THS7002 datasheet(28/40 Pages) TI

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THS6002

DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

SLOS202D– JANUARY 1998– REVISED JULY 1999

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 receiver amplifiers which are

generally used for amplifying small signals coming over a transmission line. 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 54. 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 54. 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)

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