Datasheet

e =
O
[e +(R i ) +4kTR +(R i ) +4kTR ]
N S BN G BIS G
2 2 2
2
R
L
R
G
+
1
g
m
e
n
i
bn
i
bi
R
S
R
G
√
4kTR
S
√
4kTR
S
R
L
V
O
75
70
65
60
55
50
45
40
35
30
25
20
Frequency (Hz)
Common−Mode Rejection Ratio(dB)
1k 10k 100k 1M 10M 100M 1G
Input−Referred
OPA861
www.ti.com SBOS338G –AUGUST 2005–REVISED MAY 2013
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 51
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.
Figure 50. CMRR of the ADC Driver
DESIGN-IN TOOLS
Figure 51. OTA Noise Analysis Model
DEMONSTRATION BOARDS
The total output spot noise voltage can be computed
A printed circuit board (PCB) is available to assist in
as the square root of the sum of all squared output
the initial evaluation of circuit performance using the
noise voltage contributors. Equation 8 shows the
OPA861. This module is available free, as an
general form for the output noise voltage using the
unpopulated PCB delivered with descriptive
terms shown in Figure 51.
documentation. The summary information for the
board is shown below:
LITERATURE
BOARD PART REQUEST
PRODUCT PACKAGE NUMBER NUMBER
(8)
OPA861ID SO-8 DEM-OTA-SO-1A SBOU035
THERMAL ANALYSIS
The board can be requested on the Texas
Instruments web site (www.ti.com).
Maximum desired junction temperature will set the
maximum allowed internal power dissipation as
MACROMODELS AND APPLICATIONS
described below. In no case should the maximum
SUPPORT
junction temperature be allowed to exceed 150°C.
Computer simulation of circuit performance using
Operating junction temperature (T
J
) is given by
SPICE is often useful when analyzing the
T
A
+ P
D
× θ
JA
. The total internal power dissipation
performance of analog circuits and systems. This
(P
D
) is the sum of quiescent power (P
DQ
) and
principle is particularly true for Video and RF amplifier
additional power dissipated in the output stage (P
DL
)
circuits where parasitic capacitance and inductance
to deliver output current. Quiescent power is simply
can have a major effect on circuit performance. A
the specified no-load supply current times the total
SPICE model for the OPA861 is available through the
supply voltage across the part. P
DL
will depend on the
Texas Instruments web page (www.ti.com). These
required output signal and load but would, for the
models do a good job of predicting small-signal AC
OPA861 be at a maximum when the maximum I
O
is
and transient performance under a wide variety of
being driven into a voltage source that puts the
operating conditions. They do not do as well in
maximum voltage across the output stage. Maximum
predicting the harmonic distortion. These models do
I
O
is 15mA times a 9V maximum across the output.
not attempt to distinguish between the package types
Note that it is the power in the output stage and not
in their small-signal AC performance.
into the load that determines internal power
dissipation.
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