THS6022 Datasheet, PDF (25/38 Pages) Texas Instruments – 250-mA DUAL DIFFERENTIAL LINE DRIVER

THS6022
250-mA DUAL DIFFERENTIAL LINE DRIVER
SLOS225C – SEPTEMBER 1998 – REVISED JANUARY 2000
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 52. 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 51. Noise Model
+ ^{Ǹǒ Ǔ} ) ^ǒ ) ^Ǔ ) ^{ǒ ǒ} ø ^ǓǓ ) ) ^ǒ ø ^{Ǔ The total equivalent input noise density (eni) is calculated by using the following equation:}
2
2
2
eni
en
IN
RS
IN– RF RG
4 kTRs 4 kT RF RG
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
+ + ǒ ) Ǔ ^{To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (eni) by the}
overall amplifier gain (A_V).
eno eni AV
e_ni 1
R_F
R_G
(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 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 (R_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.
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