OPA2652
www.ti.com
SBOS125A–JUNE 2000–REVISED MAY 2006
OPERATING SUGGESTIONS
Optimizing Resistor Values
causes the phase margin to approach 90° and the
bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +5, the 45MHz
bandwidth shown in the Electrical Characteristics is
close to that predicted using this simple formula.
Because the OPA2652 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 configuration isolates the inverting input
capacitance from the output pin and improves 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 OPA2652.
Above 1.5kΩ, the typical parasitic capacitance
(approximately 0.2pF) across the feedback resistor
may cause unintentional bandlimiting in the amplifier
response.
Inverting Amplifier Operation
Because the OPA2652 is
a
general-purpose,
wideband voltage feedback op amp, 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. Figure 29 shows a typical
inverting configuration.
In the inverting configuration, three key design
consideration must be noted. First, the gain resistor
(RG) 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), RG may be set equal to
the required termination value and RF 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
resulting feedback resistor value can present a
significant load to the amplifier output. For an
inverting gain of –1, setting RG to 50Ω for input
matching eliminates the need for RM but requires a
50W feedback resistor. This configuration has the
interesting advantage that the noise gain becomes
equal to 2 for a 50Ω source impedance—the same
as the noninverting circuits considered above.
However, the amplifier output now sees the 50Ω
feedback resistor in parallel with the external load. In
general, the feedback resistor should be limited to
the 200Ω to 1.5kΩ range. In this case, it is preferable
to increase both the RF and RG values as shown in
Figure 29, and then achieve the input matching
impedance with a third resistor (RM) to ground. The
total input impedance becomes the parallel
combination of RG and RM.
A good rule of thumb is to target the parallel
combination of RF and RG (see Figure 28) to be less
than approximately 300Ω. The combined impedance
RF || RG 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 RF || RG < 300Ω keeps this
pole above 250MHz. By itself, this constraint implies
that the feedback resistor RF can increase to several
kΩ at high gains. This increase is acceptable as long
as the pole formed by RF 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 is
increased. In theory, this relationship is described by
the Gain Bandwidth Product (GBP) shown in the
specifications. Ideally, dividing GBP by the
noninverting signal gain (also called the Noise Gain,
or NG) predicts the closed-loop bandwidth. In
practice, this prediction only holds true when the
phase margin approaches 90°, as it does in high
gain configurations. At low gains (increased
feedback factor), most amplifiers exhibit a wider
bandwidth and lower phase margin. The OPA2652 is
The second major consideration, touched on in the
previous paragraph, is that the signal source
impedance becomes part of the noise gain equation
and influences the bandwidth. For the example in
Figure 29, the RM value combines in parallel with the
external 50Ω source impedance, yielding an effective
driving impedance of 50Ω || 57.6Ω = 26.8Ω. This
impedance is added in series with RG for calculating
the noise gain (NG). The resulting NG is 1.94 for
Figure 29 (an ideal source would cause NG = 2.00).
compensated to give
noninverting gain of
a
flat response in
a
1
(see Figure 28). This
configuration results in a typical gain of +1 bandwidth
of 700MHz, far exceeding that predicted by dividing
the 200MHz GBP by NG = 1. Increasing the gain
The third important consideration in inverting
amplifier design is setting the bias current
cancellation resistor on the noninverting input (RB). If
this resistor is set equal to the total DC resistance
looking out of the inverting node, the output DC
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