Datasheet

OPA690
www.ti.com SBOS223F –DECEMBER 2001–REVISED FEBRUARY 2010
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES BANDWIDTH VERSUS GAIN: NONINVERTING
OPERATION
Since the OPA690 is a unity-gain stable,
voltage-feedback op amp, a wide range of resistor Voltage-feedback op amps exhibit decreasing
values may be used for the feedback and gain setting closed-loop bandwidth as the signal gain is
resistors. The primary limits on these values are set increased. In theory, this relationship is described by
by dynamic range (noise and distortion) and parasitic the Gain Bandwidth Product (GBP) shown in the
capacitance considerations. For a noninverting Electrical Characteristics. Ideally, dividing GBP by the
unity-gain follower application, the feedback noninverting signal gain (also called the Noise Gain,
connection should be made with a 25Ω resistor, not a or NG) will predict the closed-loop bandwidth. In
direct short. This will isolate the inverting input practice, this only holds true when the phase margin
capacitance from the output pin and improve the approaches 90°, as it does in high gain
frequency response flatness. Usually, for G > 1 configurations. At low gains (increased feedback
applications, the feedback resistor value should be factors), most amplifiers will exhibit a more complex
between 200Ω and 1.5kΩ. Below 200Ω, the feedback response with lower phase margin. The OPA690 is
network will present additional output loading which compensated to give a slightly peaked response in a
can degrade the harmonic distortion performance of noninverting gain of 2 (see Figure 36). This results in
the OPA690. Above 1.5kΩ, the typical parasitic a typical gain of +2 bandwidth of 220MHz, far
capacitance (approximately 0.2pF) across the exceeding that predicted by dividing the 300MHz
feedback resistor may cause unintentional GBP by 2. Increasing the gain will cause the phase
band-limiting in the amplifier response. margin to approach 90° and the bandwidth to more
closely approach the predicted value of (GBP/NG). At
A good rule of thumb is to target the parallel
a gain of +10, the 30MHz bandwidth shown in the
combination of R
F
and R
G
(see Figure 36) to be less
Electrical Characteristics agrees with that predicted
than approximately 300Ω. The combined impedance
using the simple formula and the typical GBP of
R
F
|| R
G
interacts with the inverting input capacitance,
300MHz.
placing an additional pole in the feedback network
and thus, a zero in the forward response. Assuming a The frequency response in a gain of +2 may be
2pF total parasitic on the inverting node, holding R
F
|| modified to achieve exceptional flatness simply by
R
G
< 300Ω will keep this pole above 250MHz. By increasing the noise gain to 2.5. One way to do this,
itself, this constraint implies that the feedback resistor without affecting the +2 signal gain, is to add an
R
F
can increase to several kΩ at high gains. This is 804Ω resistor across the two inputs in the circuit of
acceptable as long as the pole formed by R
F
and any Figure 36. A similar technique may be used to reduce
parasitic capacitance appearing in parallel is kept out peaking in unity-gain (voltage follower) applications.
of the frequency range of interest. 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 OPA690 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.
Copyright © 2001–2010, Texas Instruments Incorporated 19
Product Folder Link(s): OPA690