OPA3691 Datasheet, PDF (18/32 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

The total impedance going into the inverting input may be
used to adjust the closed-loop signal bandwidth. Inserting a
series resistor between the inverting input and the summing
junction will increase the feedback impedance (denominator
of Equation 2), decreasing the bandwidth. The internal buffer
output impedance for the OPA3691 is slightly influenced by
the source impedance looking out of the noninverting input
terminal. High source resistors will have the effect of increas-
ing R_I, decreasing the bandwidth. For those single-supply
applications which develop a midpoint bias at the noninverting
input through high valued resistors, the decoupling capacitor
is essential for power-supply ripple rejection, noninverting
input noise current shunting, and to minimize the high-
frequency value for R_I in Figure 9.
INVERTING AMPLIFIER OPERATION
Since the OPA3691 is a general-purpose, wideband current-
feedback op amp, most of the familiar op amp application
circuits are available to the designer. Those triple op amp
applications that require considerable flexibility in the feedback
element (for example, integrators, transimpedance, and some
filters) should consider the unity-gain stable voltage-feedback
OPA3690, since the feedback resistor is the compensation
element for a current-feedback op amp. Wideband inverting
operation (especially summing) is particularly suited to the
OPA3691. Figure 11 shows a typical inverting configuration
where the I/O impedances and signal gain from Figure 1 are
retained in an inverting circuit configuration.
50Ω
Source
V_I
R_G
187Ω
R_M
68.1Ω
+5V
Power-supply
decoupling not shown.
1/3
OPA3691
DIS
50Ω Load
V_O ^50Ω
R_F
374Ω
–5V
FIGURE 11. Inverting Gain of –2 with Impedance Matching.
In the inverting configuration, two 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 when-
ever the signal is coupled through a cable, twisted-pair, long
PC board trace or other transmission line conductor), it is
normally necessary to add an additional matching resistor to
ground. R_G by itself is normally not set to the required input
impedance since its value, along with the desired gain, will
determine a R_F which may be non-optimal from a frequency
response standpoint. The total input impedance for the
source becomes the parallel combination of R_G and R_M.
The second major consideration, touched on in the previous
paragraph, is that the signal source impedance becomes
part of the noise gain equation and will have slight effect on
the bandwidth through Equation 1. The values shown in
Figure 11 have accounted for this by slightly decreasing R_F
(from Figure 1) to re-optimize the bandwidth for the noise
gain of Figure 11 (NG = 2.73) In the example of Figure 11,
the R_M value combines in parallel with the external 50Ω
source impedance, yielding an effective driving impedance of
50Ω || 68.1Ω = 28.8Ω. This impedance is added in series with
R_G for calculating the noise gain—which gives NG = 2.73.
This value, along with the R_F of Figure 10 and the inverting
input impedance of 37Ω, are inserted into Equation 3 to get
a feedback transimpedance nearly equal to the 476Ω opti-
mum value.
Note that the noninverting input in this bipolar supply invert-
ing application is connected directly to ground. It is often
suggested that an additional resistor be connected to ground
on the noninverting input to achieve bias current error can-
cellation at the output. The input bias currents for a current
feedback op amp are not generally matched in either magni-
tude or polarity. Connecting a resistor to ground on the
noninverting input of the OPA3691 in the circuit of Figure 11
will actually provide additional gain for that input’s bias and
noise currents, but will not decrease the output DC error
since the input bias currents are not matched.
OUTPUT CURRENT AND VOLTAGE
The OPA3691 provides output voltage and current capabili-
ties that are unsurpassed in a low-cost dual monolithic op
amp. Under no-load conditions at 25°C, the output voltage
typically swings closer than 1V to either supply rail; the tested
swing limit is within 1.2V of either rail. Into a 15Ω load (the
minimum tested load), it is tested to deliver more than
±160mA.
The specifications described above, though familiar in the
industry, consider voltage and current limits separately. In
many applications, it is the voltage • current, or V-I product,
which is more relevant to circuit operation. Refer to the
Output Voltage and Current Limitations plot in the Typical
Characteristics. The X- and Y-axes of this graph show the
zero-voltage output current limit and the zero-current output
voltage limit, respectively. The four quadrants give a more
detailed view of the OPA3691’s output drive capabilities,
noting that the graph is bounded by a Safe Operating Area
of 1W maximum internal power dissipation. Superimposing
resistor load lines onto the plot shows that the OPA3691 can
drive ±2.5V into 25Ω or ±3.5V into 50Ω without exceeding the
output capabilities or the 1W dissipation limit. A 100Ω load
line (the standard test circuit load) shows the full ±3.9V
output swing capability, as shown in the Electrical Character-
istics Table.
18
OPA3691
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