Datasheet

DESIGN-IN TOOLS
Demonstration Fixtures
Macromodel and Applications Support
BANDWIDTH vs GAIN:
OPERATING SUGGESTIONS
OPA2830
SBOS309D – AUGUST 2004 – REVISED AUGUST 2008 .................................................................................................................................................. www.ti.com
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 400 Ω . The combined impedance R
F
|| R
G
interacts with the inverting input capacitance, placing
Two printed circuit boards (PCBs) are available to
an additional pole in the feedback network, and thus
assist in the initial evaluation of circuit performance
a zero in the forward response. Assuming a 2pF total
using the OPA2830 in its two package options. Both
parasitic on the inverting node, holding R
F
|| R
G
<
of these are offered free of charge as unpopulated
400 Ω will keep this pole above 200MHz. By itself, this
PCBs, delivered with a user's guide. The summary
constraint implies that the feedback resistor R
F
can
information for these fixtures is shown in Table 1 .
increase to several k Ω at high gains. This is
acceptable as long as the pole formed by R
F
and any
Table 1. Demonstration Fixtures by Package
parasitic capacitance appearing in parallel is kept out
ORDERING LITERATURE
of the frequency range of interest.
PRODUCT PACKAGE NUMBER NUMBER
In the inverting configuration, an additional design
OPA2830ID SO-8 DEM-OPA-SO-2A SBOU003
consideration must be noted. R
G
becomes the input
OPA2830IDGK MSOP-8 DEM-OPA-MSOP-2A SBOU004
resistor and therefore the load impedance to the
driving source. If impedance matching is desired, R
G
The demonstration fixtures can be requested at the
may be set equal to the required termination value.
Texas Instruments web site (www.ti.com ) through the
However, at low inverting gains, the resultant
OPA2830 product folder.
feedback resistor value can present a significant load
to the amplifier output. For example, an inverting gain
of 2 with a 50 Ω input matching resistor (= R
G
) would
Computer simulation of circuit performance using
require a 100 Ω feedback resistor, which would
SPICE is often a quick way to analyze the
contribute to output loading in parallel with the
performance of the OPA2830 and its circuit designs.
external load. In such a case, it would be preferable
This is particularly true for video and RF amplifier
to increase both the R
F
and R
G
values, and then
circuits where parasitic capacitance and inductance
achieve the input matching impedance with a third
can play a major role on circuit performance. A
resistor to ground (see Figure 84 ). The total input
SPICE model for the OPA2830 is available through
impedance becomes the parallel combination of R
G
the TI web page (www.ti.com ). The applications
and the additional shunt resistor.
department is also available for design assistance.
These models predict typical small signal AC,
transient steps, DC performance, and noise under a
NONINVERTING OPERATION
wide variety of operating conditions. The models
Voltage-feedback op amps exhibit decreasing
include the noise terms found in the electrical
closed-loop bandwidth as the signal gain is
specifications of the data sheet. These models do not
increased. In theory, this relationship is described by
attempt to distinguish between the package types in
the Gain Bandwidth Product (GBP) shown in the
their small-signal AC performance.
specifications. Ideally, dividing GBP by the
noninverting signal gain (also called the Noise Gain,
or NG) will predict the closed-loop bandwidth. In
OPTIMIZING RESISTOR VALUES
practice, this only holds true when the phase margin
Since the OPA2830 is a unity-gain stable,
approaches 90 ° , as it does in high-gain
voltage-feedback op amp, a wide range of resistor
configurations. At low gains (increased feedback
values may be used for the feedback and gain setting
factors), most amplifiers will exhibit a more complex
resistors. The primary limits on these values are set
response with lower phase margin. The OPA2830 is
by dynamic range (noise and distortion) and parasitic
compensated to give a slightly peaked response in a
capacitance considerations. For a noninverting
noninverting gain of 2 (see Figure 72 ). This results in
unity-gain follower application, the feedback
a typical gain of +2 bandwidth of 105MHz, far
connection should be made with a direct short.
exceeding that predicted by dividing the 105MHz
GBP by 2. Increasing the gain will cause the phase
Below 200 Ω , the feedback network will present
margin to approach 90 ° and the bandwidth to more
additional output loading which can degrade the
closely approach the predicted value of (GBP/NG). At
harmonic distortion performance of the OPA2830.
a gain of +10, the 10MHz bandwidth shown in the
Above 1k Ω , the typical parasitic capacitance
Electrical Characteristics agrees with that predicted
(approximately 0.2pF) across the feedback resistor
using the simple formula and the typical GBP of
may cause unintentional band limiting in the amplifier
105MHz.
response.
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