OPA3681 Burr-Brown, OPA3681 Datasheet - Page 16

• Manufacturer Part Number: OPA3681
• Description: Triple Wideband / Current-Feedback OPERATIONAL AMPLIFIER With Disable
• Manufacturer: Burr-Brown

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:
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:
If 20 • log (R
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
(which is the feedback transimpedance) gives an optimal
target of 589 . As the signal gain changes, the contribution
of the NG
change, but the total can be held constant by adjusting R
Equation 3 gives an approximate equation for optimum R
over signal gain:
As the desired signal gain increases, this equation will
eventually predict a negative R
limit to this adjustment can also be set by holding R
5V supplies. Evaluating the denominator of Equation 2
V
V
O
I
•
1
R
®
F
I
+ NG • R
OPA3681
R
term in the feedback transimpedance will
R
F
F
R
1
Z
F
R
R NG
Z
( )
I
S
R
R
( )
I
I
S
589 –
1
) were drawn on top of the open-
G
F
R
R
G
F
Loop Gain
F
NG R
. A somewhat subjective
F
1
= 499
NG
I
R
F
NG
Z
1
at NG = 2 on
R NG
( )
S
I
R
R
G
F
G
Eq. 1
Eq. 2
Eq. 3
to a
F
F
.
16
FIGURE 10. Recommended Feedback Resistor vs Noise
minimum value of 20 . Lower values will load both the
buffer stage at the input and the output stage if R
low—actually decreasing the bandwidth. Figure 10 shows
the recommended R
operation. The values shown in Figure 10 give a good
starting point for design where bandwidth optimization is
desired.
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
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
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.
600
500
400
300
200
100
0
0
Gain.
FEEDBACK RESISTOR vs NOISE GAIN
F
5
I
vs NG for both 5V and a single +5V
, decreasing the bandwidth. For those
+5V
Noise Gain
5V
10
I
in Figure 9.
15
F
gets too
20