Datasheet
SLOS274D − DECEMBER 1999 − REVISED JUNE 2001
12
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
noise calculations and noise figure (continued)
_
+
R
F
R
S
R
G
e
Rg
e
Rf
e
Rs
e
n
IN+
Noiseless
IN−
e
ni
e
no
Figure 39. 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
Ǔ
Ǔ
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).