Datasheet

OPA691
16
SBOS226D
www.ti.com
As the desired signal gain increases, this equation will
eventually predict a negative R
F
. A somewhat subjective limit
to this adjustment can also be set by holding R
G
to a
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 rec-
ommended 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.
(e.g., integrators, transimpedance, and some filters) should
consider the unity-gain stable voltage feedback OPA680,
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 OPA691. See
Figure 9 for a typical inverting configuration where the I/O
impedances and signal gain from Figure 1 are retained in an
inverting circuit configuration.
OPA691
R
F
374Ω
R
G
188Ω
DIS
+5V
–5V
50Ω
50Ω Load
V
O
Power-supply
decoupling
not shown.
V
I
50Ω
Source
R
M
68.1Ω
FIGURE 9. Inverting Gain of –2 with Impedance Matching.
FIGURE 8. Feedback Resistor vs Noise Gain.
600
500
400
300
200
100
0
Noise Gain
02010 155
Feedback Resistor (Ω)
+5V
±5V
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. This approach to
bandwidth control is used for the inverting summing circuit on
the front page. The internal buffer output impedance for the
OPA691 is slightly influenced by the source impedance
looking out of the noninverting input terminal. High source
resistors will have the effect of increasing R
I
, decreasing the
bandwidth. For those single-supply applications which de-
velop a midpoint bias at the noninverting input through high
valued resistors, the decoupling capacitor is essential for
power-supply noise rejection, noninverting input noise cur-
rent shunting, and to minimize the high frequency value for
R
I
in Figure 7.
INVERTING AMPLIFIER OPERATION
Since the OPA691 is a general-purpose, wideband current
feedback op amp, most of the familiar op amp application
circuits are available to the designer. Those applications that
require considerable flexibility in the feedback element
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 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 = 2.73) 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 = 2.73.
This value, along with the R
F
of Figure 9 and the inverting
input impedance of 35Ω, are inserted into Equation 3 to get
a feedback transimpedance nearly equal to the 472Ω 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