OPA2830 Datasheet, PDF (28/43 Pages) National Semiconductor (TI) – Dual, Low-Power, Single-Supply, Wideband OPERATIONAL AMPLIFIER

OPA2830
SBOS309D – AUGUST 2004 – 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 OPA2830 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
OPA2830ID
OPA2830IDGK
PACKAGE
SO-8
MSOP-8
ORDERING
NUMBER
DEM-OPA-SO-2A
DEM-OPA-MSOP-2A
LITERATURE
NUMBER
SBOU003
SBOU004
The demonstration fixtures can be requested at the
Texas Instruments web site (www.ti.com) through the
OPA2830 product folder.
Macromodel and Applications Support
Computer simulation of circuit performance using
SPICE is often a quick way to analyze the
performance of the OPA2830 and its circuit designs.
This is particularly true for video and RF amplifier
circuits where parasitic capacitance and inductance
can play a major role on circuit performance. A
SPICE model for the OPA2830 is available through
the TI web page (www.ti.com). The applications
department is also available for design assistance.
These models predict typical small signal AC,
transient steps, DC performance, and noise under a
wide variety of operating conditions. The models
include the noise terms found in the electrical
specifications of the data sheet. These models do not
attempt to distinguish between the package types in
their small-signal AC performance.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA2830 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.
Below 200Ω, the feedback network will present
additional output loading which can degrade the
harmonic distortion performance of the OPA2830.
Above 1kΩ, 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 (see Figure 72) to be less
than about 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 will keep this pole above 200MHz. 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 capacitance appearing in parallel is kept out
of the frequency range of interest.
In the inverting configuration, an additional design
consideration must be noted. R_G becomes the input
resistor and therefore the load impedance to the
driving source. If impedance matching is desired, R_G
may be set equal to the required termination value.
However, at low inverting gains, the resultant
feedback resistor value can present a significant load
to the amplifier output. For example, an inverting gain
of 2 with a 50Ω input matching resistor (= R_G) would
require a 100Ω feedback resistor, which would
contribute to output loading in parallel with the
external load. In such a case, it would be preferable
to increase both the R_F and R_G values, and then
achieve the input matching impedance with a third
resistor to ground (see Figure 84). The total input
impedance becomes the parallel combination of R_G
and the additional shunt resistor.
BANDWIDTH vs GAIN:
NONINVERTING 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
noninverting 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
factors), most amplifiers will exhibit a more complex
response with lower phase margin. The OPA2830 is
compensated to give a slightly peaked response in a
noninverting gain of 2 (see Figure 72). This results in
a typical gain of +2 bandwidth of 105MHz, far
exceeding that predicted by dividing the 105MHz
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 10MHz bandwidth shown in the
Electrical Characteristics agrees with that predicted
using the simple formula and the typical GBP of
105MHz.
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