OPA2681 Datasheet, PDF (15/22 Pages) Burr-Brown (TI) – Dual Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

operating conditions. They do not do as well in predicting
the harmonic distortion or dG/dP characteristics. These
models do not attempt to distinguish between the package
types in their small signal AC performance, nor do they
attempt to simulate channel-to-channel coupling.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO
OPTIMIZE BANDWIDTH
A current feedback op amp like the OPA2681 can hold an
almost constant bandwidth over signal gain settings with the
proper adjustment of the external resistor values. This is shown
in the Typical Performance Curves; the small signal bandwidth
decreases only slightly with increasing gain. Those curves also
show that the feedback resistor has been changed for each gain
setting. The resistor “values” on the inverting side of the circuit
for a current feedback op amp can be treated as frequency
response compensation elements while their “ratios” set the
signal gain. Figure 7 shows the small signal frequency response
analysis circuit for the OPA2681.
The key elements of this current feedback op amp model are:
V_I
I_ERR
α
R_I
R_G
V_O
Z_(S) I_ERR
R_F
FIGURE 7. Current Feedback Transfer Function Analysis
Circuit.
α → Buffer gain from the non-inverting input to the inverting input
R_I → Buffer output impedance
i_ERR → Feedback error current signal
Z(s) → Frequency dependent open loop transimpedance gain from i_ERR to V_O
The buffer gain is typically very close to 1.00 and is
normally neglected from signal gain considerations. It will,
however set the CMRR for a single op amp differential
amplifier configuration. For a buffer gain α < 1.0, the
CMRR = -20 x log (1– α) dB.
R_I, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA2681 is typically about
45Ω.
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 7 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 x log (R_F + NG x 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 OPA2681 is internally compensated to give a maxi-
mally flat frequency response for R_F = 402Ω at NG = 2 on
±5V supplies. Evaluating the denominator of Equation 2
(which is the feedback transimpedance) gives an optimal
target of 492Ω. As the signal gain changes, the contribution
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 = 492Ω – 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
OPA2681
15
SBOS091A