THS6062_16 Datasheet, PDF (19/36 Pages) Texas Instruments – LOW-NOISE ADSL DUAL DIFFERENTIAL RECEIVER

THS6062
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SLOS228D – JANUARY 1999 – REVISED OCTOBER 2007
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 noise-current parameters for
each input, while VFB amplifiers usually only specify one noise-current parameter. The noise model is shown in
Figure 41. 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 (e_Rx = 4 kTR_x)
R_S
e_Rs
e_n
Noiseless
e_ni
+
_
e_no
IN+
e_Rf
R_F
IN−
e_Rg
R_G
Figure 41. Noise Model
Ǹ _{ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ} ^{The total equivalent input noise density (eni) is calculated by using the following equation:}
eni +
ǒ_enǓ² _{) IN )}
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
(1)
Where:
k = Boltzmann's constant = 1.380658 × 10^-23
T = Temperature in degrees Kelvin (273 + °C)
R_F || R_G = Parallel resistance of R_F and R_G
To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (e_ni) by the
overall amplifier gain (A_V).
ǒ Ǔ eno + eni AV
+
e_ni
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
closed-loop gain is increased (by reducing R_G), 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 _S) and the internal amplifier noise voltage (e_n). 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.
For more information on noise analysis, refer to the Noise Analysis section in Operational Amplifier Circuits
Applications Report (SLVA043).
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.
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