Datasheet

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SLOS238D − MAY 1999 − REVISED AUGUST 2008
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15
APPLICATION INFORMATION
NOISE CALCULATIONS AND NOISE FIGURE (CONTINUED)
R
S
e
Rs
e
n
IN+
IN−
e
ni
Figure 43. Noise Model
The total equivalent input noise density (e
ni
) is calculated by using the following equation:
e
ni
+
ǒ
e
n
Ǔ
2
)
ǒ
IN ) R
S
Ǔ
2
)
ǒ
IN–
ǒ
R
F
ø R
G
ǓǓ
2
) 4kTR
s
) 4kT
ǒ
R
F
ø R
G
Ǔ
Ǹ
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
).
e
no
+ e
ni
A
V
+ e
ni
ǒ
1 )
R
F
R
G
Ǔ
(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).