OPA4684M Datasheet, PDF (23/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
Table 1. Evaluation Module Ordering Information
PRODUCT
OPA4684ID
OPA4684IDBW
PACKAGE
SO−14
TSSOP−14
BOARD PART NUMBER
DEM−OPA468xD
DEM−OPA46xxDPW
LITERATURE NUMBER
SBOU016
—
macromodels
Computer simulation of circuit performance using SPICE is often useful in predicting 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. Check the TI web site (www.ti.com) for SPICE
macromodels within the OPA4684 product folder. These models 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
distortion or dG/dP characteristics. These models do not attempt to distinguish between the package types in
their small-signal AC performance.
operating suggestions
setting resistor values to optimize bandwidth
Any current-feedback op amp like the OPA4684 can hold high bandwidth over signal-gain settings with the
proper adjustment of the external resistor values. A low-power part like the OPA4684 typically shows a larger
change in bandwidth due to the significant contribution of the inverting input impedance to loop-gain changes
as the signal gain is changed. Figure 54 shows a simplified analysis circuit for any current-feedback amplifier.
Figure 54. Current-Feedback Transfer Function Analysis Circuit
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
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 the buffer gain a < 1.0, the
CMRR = _20 × log(1 − α). The closed-loop input stage buffer used in the OPA4684 gives a buffer gain more
closely approaching 1.00 and this shows up in a slightly higher CMRR than previous current-feedback op amps.
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