Datasheet

DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
MACROMODELS AND APPLICATIONS
R
F
V
O
R
G
R
I
Z I
(S) ERR
I
ERR
a
V
I
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE
OPA2695
SBOS354A – APRIL 2008 – REVISED AUGUST 2008 ...................................................................................................................................................... www.ti.com
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.
Two printed circuit boards (PCBs) are available to
assist in the initial evaluation of circuit performance
The key elements of this current-feedback op amp
using the OPA2695 in its two package options. Both
model are:
of these are offered free of charge as unpopulated
α → Buffer gain from the noninverting input to the
PCBs, delivered with a user ’ s guide. The summary
inverting input
information for these fixtures is shown in Table 1 .
R
I
→ Buffer output impedance
Table 1. Demonstration Fixtures by Package
i
ERR
→ Feedback error current signal
Z
(S)
→ Frequency-dependent, open-loop
ORDERING LITERATURE
PRODUCT PACKAGE
NUMBER NUMBER
transimpedance gain from i
ERR
to V
O
OPA2695ID SO-8 DEM-OPA-SO-2E SBOU064
OPA2695IRGT QFN-16 DEM-OPA-QFN-2C SBOU061
The demonstration fixtures can be requested at the
Texas Instruments web site (www.ti.com ) through the
OPA2695 product folder .
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
Figure 76. Current-Feedback Transfer Function
the TI web site (www.ti.com ). This model does a good
Analysis Circuit
job of predicting small-signal ac and transient
performance under a wide variety of operating
The buffer gain is typically very close to 1.00 and is
conditions. They do not do as well in predicting the
normally neglected from signal gain considerations. It
harmonic distortion or dG/dP characteristics. These
also, however, sets the CMRR for a single op amp
models do not attempt to distinguish between the
differential amplifier configuration. For the buffer gain
package types in the respective small-signal ac
α < 1.0, the CMRR = – 20 × log (1 – α ).
performance, nor do they attempt to simulate
channel-to-channel coupling.
R
I
, the buffer output impedance, is a critical portion of
the bandwidth control equation. For the OPA2695, it
is typically about 29 Ω for ± 5V operation and 32 Ω for
single +5V operation.
A current-feedback op amp senses an error current in
BANDWIDTH
the inverting node (as opposed to a differential input
error voltage for a voltage-feedback op amp) and
A current-feedback op amp such as the OPA2695
passes this current on to the output through an
can hold an almost constant bandwidth over signal
internal frequency-dependent transimpedance gain.
gain settings with the proper adjustment of the
The Typical Characteristics show this open-loop
external resistor values. This performance is shown in
transimpedance response. This response is
the Typical Characteristics . The small-signal
analogous to the open-loop voltage gain curve for a
bandwidth decreases only slightly with increasing
voltage-feedback op amp.
gain. These curves also show that the feedback
resistor has been changed for each gain setting. The
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Product Folder Link(s): OPA2695