Bram Verhoef

High Performance DAC

Description of research

Conventional Digital-to-analog converters have problems with the stability of reference signals. Due to (capacitive) coupling reference signals become dependent on the output signal. This feeds to the DA cells and introduces distortion.

Conventional current-mode D/A converters use thermometer-encoded cascaded current sources with differential switching pairs. The digital input controls the switches and directs the current to either one of the differential outputs. If good high-frequency performance is required, it is important that the timing of the switches is very accurate and fast (avoid overlap and timing gaps). A disadvantage of this topology is that high voltage signals are present on the cascaded current source which feeds back to voltage references. The reference voltage may thus become dependent on the switching activity of the D/A converter. In this way, the reference voltage is dependent on the output signal; this introduces distortion. In general, crosstalk leads to what is called “memory effects” as reference voltages are dependent on the switching history.

It has been shown that D/A converters based on switched-capacitor charge-redistribution networks suffer from very little memory effect, but they produce a voltage output that needs a low output impedance buffer.

A solution to this memory effect may be found in omitting always-on current sources and switches. By modulating the current source with the input signal itself, there is very little feedback from the output to the reference lines. Furthermore, by exploiting charge-redistribution networks, voltage references can de de-coupled from their loads which leaves the references ‘floating’; this way it can settle to a stable value quickly before it is re-used. Furthermore, the output impedance of each DA cell is more constant; this also reduces distortion.

By digitally controlling the charge-redistribution, errors in voltage references and output device mismatch can be compensated. This means that smaller output devices can be used which have a smaller load on the reference networks and have less feedback from the output.

This research will investigate whether this current-source modulation can be implemented and what factors limit performance.

Advisor(s)

Prof.ir. A.J.M. van Tuijl

Prof.dr.ir. B. Nauta

Duration

15-10-2009/15-10-2013

Project

High Performance DAC

Funding institution

Axiom IC-BV, in Enschede

Strategic Research Orientation

Wireless and Sensor Systems

Links to relevant web pages:

http://icd.ewi.utwente.nl/persons/?id=290

Pictures

Description: Verhoef, B. (Bram)

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Home CTIT symposium 2013 About CTIT News NIRICT Research Meet the PhDs Working at CTIT Internal Information Conferences Open Calls (updated May 3, 2013) Library and Videos Knowledge Transfer Links Sitemap Search	Bram Verhoef High Performance DAC Description of research Conventional Digital-to-analog converters have problems with the stability of reference signals. Due to (capacitive) coupling reference signals become dependent on the output signal. This feeds to the DA cells and introduces distortion. Conventional current-mode D/A converters use thermometer-encoded cascaded current sources with differential switching pairs. The digital input controls the switches and directs the current to either one of the differential outputs. If good high-frequency performance is required, it is important that the timing of the switches is very accurate and fast (avoid overlap and timing gaps). A disadvantage of this topology is that high voltage signals are present on the cascaded current source which feeds back to voltage references. The reference voltage may thus become dependent on the switching activity of the D/A converter. In this way, the reference voltage is dependent on the output signal; this introduces distortion. In general, crosstalk leads to what is called “memory effects” as reference voltages are dependent on the switching history. It has been shown that D/A converters based on switched-capacitor charge-redistribution networks suffer from very little memory effect, but they produce a voltage output that needs a low output impedance buffer. A solution to this memory effect may be found in omitting always-on current sources and switches. By modulating the current source with the input signal itself, there is very little feedback from the output to the reference lines. Furthermore, by exploiting charge-redistribution networks, voltage references can de de-coupled from their loads which leaves the references ‘floating’; this way it can settle to a stable value quickly before it is re-used. Furthermore, the output impedance of each DA cell is more constant; this also reduces distortion. By digitally controlling the charge-redistribution, errors in voltage references and output device mismatch can be compensated. This means that smaller output devices can be used which have a smaller load on the reference networks and have less feedback from the output. This research will investigate whether this current-source modulation can be implemented and what factors limit performance. Advisor(s) Prof.ir. A.J.M. van Tuijl Prof.dr.ir. B. Nauta Duration 15-10-2009/15-10-2013 Project High Performance DAC Funding institution Axiom IC-BV, in Enschede Strategic Research Orientation Wireless and Sensor Systems Links to relevant web pages: http://icd.ewi.utwente.nl/persons/?id=290 Pictures