LMH6654MFX datasheet(14/17 Pages) NSC

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Application Information (Continued)

Total Input Noise vs. Source Resistance

The noise model for the non-inverting amplifier configuration

showing all noise sources is described in

Figure 5. In addi-

tion to the intrinsic input voltage noise (e

n) and current noise

n =in+ =in−) sources, there also exits thermal voltage noise

associated with each of the external resistors.

Equation 1 provides the general form for total equivalent

input voltage noise density (e

ni). Equation 2 is a simplifica-

tion of Equation 1 that assumes

(1)

f||Rg=Rseq for bias current cancellation. Figure 6 illustrates

the equivalent noise model using this assumption. The total

equivalent output voltage noise (e

no)iseni * AV.

(2)

If bias current cancellation is not a requirement, then R

f||Rg

does not need to equal R

seq. In this case, according to

Equation 1, R

fRg should be as low as possible in order to

minimize noise. Results similar to Equation 1 are obtained

for the inverting configuration on

Figure 2 if R

seq is replaced

by R

b and Rg is replaced by Rg +Rs. With these substitu-

tions, Equation 1 will yield an e

ni referred to the non-inverting

input. Referring to e

ni to the inverting input is easily accom-

plished by multiplying e

ni by the ration of non-inverting to

inverting gains.

Noise Figure

Noise Figure (NF) is a measure of the noise degradation

caused by an amplifier.

(3)

The noise figure formula is shown is Equation 3. The addi-

tion of a terminating resistor R

T, reduces the external ther-

mal noise but increases the resulting NF.

20016542

FIGURE 3. Non-Inverting Amplifier Configuration

20016543

FIGURE 4. Inverting Amplifier Configuration

20016544

FIGURE 5. Non-Inverting Amplifier Noise Model

20016545

FIGURE 6. Noise Model with R

f||Rg =Rseq

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