Datasheet

LMV751

SNOS468E –AUGUST 1999–REVISED MARCH 2013

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APPLICATION HINTS

Noise

There are many sources of noise in a system: thermal noise, shot noise, 1/f, popcorn noise, resistor noise, just to

name a few. In addition to starting with a low noise op amp, such as the LMV751, careful attention to detail will

result in the lowest overall noise for the system.

To invert or not invert?

Both inverting and non-inverting amplifiers employ feedback to stabilize the closed loop gain of the block being

designed. The loop gain (in decibels) equals the algebraic difference between the open loop and closed loop

gains. Feedback improves the Total Harmonic Distortion (THD) and the output impedance. The various noise

sources, when input referred, are amplified, not by the closed loop gain, but by the noise gain. For a non-

inverting amplifier, the noise gain is equal to the closed loop gain, but for an inverting amplifier, the noise gain is

equal to the closed loop gain plus one. For large gains, e.g., 100, the difference is negligible, but for small gains,

such as one, the noise gain for the inverting amplifier would be two. This implies that non-inverting blocks are

preferred at low gains.

Source impedance

Because noise sources are uncorrelated, the system noise is calculated by taking the RMS sum of the various

noise sources, that is, the square root of the sum of the squares. At very low source impedances, the voltage

noise will dominate; at very high source impedances, the input noise current times the equivalent external

resistance will dominate. For a detailed example calculation, refer to Note 1.

Bias current compensation resistor

In CMOS input op amps, the input bias currents are very low, so there is no need to use R

COMP

(see Figure 21

and Figure 22) for bias current compensation that would normally be used with early generation bipolar op amps.

In fact, inclusion of the resistor would act as another thermal noise source in the system, increasing the overall

noise.

Figure 21. Bias Current Compensation Resistor

Figure 22. Bias Current Compensation Resistor

Resistor types

Thermal noise is generated by any passive resistive element. This noise is "white"; meaning it has a constant

spectral density. Thermal noise can be represented by a mean-square voltage generator e

in series with a

noiseless resistor, where e

is given by: Where:

= 4K TRB (volts)

where

• T = temperature in °K

• R = resistor value in ohms

• B = noise bandwidth in Hz

• K = Boltzmann's constant (1.38 x 10-23 W-sec/°K) (1)

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