THS6002

DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

SLOS202D– JANUARY 1998– REVISED JULY 1999

29

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, 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

e

2

ni

e

Rs

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

voltage, we can approximate noise figure as:

NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4kTR

S

The Figure 55 shows the noise figure graph for the THS6002.

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 55. Noise Figure vs. Source Resistance