Datasheet

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SBOS249H − JUNE 2003 − REVISED AUGUST 2008
www.ti.com
20
DESIGN-IN TOOLS
DEMONSTRATION FIXTURE
A printed circuit board (PCB) is available to assist in the
initial evaluation of circuit performance using the
OPA2613. The fixture is offered free of charge as an
unpopulated PCB, delivered with a user’s guide. The
summary information for this fixture is shown in Table 2.
Table 2. Demonstration Fixture
PRODUCT PACKAGE
ORDERING
NUMBER
LITERATURE
NUMBER
OPA2613ID SO-8
DEM-OPA-SO-2A
SBOU003
The demonstration fixture can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2613
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 OPA2613 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 OPA2613 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
OPA2613. Figure 10 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
OPA2613
R
F
402
Ω
V
O
V
I
R
G
200
Ω
+6V
−
6V
50
Ω
50
Ω
Load
V
O
Power−supply
decoupling not
shown.
V
I
50
Ω
Source
R
M
66.7
Ω
R
F
R
G
=
−
=
−
2
Figure 10. Inverting Gain of −1 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 PC board 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 10, the R
M
value combines in parallel with the external 50Ω source
impedance, yielding an effective driving impedance of
50Ω || 66.7Ω = 28.6Ω. This impedance is added in series
with R
G
for calculating the noise gainwhich gives
NG = 2.76. Note that the noninverting input in this bipolar
supply inverting application is connected to ground
through a 146Ω 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.