Datasheet
OPA846
15
SBOS250E
www.ti.com
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA846ID SO-8 DEM-OPA-SO-1B SBOU026
OPA846IDBV SOT23-5 DEM-OPA-SOT-1B SBOU027
TABLE I. Demonstration Fixtures by Package.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA846
in its two package options. 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.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA846
product folder.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often a quick way to analyze the performance of the OPA846
and its circuit designs. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and induc-
tance can play a major role on circuit performance. A SPICE
model for the OPA846 is available through the TI web page
(www.ti.com). These models predict typical small-signal AC,
transient steps, and DC performance under a wide variety of
operating conditions. The models include the noise terms
found in the electrical specification of this data sheet. These
models do not attempt to distinguish between the package
types in small-signal AC performance.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE NOISE
The OPA846 provides a very low input noise voltage while
requiring a low 12.6mA quiescent current. To take full advan-
tage of this low input noise, careful attention to the other
R
F
Power-supply decoupling
not shown.
Range
2R
F
2R
F
0.01µF
R
G
V
I
OPA846
+V
CC
+12V+5V
V
O
=
– V
I
V
CC
2
R
F
R
G
FIGURE 12. Single-Supply Inverting Amplifier.
possible noise contributors is required. Figure 13 shows the
op amp noise analysis model with all the noise terms in-
cluded. In this model, all the terms are taken to be noise
voltage or current density terms in either nV/
√Hz
or pA/
√Hz
.
The total output spot noise voltage is computed as the
square root of the squared contributing terms to the output
noise voltage. This computation adds all the contributing
noise powers at the output by superposition and then takes
the square root of the terms to get back to a spot noise
voltage. Equation 8 shows the general form for this output
noise voltage using the terms of Figure 13.
E E I R kTR NG I R kTR NG
O
NI
BN
SS
BI F F
=+
(
)
+
(
)
+
(
)
+
2
222
44
(8)
Dividing this expression by the noise gain (NG = 1 + R
F
/R
G
)
gives the equivalent input-referred spot noise voltage at the
noninverting input, as shown in Equation 9.
E E I R kTR
IR
NG
kTR
NG
N
NI
BN
SS
BI F F
=+
(
)
++
+
2
2
2
4
4
(9)
Setting high resistor values into Equation 9 can quickly
dominate the total equivalent input referred noise. A 90Ω
source impedance on the noninverting input adds a Johnson
voltage noise term equal to that of the amplifier. As a
simplifying constraint, set R
G
= R
S
in Equation 9 and assume
an R
S
/2 source impedance is at the noninverting input (where
R
S
is the signal source impedance with another matching R
S
to ground on the noninverting input). This results in Equation
10, where NG > 10 is assumed to further simplify the
expression.
EE IR kT
R
N
NI
B
S
S
=+
(
)
+
2
2
5
4
4
3
2
(10)
Evaluating this expression for R
S
= 50Ω gives a total equiva-
lent input noise of 1.7nV/
√Hz
. Note that the NG has dropped
out of this expression.
This is valid only for NG > 10 as will typically be required by
stability considerations.
4kT
R
G
R
G
R
F
R
S
OPA846
I
BI
E
O
I
BN
4kT = 1.6E – 20J
at 290°K
E
RS
E
NI
√4kTR
S
√4kTR
F
FIGURE 13. Op Amp Noise Analysis Model.