OPA860 Datasheet, PDF (24/33 Pages) Burr-Brown (TI) – Wide Bandwidth OPERATIONAL TRANSCONDUCTANCE AMPLIFIER (OTA) and BUFFER

OPA860
SBOS331C – JUNE 2005 – REVISED AUGUST 2008 ....................................................................................................................................................... www.ti.com
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
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
board is shown below:
PRODUCT
OPA860ID
PACKAGE
SO-8
BOARD PART
NUMBER
DEM-OTA-SO-1A
LITERATURE
REQUEST
NUMBER
SBOU035A
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.
Ǹ^{ƪ ƫƪ Æ« ƪ Æ« e}O ⁺
e²ⁿ ) ^ǒR_Si_bn^Ǔ2 ) 4kTR_S
R_L
R_G
)
1
gm
2
)
ǒ_RGibiǓ² _{) 4kTRG}
R_L
1
gm
(12)
For the buffer, the noise model is shown in Figure 63.
Equation 13 shows the general form for the output
noise voltage using the terms shown in Figure 63.
The board can be requested on Texas Instruments
web site (www.ti.com).
MACROMODELS AND APPLICATIONS
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
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
in their small-signal AC performance.
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 62
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.
R_S
√ 4kTR_S
e_n
R_G
i_bn
√ 4kTR_S
V_O
R_L
i_bi
Figure 62. OTA Noise Analysis Model
e_n
V_O
R_S
i_n
√ 4kTR_S
Figure 63. Buffer Noise Analysis Model
Ǹ _{eO +}
e²ⁿ
)
ǒ_inR
Ǔ
S
2
)
4kTR_S
(13)
THERMAL ANALYSIS
Due to the high output power capability of the
OPA860, heatsinking or forced airflow may be
required under extreme operating conditions.
Maximum desired junction temperature will set the
maximum allowed internal power dissipation as
described below. In no case should the maximum
junction temperature be allowed to exceed +150°C.
Operating junction temperature (T_J) is given by
T_A + P_D × θ_JA. The total internal power dissipation
(P_D) is the sum of quiescent power (P_DQ) and
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_S2/(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.
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