Datasheet

_
+
R
f
R
S
R
g
e
Rg
e
Rf
e
Rs
e
n
IN+
Noiseless
IN–
e
ni
e
no
_
+
10 µF
0.1 µF
+12 V
499 Ω
+
V
I
+
_
+
10 µF
0.1 µF
–12 V
499 Ω
+
V
I
–
1:n
Telephone Line
THS3062(a)
R
Line
210 Ω
0.1 µF
THS3062(b)
R
S
R
S
R
Line
2n
2
R
Line
2n
2
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
e
ni
+
ǒ
e
n
Ǔ
2
)
ǒ
IN ) R
S
Ǔ
2
)
ǒ
IN *
ǒ
R
f
ø R
g
ǓǓ
2
) 4 kTR
s
) 4 kT
ǒ
R
f
ø R
g
Ǔ
Ǹ
e
no
+ e
ni
A
V
+ e
ni
ǒ1 )
R
f
R
g
Ǔ(Noninverting Case)
THS3061
THS3062
SLOS394B –JULY 2002–REVISED NOVEMBER 2009 www.ti.com
Figure 49. Noise Model
space
space
space
space
Figure 48. Simple Line Driver With THS3062
space
Due to the high supply voltages and the large The total equivalent input noise density (e
ni
) is
current-drive capability, the power dissipation of the calculated by using the following equation:
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
To calculate the equivalent output noise of the
package is too large to allow useful applications with
amplifier, multiply the equivalent input noise density
up to 30 V across the power-supply terminals with
(e
ni
) by the overall amplifier gain (A
V
).
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
As the previous equations show, to keep noise at a
Power Dissipation and Thermal Considerations
minimum, small value resistors should be used. As
section for more information on thermal management.
the closed-loop gain is increased (by reducing R
F
and
R
G
), the input noise is reduced considerably because
NOISE CALCULATIONS
of the parallel resistance term. This leads to the
Noise can cause errors on very small signals. This is
general conclusion that the most dominant noise
especially true for amplifying small signals coming
sources are the source resistor (R
S
) and the internal
over a transmission line or an antenna. The noise
amplifier noise voltage (e
n
). Because noise is
model for current-feedback amplifiers (CFB) is the
summed in a root-mean-squares method, noise
same as for voltage feedback amplifiers (VFB). The
sources smaller than 25% of the largest noise source
only difference between the two is that CFB
can be effectively ignored. This can greatly simplify
amplifiers generally specify different current-noise
the formula and make noise calculations much easier.
parameters for each input, while VFB amplifiers
usually only specify one noise-current parameter. The
PCB LAYOUT TECHNIQUES
noise model is shown in Figure 49. This model
FOR OPTIMAL PERFORMANCE
includes all of the noise sources as follows:
Achieving optimum performance with high-frequency
• en = Amplifier internal voltage noise (nV/√Hz)
devices in the THS306x family requires careful
• IN+ = Noninverting current noise (pA/√Hz)
attention to board layout, parasitic effects, and
• IN– = Inverting current noise (pA/√Hz)
external component types.
• eRx = Thermal voltage noise associated with each
Recommendations to optimize performance include:
resistor (eRx = 4 kTRx)
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Product Folder Link(s): THS3061 THS3062