OPA2686 Datasheet, PDF (14/18 Pages) Burr-Brown (TI) – Dual, Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER

Compensation For Improved SFDR” section of this data
sheet. The common-mode operating voltage is fed into
each amplifier’s non-inverting input. Since these are equal,
and will appear at each inverting input as well, no DC
current is produced through the transformer secondary due
to this common-mode operating voltage. Since no current
flows due to V_CM, the output will operate at V_CM as well.
This is one of the few common-mode operating point
control techniques that requires no current to flow. This
makes the common-mode control aspect of this circuit
essentially non-interactive with the signal path. To provide
a 2Vp-p differential signal operating at a 2.5V output
common-mode requires a 2.0V to 3.0V output swing on
each output. Tested performance over frequency for the
circuit of Figure 9 is shown in Figure 10.
–70
V_O = 2Vp-p Differential
2nd Harmonic
–80
0V and +2.5V DC
–90
–100
+2.5V DC
3rd Harmonic
0V DC
–110
1
10
Frequency (MHz)
FIGURE 10. Harmonic Distortion for Figure 9.
Figure 10 shows 2nd and 3rd harmonic distortion for a
2Vp-p differential output swing at both 0V output com-
mon-mode voltage and +2.5V common-mode voltage.
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 7.
The 2nd harmonic remains the dominant distortion mecha-
nism, but shows little sensitivity to the common-mode
operating voltage (improved 2nd harmonic distortion re-
sults were achieved with this circuit using two individual
OPA686N’s 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 two-tone, 3rd-order intermodulation dis-
tortion. Since this bandpass filter is also AC-coupled, the
outputs of Figure 9 may be operated ground centered,
giving the extremely low 3rd-order distortions shown in
Figure 10.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
A PC board is available to assist in the initial evaluation of
circuit performance using the OPA2686. It is available free
as an unpopulated PC board delivered with descriptive
documentation. The summary information for this board is
shown in the table below.
PRODUCT
PACKAGE
BOARD
PART
NUMBER
OPA2686U SO-8 Surface Mount DEM-OPA268xU
LITERATURE
REQUEST
NUMBER
MKT-352
Contact the Burr-Brown applications support line to request
this board.
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 induc-
tance can have a major effect on circuit performance. A
SPICE model for the OPA2686 is available through either
the Burr-Brown Internet web page (http://www.burr-
brown.com) or as one model on a disk from the Burr-Brown
Applications department (1-800-548-6132). The Applica-
tions department is also available for design assistance at this
number. 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. These models do not
attempt to distinguish between the package types in their
small-signal AC performance.
®
OPA2686
14