OPA2607 Datasheet, PDF (8/13 Pages) Burr-Brown (TI) – Dual, High Output, Current-Feedback OPERATIONAL AMPLIFIER

If 20 • log (R_F + NG • R_I) were drawn on top of the open-
loop transimpedance plot, the difference between the two
would be the loop gain at a given frequency. Eventually,
Z(s) rolls off to equal the denominator of Equation 2 at
which point the loop gain has reduced to 1 (and the curves
have intersected). This point of equality is where the
amplifier’s closed-loop frequency response given by Equa-
tion 1 will start to roll off, and is exactly analogous to the
frequency at which the noise gain equals the open-loop
voltage gain for a voltage-feedback op amp. The difference
here is that the total impedance in the denominator of
Equation 2 may be controlled somewhat separately from the
desired signal gain (or NG).
The OPA2607 is internally compensated to give a maxi-
mally flat frequency response for R_F = 1.21kΩ at NG = 8 on
±12V supplies. Evaluating the denominator of Equation 2
(which is the feedback transimpedance) gives an optimal
target of 1.44kΩ. As the signal gain changes, the contribu-
tion of the NG x R_I term in the feedback transimpedance will
change, but the total can be held constant by adjusting R_F.
Equation 3 gives an approximate equation for optimum R_F
over signal gain:
R_F = 1441Ω – NG R_I
(3)
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 3 shows the
recommended R_F versus NG. The values for R_F versus Gain
shown here are approximately equal to the values used to
generate the typical performance curves. They differ in that
the optimized values used in the typical performance curves
are also correcting for board parasitics not considered in the
simplified analysis leading to Equation 3. The values shown
in Figure 3 give a good starting point for design where
bandwidth optimization is desired.
1600
1400
1200
1000
800
600
400
200
0
0
FEEDBACK RESISTOR vs NOISE GAIN
5
10
15
20
Noise Gain, NG (V/V)
FIGURE 3. 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 OPA2607 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 2.
INVERTING AMPLIFIER OPERATION
Since the OPA2607 is a 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 feedback element
(e.g. integrators, transimpedance, some filters) should con-
sider the unity gain stable voltage-feedback OPA2680, since
the feedback resistor is the compensation element for a
current-feedback op amp. Wideband inverting operations
(and especially summing) are particularly suited to the
OPA2607. Figure 4 shows a typical inverting configuration
where the I/O impedances are 50Ω.
50Ω
Source
V_I
R_G
150Ω
R_M
75.0Ω
+12V
Power supply
de-coupling
not shown
1/2
OPA2607
50Ω Load
V_O ^49.9Ω
R_F
1.21kΩ
–12V
FIGURE 4. Inverting Gain of –8 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
(R_M) 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 combi-
nation of R_G and R_M.
®
OPA2607
8