Datasheet

OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
GBP
4 R Cp
F D
=
1
2 R Cp
F F
GBP
2 R Cp
F D
=
f
-3dB
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
MACROMODELS AND APPLICATIONS
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
Because the OPA4830 is a unity-gain stable,
this noise-gain peaking. To achieve a flat
voltage-feedback op amp, a wide range of resistor
transimpedance frequency response, this pole in the
values may be used for the feedback and gain setting
feedback network should be set to:
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
which gives a corner frequency f
– 3dB
of connection should be made with a direct short.
approximately:
Below 200 Ω , the feedback network presents
additional output loading that can degrade the
harmonic distortion performance of the OPA4830.
Above 1k Ω , 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
A printed circuit board (PCB) is available to assist in
combination of R
F
and R
G
(see Figure 74 ) to be less
the initial evaluation of circuit performance using the
than about 400 Ω . The combined impedance R
F
|| R
G
OPA4830. The fixture is offered free of charge as
interacts with the inverting input capacitance, placing
unpopulated PCB, delivered with a user ’ s guide. The
an additional pole in the feedback network, and thus
summary information for this fixture is shown in
a zero in the forward response. Assuming a 2pF total
Table 2 .
parasitic on the inverting node, holding R
F
|| R
G
<
400 Ω keeps this pole above 200MHz. By itself, this
Table 2. Demonstration Fixture
constraint implies that the feedback resistor R
F
can
increase to several k Ω at high gains. This increase is
LITERATURE
PRODUCT PACKAGE ORDERING NUMBER NUMBER
acceptable as long as the pole formed by R
F
and any
OPA4830IPW TSSOP-14 DEM-OPA-TSSOP-4A SBOU017
parasitic capacitance appearing in parallel is kept out
of the frequency range of interest.
The demonstration fixture can be requested at the
In the inverting configuration, an additional design
Texas Instruments web site (www.ti.com ) through the
consideration must be noted. R
G
becomes the input
OPA4830 product folder.
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.
SUPPORT
However, at low inverting gains, the resulting
Computer simulation of circuit performance using
feedback resistor value can present a significant load
SPICE is often a quick way to analyze the
to the amplifier output. For example, an inverting gain
performance of the OPA4830 and its circuit designs.
of 2 with a 50 Ω input matching resistor (= R
G
) would
This approach is particularly true for video and RF
require a 100 Ω feedback resistor, which would
amplifier circuits where parasitic capacitance and
contribute to output loading in parallel with the
inductance can play a major role on circuit
external load. In such a case, it would be preferable
performance. A SPICE model for the OPA4830 is
to increase both the R
F
and R
G
values, and then
available through the TI web page (www.ti.com ). Note
achieve the input matching impedance with a third
that this model is the OPA830 model applied to the
resistor to ground (see Figure 88 ). The total input
OPA4830 quad version. The applications department
impedance becomes the parallel combination of R
G
is also available for design assistance. These models
and the additional shunt resistor.
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.
Copyright © 2006 – 2008, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Link(s): OPA4830