Datasheet

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SBOS305D − JUNE 2004 − REVISED AUGUST 2008
www.ti.com
21
For the ADSL CPE upstream driver design of Figure 10,
the peak current is 150mA for a signal that requires a crest
factor of 5.33 with a target line power of 13dBm into 100Ω
(20mW). With a typical quiescent current of 12mA and a
nominal supply voltage of +12V, the total internal power
dissipation for the solution of Figure 10 will be:
P
TOT
+ 12mA
(
12V
)
)
150mA
5.33
(
12V
)
* 2
(
20mW
)
+ 400mW
DESIGN-IN TOOLS
DEMONSTRATION FIXTURE
A printed circuit board (PCB) is available to assist in the
initial evaluation of circuit performance using the
OPA2614. The fixture is offered free of charge as an
unpopulated PCB, delivered with user’s guide. The
summary information for this fixture is shown in Table 2.
Table 2. Demonstration Fixture
PRODUCT PACKAGE
ORDERING
NUMBER
LITERATURE
NUMBER
OPA2614ID SO-8
DEM-OPA-SO−2A
SBOU003
The demonstration fixture can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2614
product folder.
MACROMODELS AND APPLICATIONS
SUPPORT
Computer simulation of circuit performance using SPICE
is often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and
inductance can have a major effect on circuit performance.
A SPICE model for the OPA2614 is available through the
TI web site (www.ti.com). This model does a good job of
predicting small-signal AC and transient performance
under a wide variety of operating conditions, but does not
do as well in predicting the harmonic distortion or video
d
G
/d
P
characteristics. This model does not attempt to
distinguish between the package types in small-signal AC
performance, nor does it attempt to simulate channel-to-
channel coupling.
INVERTING AMPLIFIER OPERATION
As the OPA2614 is a general-purpose, wideband
voltage-feedback op amp, most of the familiar op amp
application circuits are available to the designer.
Wideband inverting operation is particularly suited to the
OPA2614. Figure 14 shows a typical inverting
configuration where the I/O impedances and signal gain
from Figure 1 are retained in an inverting circuit
configuration.
1/2
OPA2614
R
F
453
Ω
V
O
V
I
R
G
113
Ω
+6V
−
6V
50
Ω
50
Ω
Load
V
O
Power−supply
decoupling not
shown.
V
I
50
Ω
Source
R
M
89
Ω
110
Ω
0.01
µ
F
R
F
R
G
=
−
=
−
4
Figure 14. Inverting Gain of −4 with Impedance
Matching
In the inverting configuration, two key design
considerations must be noted. The first is that the gain
resistor (R
G
) becomes part of the input impedance. If input
impedance matching is desired (which is beneficial
whenever the signal is coupled through a cable, twisted-
pair, long PCB trace, or other transmission line conductor),
it is normally necessary to add an additional matching
resistor to ground. R
G
, by itself, is not normally set to the
required input impedance since its value, along with the
desired gain, will determine an R
F
, which may be
non-optimal from a frequency response standpoint. The
total input impedance for the source becomes the parallel
combination of R
G
and R
M
.
The second major consideration, touched on in the
previous paragraph, is that the signal source impedance
becomes part of the noise gain equation and has an effect
on the bandwidth. In the example of Figure 14, the R
M
value combines in parallel with the external 50Ω source
impedance, yielding an effective driving impedance of
50Ω || 89Ω = 32Ω. This impedance is added in series with
R
G
for calculating the noise gainwhich gives NG = 4.12.
Note that the noninverting input in this bipolar supply
inverting application is connected to ground through a
110Ω resistor. It is often suggested that an additional
resistor be connected to ground on the noninverting input
to achieve bias current error cancellation at the output.
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