High-Speed, High-Performance Converters for DSL Applications

- a research project together with Ericsson Microelectronics AB -

(Niklas U. Andersson, K. Ola Andersson, J. Jacob Wikner, and Mikael Karlsson Rudberg)

This page is under construction.

Background

This project is mainly performed at the Ericsson Microelectronics Research Center, Ericsson Microelectronics AB. But it is closely related to the research at Electronics Systems, Linköping University.

xDSL

Research areas

D/A Converters for xDSL

A/D Converters for xDSL

Linedriver Linearization

Algorithms for Improvement of Data Converter's Performance

Research status

D/A Converters for xDSL

Main focus for the research on Digital-to-analog converters (DACs) has bee set on the design of wideband DACs for telecommunications applications. Some effort has been done on the research on oversampling DACs (OSDACs).
Especially modeling of the behavior of current-steering DACs has been investigated. Both static and dynamic effects in the time- and frequency domain have been modeled.
Models and simulation environments for both Nyquist and Oversampling DACs have been developed in a Matlab framework.
A number of wideband D/A converters have been designed and processed. A large improvement in performance from previous designs have been measured.
There are 12 Nyquist DAC chip versions available and 1 Oversampled DAC chip. The Nyquist DAC supply ranges from 2.7-5.5V and the nominal resolution from 10 to 14 bits. The OSDAC is designed for a 3.3V supply and 14 input data bits.

For wideband D/A converters, we want to avoid feedback circuits, and preferably amplifiers, since they are very hard to design to meet the needed requirements. Therefore the current-steering DAC is a good candidate for high-speed. However, the chip area becomes large for a large nominal number of bits. The current- steering DAC provides a simple layout with unit elements. The DAC suffers from a large sensitivity towards matching errors and therefore this has to be managed carefully. Segmentation, spectral error shaping, and randomization are common techniques to reduce these errors.

The oversampling D/A converter is preferred when extremely high linearity is needed. With a sigma-delta modulator the number of bits in the input signal is reduced, and the additional noise is shaped to higher frequencies. When the number of bits is reduced to only one, one-bit DACs can be used. The one- bit DAC is advantageous since it provides an infinite linearity (if the start- and end-errors are signal-independent). The spectrally shaped noise introduced by the modulator has to be filtered out. This can be done in a smart way. A semi-digital FIR filter is used to convert the signal from its one-bit digital representation to the analog domain. With the filter, the signal is both filtered and weighted. The FIR filter is designed using current sources that are switched by the output bit fed through a chain of D-latches. Images are further attenuated with analog LP filters at the output. To relax the requirements on the analog filter, we choose a high oversampling ratio.

DAC Models
The work has to a large extent been about modeling of DAC performance when including mismatch and other error sources to the original design. The models help us understand how to increase the quality of the devices.
The impact on the DAC's performance of ...

output impedance
transistor matching
circuit noise
non-linear slewing

... has been modeled and successfully simulated. Matlab has been used as toolbox.

A/D Converters for xDSL

Linedriver Linearization

Algorithms for Improvement of Data Converter's Performance

Members

J Jacob Wikner, Tekn.Lic., e-mail jacobw@isy.liu.se, phone +46 70 5915938.
Mikael Karlsson Rudberg, Tekn.Lic., e-mail mikaelr@isy.liu.se, phone +46 70 8488418.
K. Ola Andersson, Tekn.Lic., e-mail olaa@isy.liu.se, phone +46 13 286698.
Niklas U. Andersson, Tekn.Lic., e-mail niklasa@isy.liu.se, phone +46 13 286698.

Publications

All publications from Electronics Systems can be found here.

Publications closely related to this research project will be listed below.

[1] J. J. Wikner and M. Vesterbacka, "D/A Conversion with Linear-Coded Weights,"
Proc. 2000 Southwest Symp. on Mixed-Signal Design, pp. 61-66, San Diego, California, Feb. 27-29, 2000.

[2] J.J. Wikner and M. Vesterbacka, "Characteristics of Linear-Coded D/A Converters,"
Proc. 2000 Southwest Symp. on Mixed-Signal Design, pp. 67-72, San Diego, California, Feb. 27-29, 2000.

[3] M. Vesterbacka and J.J. Wikner, "Dual Matrix Linear-Code D/A Converters,"
Proc. 4th Int. Analog VLSI Workshop, Stockholm, Sweden, June 2-3, 2000.
[4] N. U. Andersson and J. J. Wikner, "Comparison of Different Dynamic Element Matching Techniques for Wideband CMOS DACs,"
Proc. NorChip Conf., Oslo, Norway, Nov. 8-9, 1999.

[5] N. U. Andersson and J.J. Wikner, "A Strategy of Implementing Dynamic Element Matching in Current-Steering DACs,"
Proc. 2000 Southwest Symp. on Mixed-Signal Design, pp. 51-56, San Diego, California, Feb. 27-29, 2000.

[6] M. Karlsson Rudberg, N.U. Andersson, M. Vesterbacka and J.J. Wikner, "", submitted to ICECS'00, Lebanon
[7] M. Karlsson Rudberg, N.U. Andersson, M. Vesterbacka and J.J. Wikner, "", submitted to ICECS'00, Lebanon
[8] K.O. Andersson and J.J. Wikner, "Characterization of a CMOS Current-Steering DAC using State-Space Models", in Proc. of IEEE MWCAS'00, Lansing, Aug. 2000
[9] M. Karlsson Rudberg, "", Proc. of ISCAS'00
[10] J.J. Wikner, " Simulation and measurement of two 3-5V CMOS Current-Steering DACs," in Proc. of the IEE 3rd international A/D and D/A conf., Glasgow, Scotland, July 28, 1999
[11] Y. Gao, J. J. Wikner, and H. Tenhunen, " Design and Analysis of an Oversampling D/A Converter for DMT-ADSL Systems," in Proc. of the 3rd analog VLSI workshop, AVLSIWS'99, Taiwan, May, 1999
[12] M. Gustavsson, J.J. Wikner, and N. Tan, "CMOS Data Converters for Communications", Kluwer Academic Publishers, 2000

Patents

[13] N.U. Andersson, M. Vesterbacka, J.J. Wikner, and M. Karlsson Rudberg,
"Scrambler and Scrambler Method",
Swedish patent 00xxxxx-8 pending, June 2000.
[14] J.J. Wikner and M. Vesterbacka, "D/A Conversion Method and D/A Converter",
Swedish patent 9903500-8 pending, Oct. 1999.
[15] K.O. Andersson, "",
Swedish patent 9903500-8 pending, Oct. 1999.

For information and questions regarding this web site, please contact J. Jacob Wikner, jacobw@isy.liu.se

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