Datasheet

OPA890
www.ti.com SBOS369B –MAY 2007–REVISED DECEMBER 2009
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES the bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +10V/V, the 13MHz
Because the OPA890 is a unity-gain stable,
bandwidth shown in the Electrical Characteristics
voltage-feedback op amp, a wide range of resistor
agrees with that predicted using the simple formula
values can be used for the feedback and gain setting
and the typical GBP of 130MHz.
resistors. The primary limits on these values are set
by dynamic range (noise and distortion) and parasitic The OPA890 exhibits minimal bandwidth reduction
capacitance considerations. Usually, for G > 1 going to single-supply (+5V) operation as compared
applications, the feedback resistor value should be with ±5V. This difference in performance occurs
between 200Ω and 1.5kΩ. Below 200Ω, the feedback because the internal bias control circuitry retains
network presents additional output loading that can nearly constant quiescent current as the total supply
degrade the harmonic distortion performance of the voltage between the supply pins is changed.
OPA890. Above 1.5kΩ, the typical parasitic
capacitance (approximately 0.2pF) across the Inverting Amplifier Operation
feedback resistor may cause unintentional
The OPA890 is a general-purpose, wideband
band-limiting in the amplifier response.
voltage-feedback op amp; therefore, all of the familiar
The combined impedance of R
F
|| R
G
interacts with op amp application circuits are available to the
the inverting input capacitance, placing an additional designer. Inverting operation is one of the more
pole in the feedback network and thus, a zero in the common requirements and offers several
forward response. Assuming a 2pF total parasitic on performance benefits. Figure 51 shows a typical
the inverting node, having R
F
|| R
G
< 400Ω keeps inverting configuration where the I/O impedances and
this pole above 250MHz. By itself, this constraint signal gain from Figure 46 are retained in an inverting
implies that the feedback resistor R
F
can increase to circuit configuration.
several kΩ at high gains. This increase is acceptable,
In the inverting configuration, three key design
as long as the pole formed by R
F
and any parasitic
considerations must be noted. First, the gain resistor
capacitance appearing in parallel is kept out of the
(R
G
) becomes part of the signal channel input
frequency range of interest.
impedance. If input impedance matching is desired
(which is beneficial whenever the signal is coupled
BANDWIDTH VERSUS GAIN
through a cable, twisted-pair, long PCB trace, or
other transmission line conductor), R
G
may be set
Noninverting Amplifier Operation
equal to the required termination value and R
F
adjusted to give the desired gain. This approach is
Voltage-feedback op amps exhibit decreasing
the simplest, and results in optimum bandwidth and
closed-loop bandwidth as the signal gain is
noise performance. However, at low inverting gains,
increased. In theory, this relationship is described by
the resultant feedback resistor value can present a
the gain bandwidth product (GBP) shown in the
significant load to the amplifier output. For an
Electrical Characteristics. Ideally, dividing GBP by the
inverting gain of –2V/V, setting R
G
to 50Ω for input
noninverting signal gain (also called the noise gain, or
matching eliminates the need for R
M
but requires a
NG) predicts the closed-loop bandwidth. In practice,
100Ω feedback resistor. This option has the
this relationship only holds true when the phase
interesting advantage that the noise gain becomes
margin approaches 90°, as it does in high-gain
equal to 2V/V for a 50Ω source impedance—the
configurations. At low gains (increased feedback
same as the noninverting circuits considered in the
factors), most amplifiers exhibit a more complex
previous section. The amplifier output, however, now
response with lower phase margin. The OPA890 is
sees the 100Ω feedback resistor in parallel with the
compensated to give a slightly peaked response in a
external load. In general, the feedback resistor should
noninverting gain of 2V/V (see Figure 46). This
be limited to a range of 200Ω to 1.5kΩ. In this case, it
compensation results in a typical gain of +2V/V
is preferable to increase both the R
F
and R
G
values,
bandwidth of 115MHz, far exceeding that predicted
as shown in Figure 51, and then achieve the input
by dividing the 130MHz GBP by 2. Increasing the
matching impedance with a third resistor (R
M
) to
gain causes the phase margin to approach 90° and
ground. The total input impedance becomes the
parallel combination of R
G
and R
M
.
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