OPA3690ID Datasheet, PDF (20/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
SBOS237G – MARCH 2002 – REVISED MARCH 2010
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OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the the OPA3690 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, for G > 1
applications, the feedback resistor value should be
between 100Ω and 1.5kΩ. Below 100Ω, the feedback
network will present additional output loading which
can degrade the harmonic distortion performance of
the OPA3690. 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_F and R_G (see Figure 36) to be less
than approximately 125Ω. 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
3pF total parasitic on the inverting node, holding R_F ||
R_G < 125Ω will keep this pole above 400MHz. By
itself, this constraint implies that feedback resistor R_F
can increase to several kΩ at high gains. This 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.
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 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 OPA3690 is
compensated to give a slightly peaked response in a
noninverting gain of 2 (see Figure 36). 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
453Ω resistor across the two inputs in the circuit of
Figure 36. 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 37. Reducing the
value of the resistor across the op amp inputs will
further limit the frequency response due to increased
noise gain.
The OPA3690 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.
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