THS4051 Datasheet, PDF (15/36 Pages) Texas Instruments – 70-MHz HIGH-SPEED AMPLIFIERS

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THS4051
THS4052
SLOS238D − MAY 1999 − REVISED AUGUST 2008
APPLICATION INFORMATION
NOISE CALCULATIONS AND NOISE FIGURE (CONTINUED)
RS
eRs
en
eni
IN+
IN−
Figure 43. Noise Model
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ eni +}
ǒ_enǓ² _{) IN )}
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
Where:
k = Boltzmann’s constant = 1.380658 × 10^−23
T = Temperature in degrees Kelvin (273 +°C)
R_F || R_G = Parallel resistance of R_F and R_G
To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (e_ni) by the overall amplifier
gain (A_V).
ǒ Ǔ eno + eni AV
+
e_ni
1
)
R_F
R_G
(noninverting case)
As the previous equations show, to keep noise at a minimum, small value resistors should be used. As the closed-loop gain
is increased (by reducing R_G), the input noise is reduced considerably because of the parallel resistance term. This leads
to the general conclusion that the most dominant noise sources are the source resistor (R_S) and the internal amplifier noise
voltage (e_n). Because noise is summed in a root-mean-squares method, noise sources smaller than 25% of the largest
noise source can be effectively ignored. This can greatly simplify the formula and make noise calculations much easier to
calculate.
For more information on noise analysis, please refer to the Noise Analysis section in Operational Amplifier Circuits
Applications Report (literature number SLVA043).
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