THS6002_16 Datasheet, PDF (29/46 Pages) Texas Instruments – DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

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THS6002
DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS
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SLOS202E− JANUARY 1998− REVISED MARCH 2007
APPLICATION INFORMATION
noise calculations and noise figure (continued)
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.
This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). Noise
figure is a measure of noise degradation caused by the amplifier. The value of the source resistance must be
defined and is typically 50 Ω in RF applications.
NF + 10log_{ȧȱȲeeRnis2}^ȳȴȧ
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate noise figure as:
ǒ Ǔ _NF
+
ȱ
10log^{ȧȧȧȧȧ}1
ȧȡȢ
)
en
2
) ^ǒIN )
4 kTRS
Ȳ
RS^Ǔ2_{ȧȣȤȳȧȧȧȧȧ}
È´
The Figure 55 shows the noise figure graph for the THS6002.
NOISE FIGURE
vs
SOURCE RESISTANCE
20
TA = 25°C
18
16
14
12
10
8
6
4
2
0
10
100
1k
10k
Rs − Source Resistance − Ω
Figure 55. Noise Figure vs. Source Resistance
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