OPA2690IDBV BURR-BROWN [Burr-Brown Corporation], OPA2690IDBV Datasheet - Page 19

• Manufacturer Part Number: OPA2690IDBV
• Description: Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

MACROMODELS
Computer simulation of circuit performance using SPICE
is often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and
inductance can have a major effect on circuit performance.
A SPICE model for the OPA2690 (use two OPA690 SPICE
models) is available through the Texas Instruments web
page (http://www.ti.com). These models do a good job of
predicting small-signal AC and transient performance
under a wide variety of operating conditions. They do not
do as well in predicting the harmonic distortion or dG/dP
characteristics. These models do not attempt to
distinguish
small-signal AC performance.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
As the OPA2690 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 25Ω resistor, not a
direct short. This will isolate the inverting input
capacitance from the output pin and improve the
frequency response flatness. Usually, the feedback
resistor value should be between 200Ω and 1.5kΩ. Below
200Ω, the feedback network will present additional output
loading which can degrade the harmonic distortion
performance of the OPA2690. Above 1.5kΩ, the typical
parasitic capacitance (approximately 0.2pF) across the
feedback resistor can cause unintentional band-limiting in
the amplifier response.
A good rule of thumb is to target the parallel combination
of R
300Ω. The combined impedance R
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
250MHz. By itself, this constraint implies that the feedback
resistor R
is acceptable as long as the pole formed by R
parasitic capacitance appearing in parallel is kept out of
the frequency range of interest.
BANDWIDTH vs GAIN: NONINVERTING
OPERATION
Voltage-feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the Gain Bandwidth Product
(GBP) shown in the Electrical Characteristics. Ideally,
dividing GBP by the noninverting signal gain (also called
F
www.ti.com
and R
F
can increase to several kΩ at high gains. This
G
between the package types in their
(see Figure 1) to be less than approximately
F
 R
G
< 300Ω will keep this pole above
F
 R
G
interacts with
F
and any
the Noise Gain, or NG) will predict the closed-loop
bandwidth. In practice, this only holds true when the phase
margin approaches 90°, as it does in high gain
configurations. At low gains (increased feedback factors),
most amplifiers will exhibit a more complex response with
lower phase margin. The OPA2690 is compensated to
give a slightly peaked response in a noninverting gain of
2 (see Figure 1). This results in a typical gain of +2
bandwidth of 220MHz, far exceeding that predicted by
dividing the 300MHz GBP by 2. Increasing the gain will
cause the phase margin to approach 90° and the
bandwidth to more closely approach the predicted value of
(GBP/NG). At a gain of +10, the 30MHz bandwidth shown
in the Electrical Characteristics agrees with that predicted
using the simple formula and the typical GBP of 300MHz.
The frequency response in a gain of +2 may be modified
to achieve exceptional flatness simply by increasing the
noise gain to 2.5. One way to do this, without affecting the
+2 signal gain, is to add an 804Ω resistor across the two
inputs in the circuit of Figure 1. A similar technique may be
used to reduce peaking in unity-gain (voltage follower)
applications. For example, by using a 402Ω feedback
resistor along with a 402Ω resistor across the two op amp
inputs, the voltage follower response will be similar to the
gain of +2 response of Figure 2. Reducing the value of the
resistor across the op amp inputs will further limit the
frequency response due to increased noise gain.
The OPA2690 exhibits minimal bandwidth reduction going
to single-supply (+5V) operation as compared with ±5V.
This is because the internal bias control circuitry retains
nearly constant quiescent current as the total supply
voltage between the supply pins is changed.
INVERTING AMPLIFIER OPERATION
Since the OPA2690 is a general-purpose, wideband
voltage-feedback op amp, all of the familiar op amp
application circuits are available to the designer. Inverting
operation is one of the more common requirements and
offers several performance benefits. See Figure 12 for a
typical inverting configuration where the I/O impedances
and signal gain from Figure 1 are retained in an inverting
circuit configuration.
In
considerations must be noted. The first is that the gain
resistor (R
impedance. If input impedance matching is desired (which
is beneficial whenever the signal is coupled through a
cable, twisted-pair, long PC board trace, or other
transmission line conductor), R
required termination value and R
desired gain. This is the simplest approach and results in
optimum bandwidth and noise performance. However, at
low inverting gains, the resultant feedback resistor value
can present a significant load to the amplifier output. For
an inverting gain of −2, setting R
matching eliminates the need for R
the
inverting
G
) becomes part of the signal channel input
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
configuration,
G
may be set equal to the
F
M
adjusted to give the
G
three
but requires a 100Ω
to 50Ω for input
key
OPA2690
design
19