OPA2694ID Datasheet, PDF (16/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO
OPTIMIZE BANDWIDTH
A current-feedback op amp like the OPA2694 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 Characteristic 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 10 shows the small-signal
frequency response analysis circuit for the OPA2694.
V_I
α
R_I
i_ERR
R_G
V_O
Z_(S) i_ERR
R_F
Figure 10. Recommended Feedback Resistor
Versus Noise Gain
The key elements of this current-feedback op amp model
are:
α → Buffer gain from the noninverting input to the
inverting input
R_I → Buffer output impedance
i_ERR → Feedback error current signal
Z_(s) → Frequency dependent open-loop transimpe-
dance 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 × log (1– α) dB.
R_I, the buffer output impedance, is a critical portion of the
bandwidth control equation. R_I for the OPA2694 is typically
about 30Ω.
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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 Characteristics 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 10 gives Equation (2):
ǒ Ǔ _VO
ǒ Ǔ ^VI
+
1
a
1
)
R_F
R_G
)
R
F^)RI ¹⁾_RRGF
Z _(S)
+
1
)
aNG
R_F)R_I
Z _(S)
NG
(2)
where:
ǒ Ǔ NG +
1
)
R_F
R_G
This is written in a loop-gain analysis format, where the
errors arising from a noninfinite open-loop gain are shown
in the denominator. If Z_(S) were infinite over all frequencies,
the denominator of Equation (2) would reduce to 1 and the
ideal desired signal gain shown in the numerator would be
achieved. The fraction in the denominator of Equation (2)
determines the frequency response. Equation (3) shows
this as the loop-gain equation:
R_F
Z _(S)
) R_I
NG
+
Loop
Gain
(3)
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 (3), at which point the loop gain reduces to 1 (and
the curves intersect). This point of equality is where the
amplifier closed-loop frequency response given by
Equation (2) starts 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 (3) may be controlled somewhat
separately from the desired signal gain (or NG).
The OPA2694 is internally compensated to give a
maximally flat frequency response for R_F = 402Ω at
NG = 2 on ±5V supplies. Evaluating the denominator of
Equation (3) (which is the feedback transimpedance)
gives an optimal target of 462Ω. 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 (4) gives an
approximate equation for optimum R_F over signal gain:
R_F + 462W * NG @ R_I
(4)
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