acquisitionZONE Products for the week of November 12, 2001
Texas Instruments Says . . .
TI Establishes New Performance Level with Revolutionary
Current Feedback Amplifier for DSL, Video and Emerging Signal Processing
Systems
Setting a new level of performance in low-power, wideband operational
amplifiers, Texas Instruments (TI) announced a new family of high performance
Current FeedBack (CFB) op amps from the company's Burr-Brown division. Featuring
a significantly improved internal architecture, these amplifiers hold improved
bandwidth versus gain consistency, improve distortion and provide low power,
high output, single supply operation for DSL communications, studio cameras,
broadcast video equipment, portable systems and a wide variety of high-performance
signal processing applications.
The CFBplus OPA684 is the first Current FeedBack amplifier to use an internally closed-loop input buffer stage to improve performance significantly over earlier <2mA quiescent current CFB amplifier products. While retaining the benefits of very low power operation, this new architecture provides the benefits of a more ideal CFB topology.
"The OPA684 sets a new direction in high-speed amplifier design by providing outstanding bandwidth over a very wide gain range along with improved harmonic distortion compared to previous low power op amps," said Michael Steffes, strategic marketing manager for TI's high-speed signal processing products. "This innovation steps up the performance of low power op amps by providing significantly reduced inverting input impedance while taking no added power penalty."
Steffes added: "Typically, when significant new levels of performance capability are created in the op amp universe, the full range of applications enabled by this innovation will only be limited by the imagination of the design community."
The closed-loop input stage buffer provides a very low and linearized impedance path into the inverting input to sense the feedback error current that is fundamental to the operation of any CFB amplifier. This improved inverting node impedance gives exceptional bandwidth retention to much higher gains and improved harmonic distortion over earlier solutions that were limited by inverting input linearity.
To illustrate, the OPA684 features a 210MHz to 120MHz bandwidth change from gains of 1 to 10 (200mVp-p, 100ohm load) - this is a 1.75:1 bandwidth variation over a 10:1 gain range. The device gives more than 10dB improvement in total harmonic distortion (THD) at a gain of +2 (2Vp-p, 100ohm load, 100kHz to 1MHz), compared to previous wideband CFB parts. This improved distortion becomes even more dramatic at higher gains with the higher loop gain provided by the CFBplus amplifiers. SFDR improves by >20dB at a gains >10 versus earlier low power CFB amplifiers using open loop input buffer stages.
The output capability of the OPA684 also sets a new mark in performance for low power CFB amplifiers. Delivering a full +/-4Vp-p swing on +/-5V supplies, the OPA684 has the output current (>150mA) to support this swing into demanding filter or doubly terminated line loads. This minimal output headroom requirement is complemented by a similar 1.2V input stage headroom, giving exceptional capability for single +5V operation.
The OPA684's low 1.7mA supply current is precisely trimmed at 25C. This
trim, along with low shift over temperature and supply voltage, gives a
very robust design over a wide range of operating conditions. System power
may be further reduced by using the optional disable control pin. Leaving
this disable pin open, or holding it high, gives normal operation. If pulled
low, the OPA684 supply current drops to less than 130uA while the I/O pins
go to a high impedance state.
analogZone Says . . .
This part represents a significant change in architecture for a current-feedback op amp. The closed-loop buffer in the input stage allows for a remarkable immunization of the gain setting element(s) from interaction with bandwidth. The gain-bandwidth product actually increases with higher gains! The headroom offered by the part also signifies an amplifier that is, finally, truly suited to video work; a full +/-4.1 V swing is available on the output of a part being powered with +/- 5 V rails. The input range is from 1.32 to 3.68 V on a single 5-V supply allowing for (careful) video designs with single rail operation. The output on a single rail can be between 0.90 to 4.16 V with a 1 kilohm load. The single-rail quiescent is 1.51 mA and falls to 112 uA when the disable is activated.
The best bandwidth performances are obtained with dual rails with small-signal bandwidths ranging from 210 MHz at a gain of +1, to 104 MHz at a gain of +20. At a gain of +2 (the only point the single-rail performance is characterized on the preliminary data sheet) the dual rail bandwidth is 160 MHz, while that for the single rail is 126 MHz. The slew rate at that gain is 820 V/us (typical) and the harmonic distortion is in the 70s of dBc for realistic loads.
This is a part that will create long lines for samples from video engineers, especially those working in the high-gain arena -- such as in studio cameras and at the output of SAW filter demodulator products. This is a winner right out of the gate. TI (Burr-Brown) is planning a dual part to sample later this quarter but the company should not offer it unless the channel isolation can be guaranteed to be over 80 dB.
The (single channel) OPA684 is sampling now in
SOT-23-6 and SO-8 with production scheduled for the first quarter in 2002.
It will be priced at $1.29 in 1000-piece lots.