OPA3681 Datasheet, PDF (16/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

A current-feedback op amp senses an error current in the
inverting node (as opposed to a differential input error
voltage for a voltage feedback op amp) and passes this on to
the output through an internal frequency dependent
transimpedance gain. The Typical Performance Curves show
this open-loop transimpedance response. This is analogous
to the open-loop voltage gain curve for a voltage-feedback
op amp. Developing the transfer function for the circuit of
Figure 9 gives Equation 1:
V_O
V_I
=
α1
+
R_F
R_G


1+
R_F
+
R
I

¹
+
R_F
R_G


Eq. 1
=
1+
α NG
R_F + R_I
NG
Z_(S)
Z_(S)

^NG

≡

¹
+
R_F
R_G




This is written in a loop gain analysis format where the
errors arising from a non-infinite open-loop gain are shown
in the denominator. If Z(s) were infinite over all frequencies,
the denominator of Equation 1 would reduce to 1 and the
ideal desired signal gain shown in the numerator would be
achieved. The fraction in the denominator of Equation 1
determines the frequency response. Equation 2 shows this as
the loop gain equation:
Z_(S)
= Loop Gain
R_F + R_I NG
Eq. 2
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 OPA3681 is internally compensated to give a maxi-
mally flat frequency response for R_F = 499Ω at NG = 2 on
±5V supplies. Evaluating the denominator of Equation 2
(which is the feedback transimpedance) gives an optimal
target of 589Ω. As the signal gain changes, the contribution
of the NG • 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 = 589Ω – NG R_I
Eq. 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 10 shows
the recommended R_F vs NG for both ±5V and a single +5V
operation. The values shown in Figure 10 give a good
starting point for design where bandwidth optimization is
desired.
600
500
400
300
200
100
0
0
FEEDBACK RESISTOR vs NOISE GAIN
+5V
±5V
5
10
15
20
Noise Gain
FIGURE 10. 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 OPA3681 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 9.
INVERTING AMPLIFIER OPERATION
Since the OPA3681 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 feed-
back element (e.g., integrators, transimpedance, some fil-
ters) 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 (especially summing) is particularly suited to the
OPA3681. 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.
®
OPA3681
16