Datasheet

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