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
+VCC
voltage or current density terms in either nV/√Hz or pA/√Hz
.
+5V
+12V
Range
2RF
2RF
Power-supply decoupling
not shown.
ENI
VCC
RF
VO
=
OPA846
– VI
0.01µF
2
RG
EO
OPA846
RS
IBN
RG
RF
ERS
VI
RF
√4kTRS
√4kTRF
FIGURE 12. Single-Supply Inverting Amplifier.
IBI
RG
4kT
RG
4kT = 1.6E – 20J
at 290°K
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
FIGURE 13. Op Amp Noise Analysis Model.
Two PC boards are available to assist in the initial evaluation
of circuit performance using the OPA846 in its two package
styles. Both of these are available, free, as an unpopulated
PC board delivered with descriptive documentation. The
summary information for these boards is shown in Table I.
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.
Contact your sales representative or go to the TI web site
(www.ti.com) to request these evaluation boards.
BOARD
PART
NUMBER
LITERATURE
REQUEST
NUMBER
2
2
EO
=
E2 + IBNRS + 4kTRS NG2 + IBIRF + 4kTRFNG
(
)
(
)
(8)
(
)
NI
PRODUCT
PACKAGE
OPA846ID
SO-8
DEMOPA68XU
DEMOPA6XXN
SBOU009
SBOU010
Dividing this expression by the noise gain (NG = 1 + RF/RG)
gives the equivalent input-referred spot noise voltage at the
noninverting input, as shown in Equation 9.
OPA846IDBV
SOT23-5
TABLE I. Demo Board Part Numbers.
2
IBIRF
NG
4kTRF
NG
2
2
EN
=
ENI + IBNRS + 4kTRS
+
+
(
)
MACROMODELS AND APPLICATIONS SUPPORT
(9)
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.
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 RG = RS in Equation 9 and assume
an RS/2 source impedance is at the noninverting input (where
RS is the signal source impedance with another matching RS
to ground on the noninverting input). This results in Equation
10, where NG > 10 is assumed to further simplify the
expression.
5
4
3RS
2
2
EN
=
EN2I
+
I R
+ 4kT
(
)
(10)
B
S
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE NOISE
Evaluating this expression for RS = 50Ω gives a total equiva-
lent input noise of 1.7nV/√Hz. Note that the NG has dropped
out of this expression.
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
This is valid only for NG > 10 as will typically be required by
stability considerations.
OPA846
SBOS250C
15
www.ti.com
TI [ TEXAS INSTRUMENTS ]
