THS6002_16 Datasheet, PDF (28/46 Pages) Texas Instruments – DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

THS6002
ą
DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS
ą
SLOS202E− JANUARY 1998− REVISED MARCH 2007
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 (e_Rx = 4 kTR_x )
RS
eRs
en
Noiseless
eni
+
_
eno
IN+
eRf
RF
IN−
eRg
RG
Figure 54. Noise Model
The total equivalent input noise density (e_ni) is calculated by using the following equation:
Ǹ _{ǒ Ǔ eni + ǒenǓ}² _{) ǒIN ) RSǓ}² _{) IN– ǒRF ø RGǓ} ² _{) 4 kTRs ) 4 kTǒRF ø RGǓ}
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)
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