OPA2889 Datasheet, PDF (22/38 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373B – JUNE 2007 – REVISED AUGUST 2008 ....................................................................................................................................................... www.ti.com
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to
assist in the initial evaluation of circuit performance
using the OPA2889 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 1.
Table 1. Demonstration Fixtures by Package
PRODUCT
OPA2889ID
OPA2889IDGS
PACKAGE
SO-8
MSOP-10
ORDERING NUMBER
DEM-OPA-SO-2A
DEM-OPA-MSOP-2B
LITERATURE
NUMBER
SBOU003A
SBOU040
The demonstration fixtures can be requested at the
Texas Instruments web site (www.ti.com) through the
OPA2889 product folder.
MACROMODELS
Computer simulation of circuit performance using
SPICE is often useful when analyzing the
performance of analog circuits and systems. This
principle 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 OPA2889 is available through
the Texas Instruments web page (www.ti.com). This
model does a good job of predicting small-signal ac
and transient performance under a wide variety of
operating conditions. It does not do as well in
predicting the harmonic distortion or dG/dP
characteristics. This model does not attempt to
distinguish between the package types in their
small-signal ac performance.
OPERATING RECOMMENDATIONS
OPTIMIZING RESISTOR VALUES
Because the OPA2889 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 noninverting
unity-gain follower application, the feedback
connection should be made with a direct short.
Usually, the feedback resistor value should be
between 200Ω and 1.5kΩ. Below 200Ω, the feedback
network presents additional output loading which can
degrade the harmonic distortion performance of the
OPA2889. Above 1.5kΩ, the typical parasitic
capacitance (approximately 0.2pF) across the
feedback resistor can 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 (see Figure 50) to be less
than approximately 400Ω. The combined impedance
R_F || R_G interacts with the inverting 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, holding R_F ||
R_G < 400Ω keeps this pole above 160MHz. By itself,
this constraint implies that the feedback resistor R_F
can increase to several kΩ at high gains. This
increase in resistor size is acceptable as long as the
pole formed by R_F and any parasitic capacitance
appearing in parallel is kept out of the frequency
range of interest.
BANDWIDTH vs GAIN: NONINVERTING
OPERATION
Voltage-feedback op amps exhibit decreasing
closed-loop bandwidth as the signal gain increases.
In theory, this relationship is described by the Gain
Bandwidth Product (GBP) shown in the Electrical
Characteristics. Ideally, dividing GBP by the
noninverting signal gain (also called the Noise Gain,
or NG) predicts the closed-loop bandwidth. In
practice, this principle only holds true when the phase
margin approaches 90°, as it does in high gain
configurations. At low gains (increased feedback
factors), most amplifiers exhibit a more complex
response with lower phase margin. The OPA2889 is
compensated to give a slightly peaked response in a
noninverting gain of 2V/V (see Figure 50). This
compensation results in a typical gain of +2V/V
bandwidth of 60MHz, far exceeding that predicted by
dividing the 75MHz GBP by 2. Increasing the gain
causes 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 8MHz
bandwidth shown in the Electrical Characteristics
agrees closely with that predicted using the simple
formula and the typical GBP of 75MHz.
The frequency response in a gain of +2V/V may be
modified to achieve exceptional flatness simply by
increasing the noise gain to 2.5V/V. One way to
modify the response without affecting the +2V/V
signal gain, is to add a 750Ω resistor across the two
inputs, as shown in the circuit of Figure 50. A similar
technique may be used to reduce peaking in
unity-gain (voltage follower) applications. For
example, by using a 750Ω feedback resistor along
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