OPA2684 Datasheet, PDF (19/33 Pages) Texas Instruments – MINIMAL BANDWIDTH CHANGE VERSUS GAIN, 170MHz BANDWIDTH AT G = +2

2kΩ
+6V
+12V
1/2
OPA2684
R_F
800Ω
R_P
1.07kΩ
2Vp-p
max
R_G
931Ω
R_P
1.07kΩ
R_F
800Ω
R_O
18.2Ω
1:1
12.67Vp-p → 135Ω
V₂ max
R_O
18.2Ω
2kΩ
+6V
1/2
OPA2684
FIGURE 13. Synthetic Output Impedance xDSL Driver.
This example takes a 2Vp-p maximum differential input to a
12.67Vp-p maximum differential voltage on a 135Ω line using
a 1:1 transformer. For a nominal line at maximum target
power, each output swings a maximum 8Vp-p delivering a
peak 47mA current, on a 12V supply this leaves 2V head-
room on each output with a total amplifier power dissipation
of 163mW. Figure 14 shows the distortion for a full scale
(12.67Vp-p on the line) and 1/2 scale sinusoid signal from
100kHz to 1MHz.
DIFFERENTIAL DISTORTION vs FREQUENCY
–65
–70 _3rd-Harmonic
V_L = 12.7Vp-p
–75
–80 ^2nd-Harmonic
V_L = 12.7Vp-p
–85
2nd-Harmonic
V_L = 6.3Vp-p
–90
–95
0.1
3rd-Harmonic
V_L = 6.3Vp-p
1
Frequency (MHz)
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in the
initial evaluation of circuit performance using the OPA2684 in
its two package styles. Both of these are offered free of charge
as unpopulated PCBs, delivered with a user’s guide. The
summary information for these fixtures is shown in Table I.
PRODUCT
OPA2684ID
OPA2684IDCN
PACKAGE
SO-8
SOT23-8
ORDERING
NUMBER
DEM-OPA-SO-2A
DEM-OPA-SOT-2A
LITERATURE
NUMBER
SBOU003
SBOU001
TABLE I. Demonstration Fixtures by Package.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2684
product folder.
MACROMODELS
Computer simulation of circuit performance using SPICE is often
useful when analyzing the performance of analog circuits and
systems. This is particularly true for higher speed designs where
parasitic capacitance and inductance can have a major effect on
circuit performance. A SPICE model for the OPA2684 is avail-
able in the product folder on the TI web site (www.ti.com). This
is the single channel model for the OPA684—simply use two of
these to implement an OPA2684 simulation. These models do
a good job of predicting small-signal AC and transient perfor-
mance under a wide variety of operating conditions. They do not
do as well in predicting the harmonic distortion or dG/dP char-
acteristics. These models do not attempt to distinguish between
the package types in their small-signal AC performance.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH
Any current-feedback op amp like the OPA2684 can hold high
bandwidth over signal-gain settings with the proper adjustment
of the external resistor values. A low-power part like the OPA4684
typically shows a larger change in bandwidth due to the signifi-
cant contribution of the inverting input impedance to loop-gain
changes as the signal gain is changed. Figure 15 shows a
simplified analysis circuit for any current- feedback amplifier.
V_I
i_ERR
α
R_I
R_G
V_O
Z_(S) i_ERR
R_F
FIGURE 14. Harmonic Distortion for Figure 13.
OPA2684
SBOS239D
FIGURE 15. Current-Feedback Transfer Function Analysis
Circuit.
19
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