signals can pick up noise between

the signal source and the amplifier. By using a physically

smaller amplifier package, the LMV791 can be placed closer

to the signal source, reducing noise pickup and increasing

signal integrity.

CAPACITIVE LOAD TOLERANCE

The LMV791 can directly drive 120 pF in unity-gain without

oscillation. The unity-gain follower is the most sensitive con-

figuration 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

C

margin to the overall system. The desired performance de-

pends on the value of R

value, the more stable V

however, result in a reduced output swing and short circuit

current.

INPUT CAPACITANCE AND FEEDBACK CIRCUIT

ELEMENTS

The LMV791 has a very low input bias current (50 fA) and a

low 1/f noise corner frequency (400 Hz), which makes it ideal

for sensor applications. However, to obtain this performance

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

capacitance. Though this does not affect the DC and low

frequency performance, at higher frequencies the input ca-

pacitance interacts with the input and the feedback imped-

ances to create a pole, which results in lower phase margin

and gain peaking. This can be controlled by being selective

in the use of feedback resistors, as well as by using a

feedback capacitance. For example, in the non-inverting

amplifier shown in Figure 2,ifC

the open loop gain of the op amp is considered infinite then

the gain of the circuit is −R

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.

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