OPA2680 Datasheet, PDF (15/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available to assist in the initial evalu-
ation of circuit performance using the OPA2680 in its two
package styles. All of these are available free as an
unpopulated PC board delivered with descriptive documenta-
tion. The summary information for these boards is shown
below:
PRODUCT
OPA2680U
OPA2680N
PACKAGE
8-Lead SO-8
SO-14
BOARD
PART
NUMBER
DEM-OPA26xU
DEM-OPA26xN
ORDERING
NUMBER
MKT-352
MKT-353
Contact the Burr-Brown Applications support line to request
any of these boards.
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 OPA2680 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-
tion 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 or dG/dP characteristics. These
models do not attempt to distinguish between the package
types in their small-signal AC performance.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA2680 is a unity gain stable voltage feedback
op amp, a wide range of resistor values may be used for the
feedback and gain setting resistors. The primary limits on
these values are set by dynamic range (noise and distortion)
and parasitic capacitance considerations. For a non-inverting
unity gain follower application, the feedback connection
should be made with a 25Ω resistor, not a direct short. This
will isolate the inverting input capacitance from the output
pin and improve the frequency response flatness. Usually,
the feedback resistor value should be between 200Ω and
1.5kΩ. Below 200Ω, the feedback network will present
additional output loading which can degrade the harmonic
distortion performance of the OPA2680. Above 1.5kΩ, the
typical parasitic capacitance (approximately 0.2pF) across
the feedback resistor may cause unintentional band-limiting
in the amplifier response.
A good rule of thumb is to target the parallel combination of
R_F and R_G (Figure 1) to be less than approximately 300Ω.
The combined impedance R_F || R_G interacts with the invert-
ing input capacitance, placing an additional pole in the
feedback network and thus, a zero in the forward response.
Assuming a 2pF total parasitic on the inverting node, hold-
ing R_F || R_G < 300Ω will keep this pole above 250MHz. By
itself, this constraint implies that the feedback resistor R_F
can increase to several kΩ at high gains. This is acceptable
as long as the pole formed by R_F and any parasitic capaci-
tance appearing in parallel is kept out of the frequency range
of interest.
BANDWIDTH VS GAIN: NON-INVERTING OPERATION
Voltage feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the Gain Bandwidth Product
(GBP) shown in the specifications. Ideally, dividing GBP by
the non-inverting signal gain (also called the Noise Gain, or
NG) will predict the closed-loop bandwidth. In practice, this
only holds true when the phase margin approaches 90°, as it
does in high gain configurations. At low gains (increased
feedback factor), most amplifiers will exhibit a more com-
plex response with lower phase margin. The OPA2680 is
compensated to give a slightly peaked response in a non-
inverting gain of 2 (Figure 1). This results in a typical gain
of +2 bandwidth of 220MHz, far exceeding that predicted by
dividing the 300MHz GBP by 2. Increasing the gain will
cause the phase margin to approach 90° and the bandwidth
to more closely approach the predicted value of (GBP/NG).
At a gain of +10, the 30MHz bandwidth shown in the
Typical Specifications agrees with that predicted using the
simple formula and the typical GBP of 300MHz.
The frequency response in a gain of +2 may be modified to
achieve exceptional flatness simply by increasing the noise
gain to 2.5. One way to do this, without affecting the +2
signal gain, is to add an 804Ω resistor across the two inputs
in the circuit of Figure 1. A similar technique may be used
to reduce peaking in unity gain (voltage follower) applica-
tions. For example, by using a 402Ω feedback resistor along
with a 402Ω resistor across the two op amp inputs, the
voltage follower response will be similar to the gain of +2
response of Figure 2. Reducing the value of the resistor
across the op amp inputs will further limit the frequency
response due to increased noise gain.
The OPA2680 exhibits minimal bandwidth reduction going
to single supply (+5V) operation as compared with ±5V.
This is because the internal bias control circuitry retains
nearly constant quiescent current as the total supply voltage
between the supply pins is changed.
INVERTING AMPLIFIER OPERATION
Since the OPA2680 is a general purpose, wideband volt-
age feedback op amp, all of the familiar op amp applica-
tion circuits are available to the designer. Inverting opera-
tion is one of the more common requirements and offers
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OPA2680