Application Information

ADVANTAGES OF THE LMV641

Low Voltage and Low Power Operation

The LMV641 has performance guaranteed at supply voltages

of 2.7V and 10V. It is guaranteed to be operational at all sup-

ply voltages between 2.7V and 12.0V. The LMV641 draws a

low supply current of 138 µA. The LMV641 provides the low

voltage and low power amplification which is essential for

portable applications.

Wide Bandwidth

Despite drawing the very low supply current of 138 µA, the

LMV641 manages to provide a wide unity gain bandwidth of

10 MHz. This is easily one of the best bandwidth to power

ratios ever achieved, and allows this op amp to provide wide-

band amplification while using the minimum amount of power.

This makes the LMV641 ideal for low power signal processing

applications such as portable media players and other ac-

cessories.

Low Input Referred Noise

The LMV641 provides a flatband input referred voltage noise

density of 14 nV/

, which is significantly better than the

noise performance expected from a low power op amp. This

op amp also feature exceptionally low 1/f noise, with a very

low 1/f noise corner frequency of 4 Hz. Because of this the

LMV641 is ideal for low power applications which require de-

cent noise performance, such as PDAs and portable sensors.

Ground Sensing and Rail-to-Rail Output

The LMV641 has a rail-to-rail output stage, which provides

the maximum possible output dynamic range. This is espe-

cially important for applications requiring a large output swing.

The input common mode range of this part includes the neg-

ative supply rail which allows direct sensing at ground in a

single supply operation.

Small Size

The small footprint of the packages for the LMV641 saves

space on printed circuit boards, and enables the design of

smaller and more compact electronic products. Long traces

between the signal source and the op amp make the signal

path susceptible to noise. By using a physically smaller pack-

age, these op amps can be placed closer to the signal source,

reducing noise pickup and enhancing signal integrity.

STABILITY OF OP AMP CIRCUITS

If the phase margin of the LMV641 is plotted with respect to

the capacitive load (C

beyond 100 pF then the phase margin reduces significantly.

This is because the op amp is designed to provide the maxi-

mum bandwidth possible for a low supply current. Stabilizing

the LMV641 for higher capacitive loads would have required

either a drastic increase in supply current, or a large internal

compensation capacitance, which would have reduced the

bandwidth. Hence, if this device is to be used for driving higher

capacitive loads, it will have to be externally compensated.

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FIGURE 1. Gain vs. Frequency for an Op Amp

An op amp, ideally, has a dominant pole close to DC which

causes its gain to decay at the rate of 20 dB/decade with re-

spect to frequency. If this rate of decay, also known as the

rate of closure (ROC), remains the same until the op amp's

unity gain bandwidth, then the op amp is stable. If, however,

a large capacitance is added to the output of the op amp, it

combines with the output impedance of the op amp to create

another pole in its frequency response before its unity gain

frequency (Figure 1). This increases the ROC to 40 dB/

decade and causes instability.

In such a case, a number of techniques can be used to restore

stability to the circuit. The idea behind all these schemes is to

modify the frequency response such that it can be restored to

an ROC of 20 dB/decade, which ensures stability.

In The Loop Compensation

Figure 2 illustrates a compensation technique, known as in

the loop compensation, that employs an RC feedback circuit

within the feedback loop to stabilize a non-inverting amplifier

configuration. A small series resistance, R

the amplifier output from the load capacitance, C

capacitance, C

bypass C

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FIGURE 2. In the Loop Compensation

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