OPA2890_08 Datasheet, PDF (22/35 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
SBOS364B – DECEMBER 2007 – REVISED MAY 2008.................................................................................................................................................... www.ti.com
OPERATING RECOMMENDATIONS
OPTIMIZING RESISTOR VALUES
Because the OPA2890 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 feedback resistor isolates 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 presents
additional output loading that can degrade the
harmonic distortion performance of the OPA2890.
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 49) to be less
than approximately 400Ω. 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
2pF total parasitic on the inverting node, holding R_F ||
R_G < 400Ω keeps this pole above 160MHz. By itself,
this constraint implies that the feedback resistor R_F
can increase to several kΩ at high gains. This
increase in resistor size 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 increases.
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) predicts the closed-loop bandwidth. In
practice, this principle only holds true when the phase
margin approaches 90°, as it does in high gain
configurations. At low gains (increased feedback
factors), most amplifiers exhibit a more complex
response with lower phase margin. The OPA2890 is
compensated to give a slightly peaked response in a
noninverting gain of 2V/V (see Figure 49). This
compensation results in a typical gain of +2V/V
bandwidth of 100MHz, far exceeding that predicted
by dividing the 60MHz GBP by 2. Increasing the gain
causes the phase margin to approach 90° and the
bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +10V/V, the 12MHz
bandwidth shown in the Electrical Characteristics
agrees with that predicted using the simple formula
and the typical GBP of 120MHz.
The frequency response in a gain of +2V/V may be
modified to achieve exceptional flatness simply by
increasing the noise gain to 2.5V/V. One way to
modify the response without affecting the +2V/V
signal gain, is to add an 1.5kΩ resistor across the two
inputs, as illustrated in the circuit of Figure 49. A
similar technique may be used to reduce peaking in
unity-gain (voltage follower) applications. For
example, by using a 750Ω feedback resistor along
with a 750Ω resistor across the two op amp inputs,
the voltage follower response is similar to the gain of
+2V/V response of Figure 50. Reducing the value of
the resistor across the op amp inputs further limits the
frequency response due to increased noise gain.
The OPA2890 exhibits minimal bandwidth reduction
going to single-supply (+5V) operation as compared
with ±5V. This feature arises because the internal
bias control circuitry retains nearly constant quiescent
current as the total supply voltage between the
supply pins changes.
INVERTING AMPLIFIER OPERATION
The OPA2890 is a general-purpose, wideband,
voltage-feedback op amp; therefore, 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 59 for a typical
inverting configuration where the I/O impedances and
signal gain from Figure 49 are retained in an inverting
circuit configuration.
In the inverting configuration, three key design
considerations must be noted. The first is that the
gain resistor (R_G) becomes part of the signal channel
input impedance. If input impedance matching is
desired (which is beneficial whenever the signal is
coupled through a cable, twisted-pair, long PCB
trace, or other transmission line conductor), R_G may
be set equal to the required termination value and R_F
adjusted to give the desired gain. This consideration
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 –2V/V, setting R_G to 50Ω for
input matching eliminates the need for R_M but
requires a 100Ω feedback resistor. This consideration
has the interesting advantage that the noise gain
becomes equal to 2V/V for a 50Ω source
impedance—the same as the noninverting circuits
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