OPA890IDBV BURR-BROWN [Burr-Brown Corporation], OPA890IDBV Datasheet - Page 20

• Manufacturer Part Number: OPA890IDBV
• Description: Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

OPA890
SBOS369 – MAY 2007
20
OPTIMIZING RESISTOR VALUES
Because the OPA890 is a unity-gain stable,
voltage-feedback op amp, a wide range of resistor
values can 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. Usually, for G > 1
applications, the feedback resistor value should be
between 200
feedback network presents additional output loading
that
performance of the OPA890. Above 1.5k , the
typical parasitic capacitance (approximately 0.2pF)
across the feedback resistor may cause unintentional
band-limiting in the amplifier response.
The combined impedance of R
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, having R
this pole above 250MHz. By itself, this constraint
implies that the feedback resistor R
several
acceptable, as long as the pole formed by R
any parasitic capacitance appearing in parallel is
kept out of the frequency range of interest.
BANDWIDTH VERSUS GAIN
Noninverting Amplifier Operation
Voltage-feedback
closed-loop
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) predicts the closed-loop bandwidth. In
practice, this relationship 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
OPA890 is compensated to give a slightly peaked
response in a noninverting gain of 2V/V (see
Figure
gain of +2V/V bandwidth of 115MHz, far exceeding
that predicted by dividing the 130MHz GBP by 2.
Increasing the gain causes the phase margin to
approach 90 and the bandwidth to more closely
can
46). This compensation results in a typical
k
degrade
bandwidth
at
and 1.5k . Below 200 , the
high
op
amps
the
gains.
as
F
the
harmonic
F
exhibit
R
This
G
R
F
signal
G
can increase to
< 400
OPERATING SUGGESTIONS
interacts with
increase
decreasing
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distortion
gain
keeps
F
and
is
is
approach the predicted value of (GBP/NG). At a gain
of +10V/V, the 13MHz bandwidth shown in the
Electrical Characteristics agrees with that predicted
using the simple formula and the typical GBP of
130MHz.
The OPA890 exhibits minimal bandwidth reduction
going to single-supply (+5V) operation as compared
with
because the internal bias control circuitry retains
nearly constant quiescent current as the total supply
voltage between the supply pins is changed.
Inverting Amplifier Operation
The OPA890 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
performance benefits.
inverting configuration where the I/O impedances
and signal gain from
inverting circuit configuration.
In the inverting configuration, three key design
considerations must be noted. First, the gain resistor
(R
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
equal to the required termination value and R
adjusted to give the desired gain. This approach is
the simplest, 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
matching eliminates the need for R
100
interesting advantage that the noise gain becomes
equal to 2V/V for a 50
same as the noninverting circuits considered in the
previous section. The amplifier output, however, now
sees the 100
external load. In general, the feedback resistor
should be limited to a range of 200
case, it is preferable to increase both the R
values, as shown in
input matching impedance with a third resistor (R
to ground. The total input impedance becomes the
parallel combination of R
G
) becomes part of the signal channel input
5V. This difference in performance occurs
feedback resistor. This option has the
requirements
feedback resistor in parallel with the
Figure
Figure 46
Figure 51
G
and R
51, and then achieve the
source impedance—the
and
M
are retained in an
.
G
shows a typical
M
offers
to 50
to 1.5k . In this
G
but requires a
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may be set
F
for input
and R
several
M
G
F
)