THS6092SLASH3 datasheet(19/46 Pages) TI1

19 / 46 page

THS6042, THS6043

350 mA, ±12 V ADSL CPE LINE DRIVERS

SLOS264G − MARCH 2000 − REVISED DECEMBER 2001

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

noise calculations and noise figure

Noise can cause errors on very small signals. This is especially true for the amplifying small signals. The noise

model for current feedback amplifiers (CFB) is the same as voltage feedback amplifiers (VFB). The only

difference between the two is that the CFB amplifiers generally specify different current noise parameters for

each input, while VFB amplifiers usually only specify one noise current parameter. The noise model is shown

in Figure 38. This model includes all of the noise sources as follows:

• e

n = Amplifier internal voltage noise (nV/√Hz)

• IN+ = Noninverting current noise (pA/√Hz)

• IN− = Inverting current noise (pA/√Hz)

• e

Rx = Thermal voltage noise associated with each resistor (eRx = 4 kTRx )

eRg

eRf

eRs

IN+

Noiseless

IN−

eni

eno

Figure 38. Noise Model

The total equivalent input noise density (eni) is calculated by using the following equation:

ni +

) IN )

) IN–

F ø

) 4kTRs ) 4kT RF ø RG

Where:

k = Boltzmann’s constant = 1.380658

× 10−23

T = Temperature in degrees Kelvin (273 +

°C)

RF || RG = Parallel resistance of RF and RG

To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (eni) by the

overall amplifier gain (AV).

eno + eni AV + eni 1 )

(Noninverting Case)

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 RG), 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 (RS) and the internal amplifier noise voltage (en). 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.

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