C o n t e n t s

ADC with Configurable Filtering Adds Flexibility >>

High Speed ADC Portfolio Responds to Market Demands >>

High Performance ADC Dissipates Only 15 mW to Solve Heat Dissipation >>

Smallest SAR ADC Targets Space-Constrained Applications >>

ADC Selection Guide >>

ADC Selection Guide, continued >>

ADC with Integrated Quadrature Error Correction Minimizes Errors >>

Why Use an ADC Driver Amplifier? >>

Circuits from the Lab—Tested Design Resource >>

Buffer Enhances Clock Integrity to Enable Rated Performance from High Performance >>

Diff Amp Calculator Design Tool >>

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YOUR SEMICONDUCTOR SOLUTIONS RESOURCEVolume 10, Issue 2

ANALOG-TO-DIGITAL
CONVERTER AND DRIVER ICs

ADC with Integrated Quadrature Error Correction and DC Offset Digital Processing Blocks Dynamically, Minimizes Errors Produced by I/Q Complex Signal Receivers

In today's multichannel, high data rate communications systems, quadrature and gain errors can be produced by mismatching and leakage in the individual components in the I/Q receiver signal chain stages ahead of the ADC function. These errors can affect quality of service, network reliability, and overall system functionality.

Solution
The AD9269 is the industry's first dual 16-bit, 80 MSPS ADC, and it includes a QEC and a dc offset digital processing block specifically designed to address this problem. This functional block dynamically minimizes the errors produced in an in-phase/quadrature (I/Q) complex signal receiver system. By utilizing the QEC functional block, system designers can relax component testing requirements by enabling this function to minimize gain and phase errors due to component mismatches. The net result can be a more robust receiver design. Depending on operating conditions, enabling the QEC function of the AD9269 dual ADC can result in as much as 60 dB of image suppression.

The AD9269 operates from a 1.8 V supply and contains several additional features designed to maximize flexibility and minimize system cost, such as programmable clock and data alignment and programmable digital test pattern generation.

Integrated Quadrature Error Correction (QEC Minimizes I/Q Complex Rx Errors)

Why Use an ADC Driver Amplifier Instead of a Transformer in Front of an ADC?

In order to properly condition a signal for an analog-to-digital converter (ADC), four considerations must be made. First, if the signal is single-ended, it needs to be converted to a differential signal. Sending a single-ended (ground-referenced) signal into a differential input ADC results in the immediate loss of 6 dB of dynamic range, which means a bit is lost before the conversion even begins. Second, most ADCs have large sampling capacitors on the input to hold the sampled value during the conversion cycle. When the ADC switches back from hold mode to track mode, the capacitor is reconnected to the input and a large charge injection can occur, injecting noise back into the signal path. Reverse isolation is required to prevent the charge injection. Third, unless the dc level of the incoming signal already exactly matches the required common-mode input level, dc level translation must occur. Finally, ADC inputs can load signal sources, especially sources with high source impedances, and an impedance transformation stage may be required.

Of these four functions, a transformer can only perform one: the single-ended-to-differential conversion. There is no reverse isolation on a transformer. As a passive device, there is no ability to provide power gain or impedance transformation. A voltage gain results in a corresponding current reduction. While some circuit tricks involving ac coupling and rebiasing a signal can be employed to interface dc levels through a transformer, this approach means that low frequency content (below the ac coupling high-pass response) will be lost.

A differential amplifier acting as an ADC driver, however, can provide all four functions. Analog Devices offers a wide assortment of differential amplifiers for ADC driver functions in a variety of applications. There are the ADL5561 and ADL5562 for high IF signals and the ADA4937 and ADA4939 for dc to low-/mid-IF signals. The ADA4927 also serves this space and offers the additional benefit of high performance at high gains. There are the ADA4932 and ADA4950 for low power applications and many more. There is even the ADA4941 for the precision converter space. Whatever the application, Analog Devices has a differential amplifier that will enhance circuit performance and simplify design efforts.

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