OPA2677IDDA Datasheet, PDF (18/38 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier

DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
A printed circuit board (PCB) is available to assist in the initial
evaluation of circuit performance using the OPA2677. The
fixture is offered free of charge as unpopulated PCB, deliv-
ered with a user’s guide. The summary information for this
fixture is shown in Table II.
PRODUCT
OPA2677U
OPA2677IDDA
OPA2677T
OPA2677IRGV
PACKAGE
SO-8
HSOP-8
SO-16
QFN-16
ORDERING
NUMBER
DEM-OPA-SO-2A
Not Available
Not Available
Not Available
LITERATURE
NUMBER
SBOU003
Not Available
Not Available
Not Available
TABLE II. Demonstration Fixtures by Package.
This demonstration fixture can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2677
product folder.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and RF
amplifier circuits where parasitic capacitance and inductance
can have a major effect on circuit performance. A SPICE
model for the OPA2677 is available through the TI web site
(www.ti.com). This model does a good job of predicting
small-signal AC and transient performance under a wide
variety of operating conditions, but does not do as well in
predicting the harmonic distortion or dG/dP characteristics.
This model does not attempt to distinguish between the
package types in small-signal AC performance, nor does it
attempt to simulate channel-to-channel coupling.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO
OPTIMIZE BANDWIDTH
A current-feedback op amp like the OPA2677 can hold an
almost constant bandwidth over signal gain settings with the
proper adjustment of the external resistor values, which is
shown in the Typical Characteristics; the small-signal band-
width decreases only slightly with increasing gain. These
curves also show that the feedback resistor is 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, whereas their
ratios set the signal gain. Figure 11 shows the small-signal
frequency response analysis circuit for the OPA2677.
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 transimpedance
gain from i_ERR to V_O
V_I
I_ERR
α
R_I
V_O
Z_(S) I_ERR
R_F
R_G
FIGURE 11. Current Feedback Transfer Function Analysis
Circuit.
The buffer gain is typically very close to 1.00 and is normally
neglected from signal gain considerations. It sets the CMRR,
however, for a single op amp differential amplifier configura-
tion. 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. The OPA2677 inverting input
resistor is typically 22Ω.
A current-feedback op amp senses an error current in the
inverting node (as opposed to a differential input error volt-
age for a voltage-feedback op amp) and passes this on to the
output through an internal frequency dependent transimped-
ance gain. The Typical Characteristics show this open-loop
transimpedance response, which is analogous to the open-
loop voltage gain curve for a voltage-feedback op amp.
Developing the transfer function for the circuit of Figure 11
gives Equation 14:
V_O
V_I
=
α1 +
R_F
R_G


1+
R_F
+
R_I
_{1 +}
R_F
R_G


=
1+
α • NG
R_F + R_ING
Z(s)
Z(s)
(14)

^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) is infinite over all frequencies, the denomi-
nator of Equation 14 reduces to 1 and the ideal desired signal
gain shown in the numerator is achieved. The fraction in the
denominator of Equation 14 determines the frequency re-
sponse. Equation 15 shows this as the loop-gain equation:
Z(s) _{= Loop Gain}
R_F + R_ING
(15)
If 20log(R_F + NG • R_I) is 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 15, at which point the
loop gain has reduced to 1 (and the curves have intersected).
This point of equality is where the amplifier closed-loop
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OPA2677
www.ti.com
SBOS126I