OPA2846_14 Datasheet, PDF (18/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
−80
V_O = 2V_PP Differential
−85
2nd−Harmonic, 0V_DC
−90
3rd−Harmonic, +2.5V_DC
−95
−100
2nd−Harmonic, +2.5V_DC
−105
−110
1
3rd−Harmonic, 0V_DC
10
Frequency (MHz)
Figure 11. Harmonic Distortion for Figure 10
Figure 11 shows 2nd- and 3rd-harmonic distortion for a
2V_PP differential output swing at both 0V output
common-mode voltage and +2.5V common-mode volt-
age. Since there is no DC current required from the output
to level shift to +2.5V in this circuit, no pull-up resistors to
the power supply were used as in the circuit of Figure 8.
The 2nd harmonic remains the dominant distortion
mechanism, but shows little sensitivity to the common-
mode operating voltage (improved 2nd-harmonic distor-
tion results were achieved with this circuit using two
individual OPA846s with an extremely symmetrical
layout). The 3rd harmonic is essentially unmeasureable
for the ground-centered output swing, but increases as the
output is shifted to a +2.5V DC output. Narrowband
systems, where a bandpass filter less than an octave wide
can be inserted between the amplifier and the converter,
will only be concerned about 2-tone, 3rd-order inter-
modulation distortion. Since this bandpass filter is also
AC-coupled, the outputs of Figure 10 may be operated
ground-centered, giving the extremely low 3rd-order
distortions of Figure 11.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
A printed circuit board (PCB) is available to assist in the
initial evaluation of circuit performance using the
OPA2846. The fixture is offered free of charge as an
unpopulated PCB, delivered with a user’s guide. The
summary information for this board is shown in Table 1.
www.ti.com
PRODUCT
OPA2846ID
PACKAGE
ORDERING
NUMBER
SO-8 Surface-Mount DEM-OPA-SO-2A
LITERATURE
NUMBER
SBOU003
Table 1. Evaluation Module Ordering Information
The demonstration fixture can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2846
product folder.
MACROMODELS AND APPLICATIONS
SUPPORT
Computer simulation of circuit performance using SPICE
is often useful when analyzing the performance of analog
circuits and systems. This 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 OPA2846 is available through the
Texas Instruments web page (http://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 characteristics.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE
NOISE
The OPA2846 provides a very low input noise voltage
while requiring a low 12.6mA/channel quiescent current.
To take full advantage of this low input noise, careful
attention to the other possible noise contributors is
required. Figure 12 shows the op amp noise analysis
model with all the noise terms included. In this model, all
the noise terms are taken to be noise voltage or current
density terms in either nV/√Hz or pA/√Hz.
E_NI
1/2
R_S
I_BN
OPA2846
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√ 4kTR_F
R_G
I_BI
4kT = 1.6E − 20J
at 290_K
Figure 12. Op Amp Noise Analysis Model
18