THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

16

POST OFFICE BOX 655303

APPLICATION INFORMATION

noise calculations and noise figure (continued)

_

+

RF

RS

RG

eRg

eRf

eRs

en

IN+

Noiseless

IN–

eni

eno

Figure 52. Noise Model

e

ni +

en

2

) IN )

R

S

2

) IN–

R

F ø

R

G

2

) 4kTRs ) 4kT RF ø RG

Where:

k = Boltzmann’s constant = 1.380658

T = temperature in degrees Kelvin (273 +

°C)

(1)

R

F

R

G

(Noninverting Case)

(2)

As the previous equations show, to keep noise at a minimum, small value resistors should be used. As the

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

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: