Datasheet
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
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MACROMODELS AND APPLICATIONS
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(13)
THERMAL ANALYSIS
NOISE PERFORMANCE
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OPA860
SBOS331C – JUNE 2005 – REVISED AUGUST 2008 .......................................................................................................................................................
www.ti.com
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 12 shows the
general form for the output noise voltage using the
terms shown in Figure 62 .
A printed circuit board (PCB) is available to assist in
the initial evaluation of circuit performance using the
OPA860. This module is available free, as an
unpopulated PCB delivered with descriptive
documentation. The summary information for the
(12)
board is shown below:
For the buffer, the noise model is shown in Figure 63 .
LITERATURE
BOARD PART REQUEST
Equation 13 shows the general form for the output
PRODUCT PACKAGE NUMBER NUMBER
noise voltage using the terms shown in Figure 63 .
OPA860ID SO-8 DEM-OTA-SO-1A SBOU035A
The board can be requested on Texas Instruments
web site (www.ti.com ).
SUPPORT
Computer simulation of circuit performance using
SPICE is often useful when analyzing the
performance 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 OPA860 is available through the
Figure 63. Buffer Noise Analysis Model
Texas Instruments web page (www.ti.com ). These
models do a good job of predicting small-signal AC
and 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
Due to the high output power capability of the
in their small-signal AC performance.
OPA860, heatsinking or forced airflow may be
required under extreme operating conditions.
Maximum desired junction temperature will set the
The OTA noise model consists of three elements: a
maximum allowed internal power dissipation as
voltage noise on the B-input; a current noise on the
described below. In no case should the maximum
B-input; and a current noise on the E-input. Figure 62
junction temperature be allowed to exceed +150 ° C.
shows the OTA noise analysis model with all the
Operating junction temperature (T
J
) is given by
noise terms included. In this model, all noise terms
T
A
+ P
D
× θ
JA
. The total internal power dissipation
are taken to be noise voltage or current density terms
(P
D
) is the sum of quiescent power (P
DQ
) and
in either nV/ √ Hz or pA/ √ Hz.
additional power dissipated in the output stage (P
DL
)
to deliver load power. Quiescent power is simply the
specified no-load supply current times the total supply
voltage across the part. P
DL
will depend on the
required output signal and load but would, for a
grounded resistive load, be at a maximum when the
output is fixed at a voltage equal to 1/2 of either
supply voltage (for equal bipolar supplies). Under this
condition, P
DL
= V
S
2
/(4 × R
L
) where R
L
includes
feedback network loading.
Note that it is the power in the output stage and not
into the load that determines internal power
dissipation.
Figure 62. OTA Noise Analysis Model
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