OPA2695 Datasheet, PDF (26/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
SBOS354 – APRIL 2008..................................................................................................................................................................................................... www.ti.com
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to
assist in the initial evaluation of circuit performance
using the OPA2695 in its two package options. Both
of these are offered free of charge as unpopulated
PCBs, delivered with a user’s guide. The summary
information for these fixtures is shown in Table 2.
Table 2. Demonstration Fixtures by Package
PRODUCT
OPA2695ID
OPA2695IRGT
PACKAGE
SO-8
QFN-16
ORDERING
NUMBER
DEM-OPA-SO-2E
DEM-OPA-SO-2C
LITERATURE
NUMBER
SBOU064
SBOU061
The demonstration fixtures can be requested at the
Texas Instruments web site (www.ti.com) through the
OPA2695 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
practice 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 OPA2695 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. 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 the respective 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 such as the OPA2695
can hold an almost constant bandwidth over signal
gain settings with the proper adjustment of the
external resistor values. This performance is shown in
the Typical Characteristics. The small-signal
bandwidth decreases only slightly with increasing
gain. These 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 the ratios set
the signal gain. Figure 76 shows the analysis circuit
for the OPA2695 small-signal frequency response.
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,
transimpedance gain from i_ERR to V_O
open-loop
V_I
I_ERR
a
R_I
V_O
Z_(S) I_ERR
R_F
R_G
Figure 76. 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
also, however, sets the CMRR for a single op amp
differential amplifier configuration. For the buffer gain
α < 1.0, the CMRR = –20 × log (1 – α).
R_I, the buffer output impedance, is a critical portion of
the bandwidth control equation. For the OPA2695, it
is typically about 29Ω for ±5V operation and 32Ω for
single +5V operation.
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 current on to the output through an
internal frequency-dependent transimpedance gain.
The Typical Characteristics show this open-loop
transimpedance response. This response is
analogous to the open-loop voltage gain curve for a
voltage-feedback op amp.
26
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