OPA4872 Datasheet, PDF (13/27 Pages) Texas Instruments – 4:1 High-Speed Multiplexer

OPA4872
www.ti.com
SBOS346C – JUNE 2007 – REVISED MARCH 2011
Solving for R_O, with n devices connected together,
results in Equation 3:
75 ´ (n - 1) + 804
R_O =
2
´
1+
241200
-1
[75 ´ (n - 1) + 804]²
(3)
Results for n varying from 2 to 6 are given in Table 3.
Table 3. Series Resistance versus
Number of Parallel Outputs
NUMBER OF OPA4872s
2
3
4
5
6
R_O (Ω)
69
63.94
59.49
55.59
52.15
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE
BANDWIDTH
The output stage of the OPA4872 is a
current-feedback op amp, meaning it can hold an
almost constant bandwidth over signal gain settings
with the proper adjustment of the external resistor
values. This performance is shown in the Typical
Characteristic curves; the small-signal bandwidth
decreases only slightly with increasing gain. These
curves also show that the feedback resistor has been
changed for each gain setting. The resistor values on
the feedback path can be treated as frequency
response compensation elements while the ratio sets
the signal gain of the feedback resistor divided by the
gain resistor. Figure 32 shows the small-signal
frequency response analysis circuit for a current
feedback amplifier.
The two major limitations of this circuit are the device
requirements for each OPA4872 and the acceptable
return loss resulting from the mismatch between the
load and the matching resistor.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURE
A printed circuit board (PCB) is available to assist in
the initial evaluation of circuit performance using the
OPA4872. 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 4.
Table 4. OPA4872 Demonstration Fixture
PRODUCT
OPA4872
PACKAGE
SO-14
ORDERING NUMBER
DEM-OPA-SO-1E
LITERATURE
NUMBER
SBOU045
The demonstration fixture can be requested at the
Texas Instruments web site at (www.ti.com) through
the OPA4872 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
practice 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 OPA4872 is available through
the Texas Instruments web site at www.ti.com. This
model does a good job of predicting small-signal ac
and transient performance under a wide variety of
operating conditions. It does not do as well in
predicting the harmonic distortion or dG/dP
characteristics.
V_I
i_ERR
a
R_I
R_G
V_O
Z_(S) i_ERR
R_F
Figure 32. Recommended Feedback Resistor
versus Noise Gain
The key elements of this current-feedback op amp
model are:
α → Buffer gain from the noninverting input to the
inverting input
R_I → Buffer output impedance
i_ERR → Feedback error current signal
Z_(s)
→
Frequency-dependent
transimpedance gain from i_ERR to V_O
open-loop
The buffer gain is typically very close to 1.00 and is
normally neglected from signal gain considerations. It
will, however, set the CMRR for a single op amp
differential amplifier configuration. For a buffer gain
α < 1.0, the CMRR = –20 × log (1 – α) dB.
Copyright © 2007–2011, Texas Instruments Incorporated
Product Folder Link(s): OPA4872
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