Datasheet

OPA2890
SBOS364C –DECEMBER 2007–REVISED DECEMBER 2009 www.ti.com
OPERATING RECOMMENDATIONS
bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +10V/V, the 12MHz
OPTIMIZING RESISTOR VALUES
bandwidth shown in the Electrical Characteristics
Because the OPA2890 is a unity-gain stable,
agrees with that predicted using the simple formula
voltage-feedback op amp, a wide range of resistor
and the typical GBP of 120MHz.
values may be used for the feedback and gain setting
The frequency response in a gain of +2V/V may be
resistors. The primary limits on these values are set
modified to achieve exceptional flatness simply by
by dynamic range (noise and distortion) and parasitic
increasing the noise gain to 2.5V/V. One way to
capacitance considerations. For a noninverting
modify the response without affecting the +2V/V
unity-gain follower application, the feedback
signal gain, is to add an 1.5kΩ resistor across the two
connection should be made with a 25Ω resistor, not a
inputs, as illustrated in the circuit of Figure 49. A
direct short. This feedback resistor isolates the
similar technique may be used to reduce peaking in
inverting input capacitance from the output pin and
unity-gain (voltage follower) applications. For
improve the frequency response flatness. Usually, the
example, by using a 750Ω feedback resistor along
feedback resistor value should be between 200Ω and
with a 750Ω resistor across the two op amp inputs,
1.5kΩ. Below 200Ω, the feedback network presents
the voltage follower response is similar to the gain of
additional output loading that can degrade the
+2V/V response of Figure 50. Reducing the value of
harmonic distortion performance of the OPA2890.
the resistor across the op amp inputs further limits the
Above 1.5kΩ, the typical parasitic capacitance
frequency response due to increased noise gain.
(approximately 0.2pF) across the feedback resistor
can cause unintentional band-limiting in the amplifier
The OPA2890 exhibits minimal bandwidth reduction
response.
going to single-supply (+5V) operation as compared
with ±5V. This feature arises because the internal
A good rule of thumb is to target the parallel
bias control circuitry retains nearly constant quiescent
combination of R
F
and R
G
(see Figure 49) to be less
current as the total supply voltage between the
than approximately 400Ω. The combined impedance
supply pins changes.
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 INVERTING AMPLIFIER OPERATION
2pF total parasitic on the inverting node, holding R
F
||
The OPA2890 is a general-purpose, wideband,
R
G
< 400Ω keeps this pole above 160MHz. By itself,
voltage-feedback op amp; therefore, all of the familiar
this constraint implies that the feedback resistor R
F
op amp application circuits are available to the
can increase to several kΩ at high gains. This
designer. Inverting operation is one of the more
increase in resistor size is acceptable as long as the
common requirements and offers several
pole formed by R
F
and any parasitic capacitance
performance benefits. See Figure 59 for a typical
appearing in parallel is kept out of the frequency
inverting configuration where the I/O impedances and
range of interest.
signal gain from Figure 49 are retained in an inverting
circuit configuration.
BANDWIDTH vs GAIN: NONINVERTING
OPERATION In the inverting configuration, three key design
considerations must be noted. The first is that the
Voltage-feedback op amps exhibit decreasing
gain resistor (R
G
) becomes part of the signal channel
closed-loop bandwidth as the signal gain increases.
input impedance. If input impedance matching is
In theory, this relationship is described by the Gain
desired (which is beneficial whenever the signal is
Bandwidth Product (GBP) shown in the Electrical
coupled through a cable, twisted-pair, long PCB
Characteristics. Ideally, dividing GBP by the
trace, or other transmission line conductor), R
G
may
noninverting signal gain (also called the Noise Gain,
be set equal to the required termination value and R
F
or NG) predicts the closed-loop bandwidth. In
adjusted to give the desired gain. This consideration
practice, this principle only holds true when the phase
is the simplest approach and results in optimum
margin approaches 90°, as it does in high gain
bandwidth and noise performance. However, at low
configurations. At low gains (increased feedback
inverting gains, the resultant feedback resistor value
factors), most amplifiers exhibit a more complex
can present a significant load to the amplifier output.
response with lower phase margin. The OPA2890 is
For an inverting gain of –2V/V, setting R
G
to 50Ω for
compensated to give a slightly peaked response in a
input matching eliminates the need for R
M
but
noninverting gain of 2V/V (see Figure 49). This
requires a 100Ω feedback resistor. This consideration
compensation results in a typical gain of +2V/V
has the interesting advantage that the noise gain
bandwidth of 100MHz, far exceeding that predicted
becomes equal to 2V/V for a 50Ω source
by dividing the 60MHz GBP by 2. Increasing the gain
impedance—the same as the noninverting circuits
causes the phase margin to approach 90° and the
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