OPA2607 Datasheet, PDF (7/13 Pages) Burr-Brown (TI) – Dual, High Output, Current-Feedback OPERATIONAL AMPLIFIER

DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available to assist in the initial
evaluation of circuit performance using the OPA2607 in its
3 package styles. All are available free as an unpopulated
PC board delivered with descriptive documentation. The
summary information for these boards is shown in Table I.
PRODUCT
OPA2607U
OPA2607N
OPA2607H
TABLE I.
PACKAGE
SO-8
SO-14 SO-Cool
SO-8 SO-Cool
DEMO BOARD
NUMBER
DEM-OPA268xU
DEM-OPA2607N
DEM-OPA2607H
ORDERING
NUMBER
MKT-352
MKT-367
MKT-366
Contact the Burr-Brown applications support line to request
any of these boards.
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 induc-
tance can have a major effect on circuit performance. SPICE
models for some op amps are available through the Burr-
Brown web site (http://www.burr-brown.com). These mod-
els do a good job of predicting small-signal AC and transient
performance under a wide variety of operating conditions.
They do not do as well in predicting the harmonic distortion,
dG/dP, or temperature characteristics. These models do not
attempt to distinguish between the package types in their
small-signal AC performance, nor do they attempt to simu-
late channel-to-channel coupling.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO
OPTIMIZE BANDWIDTH
A current-feedback op amp like the OPA2607 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 2 shows the small-
signal frequency-response analysis circuit for the OPA2607.
The key elements of this current feedback op amp model are:
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
V_I
I_ERR
α
R_I
R_G
V_O
Z_(S) I_ERR
R_F
FIGURE 2. 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 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. The OPA2607 has an R_I typi-
cally about 33Ω.
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 2 gives Equation 1:
V_O
V_I
=
1+
α

¹
+
R_F
R_G


R_F
+
R_I

¹
+
R_F
R_G


=
1+
α NG
R_F + R_I
NG
Z_(S)
(1)
Z_(S)

^NG

≡

¹
+
R_F
R_G




This is written in a loop-gain analysis format where the
errors arising from a finite 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
(2)
R_F + R_I NG
®
7
OPA2607