Using sigma-delta A/D methods, high resolution can be obtained for only low to medium signal bandwidths. This article describes conventional A/D conversion, as well as its performance modeling. We then look at the technique of oversampling, which can be used to improve the resolution of classical A/D methods. We discuss how sigma-delta converters use the technique of noise shaping in addition to oversampling to allow high resolution conversion of relatively low bandwidth signals. We examine the use of sigma-delta converters to convert narrowband bandpass signals with high resolution. Several parallel sigma-delta converters, which offer the potential of extending high resolution conversion to signals with higher bandwidths, are also described.

An overview of sigma-delta converters

A dynamic element matching algorithm, data weighted averaging, is introduced for use in multibit /spl Delta//spl Sigma/ data converters. Using this algorithm, distortion spectra from DAC linearity errors are shaped by first-order noise shaping, resulting in a dynamic range improvement of 9 dB/octave when DAC errors dominate. Combining this technique with random dithering nearly eliminates the aliasing of the DAC errors into the baseband. Simulations show that with only 1% element matching 110 dB signal-to-noise ratio (18 b) is achieved for a third-order 3-b modulator with an oversampling ratio of 128.

Linearity enhancement of multibit /spl Delta//spl Sigma/ A/D and D/A converters using data weighted averaging

Recently, various multibit noise-shaping digital-to-analog converters (DACs) have been proposed that use digital signal processing techniques to cause the DAC noise arising from analog component mismatches to be spectrally shaped. Such DACs have the potential to significantly increase the present precision limits of /spl Delta//spl Sigma/ data converters by eliminating the need for one-bit quantization in delta-sigma modulators. This paper extends the practicality of the noise-shaping DAC approach by presenting a general noise-shaping DAC architecture along with two special-case configurations that achieve first- and second-order noise-shaping, respectively. The second-order DAC configuration, in particular, is the least complex of those currently known to the author. Additionally, the paper provides a rigorous explanation of the apparent paradox of how the DAC noise can be spectrally shaped even though the sources of the DAC noise-the errors introduced by the analog circuitry-are not known to the noise-shaping algorithm.

Spectral shaping of circuit errors in digital-to-analog converters

Oversampled sigma-delta (EA) modulators offer numerous advantages for the realization of high-resolution analog-to-digital (A/D) converters. This paper explores how oversampling and feedback can be employed in high-resolution /spl Sigma//spl Delta/ modulators to extend the signal bandwidth into the range of several megahertz when the oversampling ratio is constrained by technology limitations. A 2-2-1 cascaded multibit architecture suitable for operation from a 2.5-V power supply is presented, and a linearization technique referred to as partitioned data weighted averaging is introduced to suppress in-band digital-to-analog converter (DAC) errors. An experimental prototype based on the proposed topology has been integrated in a 0.5-/spl mu/m double-poly triple-metal CMOS technology. Fully differential double-sampled switched-capacitor integrators enable the modulator to achieve 95-dB dynamic range at a 4-Msample/s Nyquist conversion rate with an oversampling ratio of 16. The experimental modulator dissipates 150 mW from a 2.5-V supply.

A 2.5-V sigma-delta modulator for broadband communications applications

A systematic study of single-loop, cascaded, and multibit /spl Delta//spl Sigma/ modulators of second to fourth order is presented, based on a combination of behavioral simulations and linear modeling. Constraints for optimal performance and precise guidelines for the choice of parameters are derived. Moreover, the optimal parameters and the corresponding performance are found and given in tables. A graph showing the maximal achievable performance of each topology as a function of the oversampling ratio is presented, offering a valuable help for the design of analog-to-digital converters.

Optimal parameters for /spl Delta//spl Sigma/ modulator topologies

A dynamic element matching technique is applied to multibit sigma-delta modulators. The approach translates the harmonic distortion components of a nonideal digital-to-analog converter (DAC) in the feedback loop of a sigma-delta modulator to high-frequency components, which can then be filtered out by the decimation filter. Computer simulations have confirmed that with this approach a third-order sigma-delta modulator employing a 3-bit forward ADC, a 3-bit feedback DAC with a random mismatch of 0.1% can achieve a 104-dB (17+bit) dynamic range and a harmonic distortion below 100 dB, with an oversampling ratio of 64. The technique does not generate any tone in the passband. >

S. Sutarja

Papers

Multibit Sigma - Delta A/D converter incorporating a novel class of dynamic element matching techniques