Application Hints

1.0 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 low-

est overall noise for the system.

1.1 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 im-

proves 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 pre-

ferred at low gains.

1.2 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 domi-

nate; at very high source impedances, the input noise cur-

rent times the equivalent external resistance will dominate.

For a detailed example calculation, refer to Note 1.

1.3 Bias current compensation resistor

In CMOS input op amps, the input bias currents are very low,

so there is no need to use R

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, in-

creasing the overall noise.

1.4 Resistor types

Thermal noise is generated by any passive resistive ele-

ment. This noise is

″white″; meaning it has a constant spec-

tral density. Thermal noise can be represented by a mean-

square voltage generator e

R

resistor, where e

R

e

R

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)

Actual resistor noise measurements may have more noise

than the calculated value. This additional noise component is

known as excess noise. Excess noise has a 1/f spectral re-

sponse, and is proportional to the voltage drop across the re-

sistor. It is convenient to define a noise index when referring

to excess noise in resistors. The noise index is the RMS

value in uV of noise in the resistor per volt of DC drop across

the resistor in a decade of frequency. Noise index expressed

in dB is:

NI = 20 log ((E

Where: E

cade.

V

Excess noise in carbon composition resistors corresponds to

a large noise index of +10 dB to -20 dB. Carbon film resistors

have a noise index of -10 dB to -25 dB. Metal film and wire

wound resistors show the least amount of excess noise, with

a noise index figure of -15 dB to -40 dB.

1.5 Other noise sources:

As the op amp and resistor noise sources are decreased,

other noise contributors will now be noticeable. Small air cur-

rents across thermocouples will result in low frequency

variations. Any two dissimilar metals, such as the lead on the

IC and the solder and copper foil of the pc board, will form a

thermocouple. The source itself may also generate noise. An

example would be a resistive bridge. All resistive sources

generate thermal noise based on the same equation listed

above under

″resistor types″.(2)

DS101081-23

Figure 1

DS101081-24

Figure 2

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