OPA2681 Datasheet, PDF (16/21 Pages) Burr-Brown (TI) – Dual Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

minimum value of 20Ω. Lower values will load both the
buffer stage at the input and the output stage if R_F gets too
low—actually decreasing the bandwidth. Figure 8 shows the
recommended R_F vs NG for both ±5V and a single +5V
operation. The values for R_F vs Gain shown here are ap-
proximately equal to the values used to generate the Typical
Performance Curves. They differ in that the optimized val-
ues used in the Typical Performance Curves are also correct-
ing 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.
FEEDBACK RESISTOR vs NOISE GAIN
600
500
+5V
400
300
±5V
200
100
0
0
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 OPA2681 is slightly influ-
enced by the source impedance looking out of the non-
inverting input terminal. High source resistors will have the
effect of increasing R_I, decreasing the bandwidth. For those
single supply applications which develop a midpoint bias at
the non-inverting input through high valued resistors, the
decoupling capacitor is essential for power supply ripple
rejection, non-inverting input noise current shunting, and to
minimize the high frequency value for R_I in Figure 7.
INVERTING AMPLIFIER OPERATION
Since the OPA2681 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 (e.g. integrators, transimpedance, some filters)
should consider the unity gain stable voltage feedback
OPA2680, since the feedback resistor is the compensation
element for a current feedback op amp. Wideband inverting
operation (and especially summing) is particularly suited to
the OPA2681. Figure 9 shows a typical inverting configura-
tion where the I/O impedances and signal gain from Figure
1 are retained in an inverting circuit configuration.
50Ω
Source
V_I
R_G
182Ω
R_M
68.1Ω
+5V
Power supply
de-coupling
not shown
1/2
OPA2681
50Ω Load
V_O ^50Ω
R_F
365Ω
–5V
FIGURE 9. Inverting Gain of –2 with Impedance Matching.
In the inverting configuration, two key design consider-
ations 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 an R_F which may be non-optimal from a fre-
quency 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 45Ω, are inserted into Equation 3 to get
a feedback transimpedance nearly equal to the 492Ω opti-
mum value.
Note that the non-inverting input in this bipolar supply
inverting application is connected directly to ground. It is
often suggested that an additional resistor be connected to
ground on the non-inverting input to achieve bias current
error cancellation at the output. The input bias currents for
a current feedback op amp are not generally matched in
either magnitude or polarity. Connecting a resistor to ground
on the non-inverting input of the OPA2681 in the circuit of
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OPA2681
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