OPA2832 Datasheet, PDF (24/37 Pages) National Semiconductor (TI) – Dual, Low-Power, High-Speed, Fixed-Gain Operational Amplifier

OPA2832
SBOS327C – FEBRUARY 2005 – 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 OPA2832 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
OPA2832ID
OPA2832IDGK
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
OPA2832 product folder.
MACROMODEL AND APPLICATIONS
SUPPORT
Computer simulation of circuit performance using
SPICE is often a quick way to analyze the
performance of the OPA2832 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 OPA2832 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
OUTPUT CURRENT AND VOLTAGES
The OPA2832 provides outstanding output voltage
capability. For the +5V supply, under no-load
conditions at +25°C, the output voltage typically
swings closer than 90mV to either supply rail.
The minimum specified output voltage and current
specifications over temperature are set by worst-case
simulations at the cold temperature extreme. Only at
cold startup will the output current and voltage
decrease to the numbers shown in the ensured
tables. As the output transistors deliver power, their
junction temperatures will increase, decreasing their
V_BEs (increasing the available output voltage swing)
and increasing their current gains (increasing the
available output current). In steady-state operation,
the available output voltage and current will always
be greater than that shown in the over-temperature
specifications, since the output stage junction
temperatures will be higher than the minimum
specified operating ambient.
To maintain maximum output stage linearity, no
output short-circuit protection is provided. This will not
normally be a problem, since most applications
include a series matching resistor at the output that
will limit the internal power dissipation if the output
side of this resistor is shorted to ground. However,
shorting the output pin directly to the adjacent
positive power-supply pin (8-pin packages) will, in
most cases, destroy the amplifier. If additional
short-circuit protection is required, consider a small
series resistor in the power-supply leads. This will
reduce the available output voltage swing under
heavy output loads.
DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common
load conditions for an op amp is capacitive loading.
Often, the capacitive load is the input of an
ADC—including additional external capacitance which
may be recommended to improve ADC linearity. A
high-speed, high open-loop gain amplifier like the
OPA2832 can be very susceptible to decreased
stability and closed-loop response peaking when a
capacitive load is placed directly on the output pin.
When the primary considerations are frequency
response flatness, pulse response fidelity, and/or
distortion, the simplest and most effective solution is
to isolate the capacitive load from the feedback loop
by inserting a series isolation resistor between the
amplifier output and the capacitive load.
The Typical Characteristic curves show the
recommended R_S versus capacitive load and the
resulting frequency response at the load. Parasitic
capacitive loads greater than 2pF can begin to
degrade the performance of the OPA2832. Long PC
board traces, unmatched cables, and connections to
multiple devices can easily exceed this value. Always
consider this effect carefully, and add the
recommended series resistor as close as possible to
the output pin (see the Board Layout Guidelines
section).
The criterion for setting this R_S resistor is a maximum
bandwidth, flat frequency response at the load. For a
gain of +2, the frequency response at the output pin
is already slightly peaked without the capacitive load,
requiring relatively high values of R_S to flatten the
response at the load. Increasing the noise gain will
also reduce the peaking (see Figure 24).
24
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