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D-SPACING: 0px; PADDING-TOP: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px">A calculation of the maximum output DC offset error band at 25 °C for the OPA684 shows the relatively poor DC precision offered by the CFB implementation. That calculation must treat the two input bias currents separately, because they are not physically related in the CFB input stage as they are in most VFB input stages. Leaving the 90.9 Ω resistor on the non-inverting input, the total output DC error band, using the OPA684 specified maximum DC errors at 25 °C, is:

Maximum V_os=
±3.5 mV·11 V/V±10μA·91 Ω·11 V/V± 16μA·1 kΩ =
±64.5 mV

So, while the OPA684 can match the speed of the OPA846 in this higher gain application, if output DC precision (or lower noise) is desired, the VFB design offers considerable advantages.

Conclusions to Part 1

In Part 1, we reviewed the key internal differences between VFB and CFB op amps as a means of identifying applications that must use the VFB topology over the CFB devices. These examples generally fall into two categories: applications that need a capacitive feedback for some reason; and circuits that need to emphasize DC precision and/or the lowest output noise.Part 2will continue with applications uniquely suited to the CFB topology, and then introduce the newest member of the wideband op amp universe—the fully differential amplifier. Part 2 will conclude with illustrations of a few applications uniquely suited to this amplifier.

(To go to Part 2 of this article, click here)

About the author
Michael Steffesis the Market Development Manager for High-Speed Signal Conditioning, and a Distinguished Member of the Technical Staff, at Texas Instruments Inc. With more than 25 years of experience in high-speed amplifier design, applications, and marketing, Michael currently provides product definition and customer design-in support.

Michael earned a BSEE from the University of Kansas and an MBA from Colorado State University. He shares several basic patents in high-speed op amp designs and has written more than 85 product data sheets, scores of contributed articles, applications notes and conference papers. You can reach Michael at ti8728499steffes@list.ti.com.

References
1. "Voltage Feedback vs. Current Feedback Op Amps," Jim Karki, Texas Instruments Application Note, SLVA051 (Voltage Feedback vs. Current Feedback Op Amps or click here).

2. "Fully Differential Amplifiers," Jim Karki , Texas Instruments Application Note SLOA054D (Fully-Differential Amplifiers (Rev. D) or click here).

3. "Control Frequency Response and Noise in Broadband, Photodetector Transimpedance Amplifiers," Michael Steffes, EDN, Design Feature, July 4th, 1996 pp. 113-125.

4. "Design Methodology for MFB Filters in ADC Interface Applications," Michael Steffes. TI Application Note, SBOA114 (Design Methodology for MFB Filters in ADC Interface Applications or click here).

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