THS3061_16 Datasheet, PDF (14/33 Pages) Texas Instruments – LOW-DISTORTION, HIGH SLEW RATE, CURRENT-FEEDBACK AMPLIFIERS

THS3061
THS3062
SLOS394B – JULY 2002 – REVISED NOVEMBER 2009
+12 V
THS3062(a)
VI+
+
_
0.1 µF
+
10 µF
499 Ω
0.1 µF
210 Ω
RS
RLine
2n²
1:n
Telephone Line
RLine
R_S
e_Rs
e_n
e_ni
IN+
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Noiseless
+
_
e_no
e_Rf
R_f
IN–
e_Rg
R_g
THS3062(b)
VI–
+
_
499 Ω
–12 V
0.1 µF
10 µF
+
RS
RLine
2n²
Figure 48. Simple Line Driver With THS3062
Due to the high supply voltages and the large
current-drive capability, the power dissipation of the
amplifier must be carefully considered. To have as
much power dissipation as possible in a small
package, the THS3062 is available only in a MSOP-8
PowerPAD package (DGN), and an even lower
thermal-impedance SOIC-8 PowerPAD package
(DDA). The thermal impedance of a standard SOIC
package is too large to allow useful applications with
up to 30 V across the power-supply terminals with
this dual amplifier. But the THS3061 (a single
amplifier) can be used in the standard SOIC package.
Again, the amplifier power dissipation must be
carefully examined, or else the amplifiers could
overheat, severely degrading performance. See the
Power Dissipation and Thermal Considerations
section for more information on thermal management.
NOISE CALCULATIONS
Noise can cause errors on very small signals. This is
especially true for amplifying small signals coming
over a transmission line or an antenna. The noise
model for current-feedback amplifiers (CFB) is the
same as for voltage feedback amplifiers (VFB). The
only difference between the two is that 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 49. This model
includes all of the noise sources as follows:
• en = Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN– = Inverting current noise (pA/√Hz)
• eRx = Thermal voltage noise associated with each
resistor (eRx = 4 kTRx)
Figure 49. Noise Model
space
space
space
space
space
The total equivalent input noise density (e_ni) is
calculated by using the following equation:
Ǹ _{ǒ Ǔ ǒ Ǔ ǒ Ǔ ǒ Ǔ eni +}
ǒe_nǓ² ) IN )
2
RS ) IN *
2
Rf ø Rg ) 4 kTRs ) 4 kT Rf ø 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
To calculate the equivalent output noise of the
amplifier, multiply the equivalent input noise density
(e_ni) by the overall amplifier gain (A_V).
ǒ Ǔ eno + eni AV
+ eni
1
)
Rf
Rg
(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_F and
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.
PCB LAYOUT TECHNIQUES
FOR OPTIMAL PERFORMANCE
Achieving optimum performance with high-frequency
devices in the THS306x family requires careful
attention to board layout, parasitic effects, and
external component types.
Recommendations to optimize performance include:
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