THS6007 Datasheet, PDF (27/36 Pages) Texas Instruments – DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS

THS6007
DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS
APPLICATION INFORMATION
SLOS334– DECEMBER 2000
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, 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.
+ _ȧȱȲ ^ȳȴȧ NF
10log
eni ²
eRs
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate the noise figure as:
ȧȧȧȧȱȲ ^{ȧȡȢǒ Ǔ ǒ Ǔ ȧȣȤȳȴȧȧȧȧ} _{+ ) NF 10log 1}
) ) ²
en
IN
4 kTRS
2
RS
The Figure 83 shows the noise figure graph for the drivers of the THS6007. Figure 84 shows the noise figure
graph for the receivers of the THS6007.
DRIVER 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 83
RECEIVER NOISE FIGURE
vs
SOURCE RESISTANCE
40
f = 10 kHz
35 _{TA = 25°C}
30
25
20
15
10
5
0
10
100
1k
10k
Source Resistance – RS (Ω)
Figure 84
100k
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