OPA2691 Datasheet, PDF (16/30 Pages) Texas Instruments – Dual Wideband, Current-Feedback OPERATIONAL AMPLIFIER

the bandwidth. Figure 8 shows the recommended R_F versus
NG for both ±5V and a single +5V operation. The values for R_F
versus Gain shown here are approximately equal to the values
used to generate the Typical Characteristics. They differ in that
the optimized values used in the Typical Characteristics are
also correcting for board parasitics not considered in the
simplified analysis leading to Equation 3. The values shown in
Figure 8 give a good starting point for design where bandwidth
optimization is desired.
600
500
400
300
200
100
0
0
+5V
±5V
5
10
15
20
Noise Gain
FIGURE 8. Recommended Feedback Resistor vs Noise Gain.
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 OPA2691 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 7.
INVERTING AMPLIFIER OPERATION
Since the OPA2691 is a general-purpose, wideband current-
feedback op amp, most of the familiar op amp application
circuits are available to the designer. Those dual op amp
applications that require considerable flexibility in the feed-
back element (for example, integrators, transimpedance, and
some filters) should consider the unity-gain stable voltage-
feedback OPA2690, since the feedback resistor is the com-
pensation element for a current-feedback op amp. Wideband
inverting operation (and especially summing) is particularly
suited to the OPA2691. Figure 9 shows a typical inverting
configuration where the I/O impedances and signal gain from
Figure 1 are retained in an inverting circuit configuration.
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
+5V
Power-supply
decoupling not shown.
50Ω
Source
V_I
R_G
182Ω
R_M
68.1Ω
1/2
OPA2691
50Ω Load
V_O ^50Ω
R_F
374Ω
–5V
FIGURE 9. Inverting Gain of –2 with Impedance Matching.
impedance matching is desired (which is beneficial when-
ever the signal is coupled through a cable, twisted-pair, long
PCB trace, or other transmission line conductor), it is nor-
mally 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 an 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 9 have accounted for this by slightly decreasing R_F
(from Figure 1) to re-optimize the bandwidth for the noise
gain of Figure 9 (NG = 2.74) In the example of Figure 9, the
R_M value combines in parallel with the external 50Ω source
impedance, yielding an effective driving impedance of
50Ω || 68Ω = 28.8Ω. This impedance is added in series with
R_G for calculating the noise gain—which gives NG = 2.74.
This value, along with the R_F of Figure 8 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 OPA2691 in the circuit of Figure 9
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 OPA2691 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
16
OPA2691
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