OPA861
www.ti.com
SBOS338–AUGUST 2005
DESIGN-IN TOOLS
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 8 shows the
general form for the output noise voltage using the
terms shown in Figure 48.
DEMONSTRATION BOARDS
A printed circuit board (PCB) is available to assist in
the initial evaluation of circuit performance using the
OPA861. This module is available free, as an
unpopulated PCB delivered with descriptive docu-
mentation. The summary information for the board is
shown below:
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LITERATURE
REQUEST
NUMBER
THERMAL ANALYSIS
BOARD PART
NUMBER
PRODUCT
PACKAGE
Maximum desired junction temperature will set the
maximum allowed internal power dissipation as de-
scribed below. In no case should the maximum
junction temperature be allowed to exceed 150°C.
OPA861ID
SO-8
DEM-OPA86xD
SBOU035
The board can be requested on Texas Instruments
web site (www.ti.com).
Operating junction temperature (TJ) is given by
TA + PD × θJA. The total internal power dissipation
(PD) is the sum of quiescent power (PDQ) and
MACROMODELS AND APPLICATIONS
SUPPORT
additional power dissipated in the output stage (PDL
)
Computer simulation of circuit performance using
SPICE is often useful when analyzing the perform-
ance of analog circuits and systems. This principle 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 OPA861 is available through the Texas
Instruments web page (www.ti.com). These models
do a good job of predicting small-signal AC and
to deliver output current. Quiescent power is simply
the specified no-load supply current times the total
supply voltage across the part. PDL will depend on the
required output signal and load but would, for the
OPA861 be at a maximum when the maximum IO is
being driven into a voltage source that puts the
maximum voltage across the output stage. Maximum
IO is 15mA times a 9V maximum across the output.
Note that it is the power in the output stage and not
into the load that determines internal power dissi-
pation.
transient performance under
a wide variety of
operating conditions. They do not do as well in
predicting the harmonic distortion. These models do
not attempt to distinguish between the package types
in their small-signal AC performance.
As a worst-case example, compute the maximum TJ
using an OPA861IDBV in the circuit of Figure 29b
operating at the maximum specified ambient tem-
perature of +85°C and driving a –1V voltage refer-
ence.
NOISE PERFORMANCE
The OTA noise model consists of three elements: a
voltage noise on the B-input; a current noise on the
B-input; and a current noise on the E-input. Figure 48
shows the OTA noise analysis model with all the
noise terms included. In this model, all noise terms
are taken to be noise voltage or current density terms
in either nV/√Hz or pA/√Hz.
PD = 10V × 5.4mA + (15mA × 9V) = 185mW
Maximum TJ = +85°C + (0.19W × 150°C/W) = 114°C.
Although this is still well below the specified maxi-
mum junction temperature, system reliability con-
siderations may require lower tested junction tem-
peratures. The highest possible internal dissipation
will occur if the load requires current to be forced into
the output for positive output voltages or sourced
from the output for negative output voltages. This
puts a high current through a large internal voltage
drop in the output transistors.
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BOARD LAYOUT GUIDELINES
√
√
4kTRS
4kTRS
Achieving
optimum
performance
with
a
high-frequency amplifier like the OPA861 requires
careful attention to board layout parasitics and exter-
nal component types. Recommendations that will
optimize performance include:
Figure 48. OTA Noise Analysis Model
23
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