Datasheet

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  
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SLOS214C − OCTOBER 1998 − REVISED MARCH 2007
16
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 52. 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
(1)
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)
(2)
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
F
+ R
G
), the input noise can be 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.
By using the low noise preamplifiers as the first element in the signal chain, the input signal’s signal-to-noise
ratio (SNR) is maintained throughout the entire system. This is because the dominant system noise is due to
the first amplifier. This can be seen with the following example: