THS6007

DUAL DIFFERENTIAL LINE DRIVERS AND LOW-POWER RECEIVERS

SLOS334– DECEMBER 2000

27

POST OFFICE BOX 655303

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

resistance term. This leads to the general conclusion that the most dominant noise sources are the source

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.

NF

+ 10log

e

2

ni

e

Rs

Because the dominant noise components are generally the source resistance and the internal amplifier noise

voltage, we can approximate the noise figure as:

NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4kTR

S

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.

Figure 83

DRIVER NOISE FIGURE

vs

SOURCE RESISTANCE

18

14

0

20

16

12

Rs – Source Resistance – Ω

10

100

1k

10k

TA = 25°C

8

4

10

6

2

Figure 84

0

5

10

15

20

25

30

35

40

Source Resistance – RS (Ω)

f = 10 kHz

TA = 25°C

RECEIVER NOISE FIGURE

vs

SOURCE RESISTANCE

10

100

1k

10k

100k