Datasheet

OPA842
www.ti.com SBOS267D –NOVEMBER 2002–REVISED SEPTEMBER 2010
MACROMODELS AND APPLICATIONS SUPPORT
DESIGN-IN TOOLS
Computer simulation of circuit performance using
DEMONSTRATION FIXTURES
SPICE is often a quick way to analyze the
performance of the OPA842 and its circuit designs.
Two printed circuit boards (PCBs) are available to
This is particularly true for video and RF amplifier
assist in the initial evaluation of circuit performance
circuits where parasitic capacitance and inductance
using the OPA842 in its two package options. Both of
can play a major role on circuit performance. A
these are offered free of charge as unpopulated
SPICE model for the OPA842 is available through the
PCBs, delivered with a user's guide. The summary
TI web page (www.ti.com). The applications group is
information for these fixtures is shown in Table 1.
also available for design assistance. These models
predict typical small-signal ac, transient steps, dc
Table 1. Demonstration Fixtures by Package
performance, and noise under a wide variety of
ORDERING LITERATURE
operating conditions. The models include the noise
PRODUCT PACKAGE NUMBER NUMBER
terms found in the Electrical Characteristics of the
OPA842ID SO-8 DEM-OPA-SO-1A SBOU009
data sheet. These models do not attempt to
OPA842IDBV SOT23-5 DEM-OPA-SOT-1A SBOU010
distinguish between the package types in the
small-signal ac performance.
The demonstration fixtures can be requested at the
Texas Instruments web site (www.ti.com) through the
OPA842 product folder.
OPERATING SUGGESTIONS
In the inverting configuration, an additional design
OPTIMIZING RESISTOR VALUES
consideration must be noted. R
G
becomes the input
resistor and therefore the load impedance to the
Since the OPA842 is a unity-gain stable,
driving source. If impedance matching is desired, R
G
voltage-feedback op amp, a wide range of resistor
may be set equal to the required termination value.
values may be used for the feedback and gain setting
However, at low inverting gains, the resultant
resistors. The primary limits on these values are set
feedback resistor value can present a significant load
by dynamic range (noise and distortion) and parasitic
to the amplifier output. For example, an inverting gain
capacitance considerations. For a noninverting
of 2 with a 50Ω input matching resistor (equal to R
G
)
unity-gain follower application, the feedback
would require a 100Ω feedback resistor, which would
connection should be made with a 25Ω resistor—not
contribute to output loading in parallel with the
a direct short. This will isolate the inverting input
external load. In such a case, it would be preferable
capacitance from the output pin and improve the
to increase both the R
F
and R
G
values, and then
frequency response flatness. Usually, the feedback
achieve the input matching impedance with a third
resistor value should be between 200Ω and 1kΩ.
resistor to ground (see Figure 38). The total input
Below 200Ω, the feedback network will present
impedance becomes the parallel combination of R
G
additional output loading which can degrade the
and the additional shunt resistor.
harmonic distortion performance of the OPA842.
Above 1kΩ, the typical parasitic capacitance
(approximately 0.2pF) across the feedback resistor BANDWIDTH vs GAIN
may cause unintentional band limiting in the amplifier
Voltage-feedback op amps exhibit decreasing
response.
closed-loop bandwidth as the signal gain is
A good rule of thumb is to target the parallel increased. In theory, this relationship is described by
combination of R
F
and R
G
(see Figure 37) to be less the GBP shown in the specifications. Ideally, dividing
than about 200Ω. The combined impedance R
F
|| R
G
GBP by the noninverting signal gain (also called the
interacts with the inverting input capacitance, placing Noise Gain, or NG) will predict the closed-loop
an additional pole in the feedback network, and thus bandwidth. In practice, this only holds true when the
a zero in the forward response. Assuming a 2pF total phase margin approaches 90 degrees, as it does in
parasitic on the inverting node, holding R
F
|| R
G
< high-gain configurations. At low signal gains, most
200Ω will keep this pole above 400MHz. By itself, this amplifiers will exhibit a more complex response with
constraint implies that the feedback resistor R
F
can lower phase margin. The OPA842 is optimized to
increase to several kΩ at high gains. This is give a maximally flat second-order Butterworth
acceptable as long as the pole formed by R
F
and any response in a gain of 2. In this configuration, the
parasitic capacitance appearing in parallel is kept out OPA842 has approximately 60 degrees of phase
of the frequency range of interest. margin and will show a typical –3dB bandwidth of
150MHz. When the phase margin is 60 degrees, the
closed-loop bandwidth is approximately √2 greater
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Product Folder Link(s): OPA842