OPA4830 Datasheet, PDF (31/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
www.ti.com.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008
The dc gain for this circuit is equal to R_F. At high
frequencies, the DAC output capacitance (C_D)
produces a zero in the noise gain for the OPA4830
that may cause peaking in the closed-loop frequency
response. C_F is added across R_F to compensate for
this noise-gain peaking. To achieve a flat
transimpedance frequency response, this pole in the
feedback network should be set to:
1 ₌ GBP
2pR_FC_F
4pR_FC_D
which gives a corner frequency f_–3dB of
approximately:
f_-3dB =
GBP
2pR_FC_D
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
A printed circuit board (PCB) is available to assist in
the initial evaluation of circuit performance using the
OPA4830. The fixture is offered free of charge as
unpopulated PCB, delivered with a user’s guide. The
summary information for this fixture is shown in
Table 2.
Table 2. Demonstration Fixture
PRODUCT
OPA4830IPW
PACKAGE
TSSOP-14
ORDERING NUMBER
DEM-OPA-TSSOP-4A
LITERATURE
NUMBER
SBOU017
The demonstration fixture can be requested at the
Texas Instruments web site (www.ti.com) through the
OPA4830 product folder.
MACROMODELS AND APPLICATIONS
SUPPORT
Computer simulation of circuit performance using
SPICE is often a quick way to analyze the
performance of the OPA4830 and its circuit designs.
This approach 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 OPA4830 is
available through the TI web page (www.ti.com). Note
that this model is the OPA830 model applied to the
OPA4830 quad version. 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. This model does not attempt to distinguish
between the package types in their small-signal ac
performance.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Because the OPA4830 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 presents
additional output loading that can degrade the
harmonic distortion performance of the OPA4830.
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 74) 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 keeps this pole above 200MHz. By itself, this
constraint implies that the feedback resistor R_F can
increase to several kΩ at high gains. This increase 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 resulting
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 88). The total input
impedance becomes the parallel combination of R_G
and the additional shunt resistor.
Copyright © 2006–2008, Texas Instruments Incorporated
Product Folder Link(s): OPA4830
Submit Documentation Feedback
31