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SBOS249D − JUNE 2003− REVISED APRIL 2004
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
+6V
Power−supply
decoupling not
shown.
A PC board is available to assist in the initial evaluation of
circuit performance using the OPA2613 in its two package
styles. It is available, free, as an unpopulated PC board
delivered with descriptive documentation. The summary
information for this unit is shown in Table 2.
Ω
50 Load
Ω
50
VO
1/2
OPA2613
Ω
50
Source
RG
RF
Ω
Ω
402
200
Check the TI web site (www.ti.com) to request this board.
VI
VO
RF
RG
RM
66.7
Table 2. Demonstration Board Ordering
Information
−
−
2
=
=
Ω
VI
−
6V
DEMO BOARD
NUMBER
ORDERING
NUMBER
PRODUCT
PACKAGE
Figure 10. Inverting Gain of −1 with Impedance
Matching
OPA2613ID
SO-8
SBOU003
DEM-OPA268XU
In the inverting configuration, two key design
considerations must be noted. The first is that the gain
resistor (RG) 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. RG, by itself, is not normally
set to the required input impedance since its value, along
with the desired gain, will determine an RF, which may be
non-optimal from a frequency response standpoint. The
total input impedance for the source becomes the parallel
combination of RG and RM.
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
dG/dP 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.
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 RM
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 RG for calculating the noise gainwhich 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.
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.
20
TI [ TEXAS INSTRUMENTS ]
