Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2003-01-08
2004-08-17
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S153000
Reexamination Certificate
active
06778115
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a digital-to-analog converter and to a method for reducing harmonic distortion in a digital-to-analog converter.
BACKGROUND OF THE INVENTION
Digital-to-analog (D/A) converters are used in a variety of applications for converting digital signals into corresponding analog signals. They are employed for example in base stations and in radio relay transmitters. The purity of the analog output signal is often of great significance for the performance of the application.
In current-steering D/A converters, the analog output signal is formed by connecting a number of current sources to a current output. In most of the applications, each current source is steered to one of two current outputs, resulting in a differential output signal current.
FIG. 1
shows as an example a segmented current steering 10-bit D/A converter with a 6-bit MSB (most significant bits) block formed of 63 unweighted current sources
1
and a 4-bit LSB (least significant bits) block formed of four binary-weighted current sources. To each current source there is assigned a differential switch pair S controlled by a current switch circuit and used to steer the respective current source to one of two current outputs OUT and XOUT. The four current sources of the LSB block output a one-, two-, four- and eight-fold predetermined current respectively if selected, thereby enabling an output of 8 different current values. Each of the current sources of the MSB block outputs a 16-fold predetermined current if selected. Each current source of the MSB block is responsible for a stepwise increase of the output current signal by a 16-fold predetermined current
16
I when selected, thereby enabling an output of 64 different current values. The currents of the MSB and the LSB blocks are summed to form the output signal.
Today it is possible to design an integrated current steering D/A converter for sampling frequencies of up to several hundreds of megasamples per second with a resolution of up to 14 bit. However, with resolutions of 10 bit or more, the full resolution bandwidth is limited to several megahertz. In telecommunications applications a signal bandwidth of several megahertz is required.
The limiting factor for an effective resolution with high frequency signals is distortion. Timing errors and code dependency of the output impedance contribute to distortion. The most common cause of distortion, however, is asymmetrical glitches that occur during the state changes in the differential switches. If the resolution of a D/A converter is equal to or more than 10 bit, distortion starts to limit the dynamic linearity rapidly after some critical point of usually less than 10 MHz.
For illustration,
FIG. 2
shows a simulated spectrum of the D/A converter of
FIG. 1
, the output voltage V
out
being depicted over the frequency f/Hz of the outputted signal. The used sampling rate is 200 MHz and the signal frequency 20 MHz. Even though the output signal is differential, the even order harmonic components are high. The 2
nd
order harmonic limits the SFDR (spurious free dynamic range) to 53 dB. This illustrates that the differential output is not symmetrical in practice.
Attempts to improve the spectrum are known from the state of the art. Most published methods focus on decreasing the glitch energy in order to improve the spectral purity, but the 2
nd
harmonic component can still appear in the spectrum. Moreover, the 2
nd
harmonic usually dominates the distortion. Such methods are described for example in Analog Devices AD9754 Datasheet: “14-bit, 125MSPS High Performance TxDAC D/A Converter”, Analog Devices, Inc., 1999; J. Bastos, A. M. Marques, M. S. J. Steyaert, W. Sansen: “A 12-bit Intrinsic Accuracy High-Speed CMOS DAC”, IEEE J. Solid-State Circuits, vol. 33, no. 12, December 1998, pp. 1959-1969; J. Vandenbussche, G. Van der Plas, A. Van den Bosch, W. Daems, G. Gielen, M. Steyaert, W. Sansen: “A 14-bit 150 MSamples/s Update Rate Q
2
Random walk CMOS DAC”, Proc. IEEE Int. Solid-State Circuits Conf., February 1999, pp. 146-147; and A. Van den Bosch, M. Borremans, J. Vandenbussche, G. Van der Plas, A. Marques, J. Bastos, M. Steyaert, G. Gielen, W. Sansen: “A 12-bit 200 MHz Low Glitch CMOS D/A Converter”, Proc. Custom Integrated Circuits Conference, 1998, pp. 249-252.
Another proposed possibility is to use track-and-hold circuitry in the output, as disclosed in A. R. Bugeja, B. -S. Song, P. L. Rakers, S. F. Gilling: “A 14-bit 100 MSamples/s CMOS DAC Designed for Spectral Performance”, Proc. IEEE Int. Solid-State Circuits Conference, February 1999, pp. 148-149 and A. Bugeja, B. -S. Song: “A Self-Trimming
14
b
100 MSample/s CMOS DAC”, Proc. IEEE Solid-State Circuits Conference, February 2000, pp. 44-45. The settled signal from the output is tracked so that the state change phase cannot be seen in the output signal. The usage of a track-and-hold circuit in the output increases the complexity of the D/A converter and the current consumption and is not very practical in mobile terminal units. In addition the speed of the system is limited by the sampling circuitry.
Finally, a few published solutions, like U.S. Pat. No. 6,031,477 and D. A. Mercer, L. Singer: “12-bit 125MSPS CMOS D/A Designed For Spectral Performance”, International Symposium on Low Power Electronics and Design, 1996, pp. 243-246, focus on improving the timing accuracy in the single current switch circuits. With these methods, however, there remains some finite signal frequency dependent distortion.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce distortion in the output of a segmented current steering D/A converter.
The object is reached on the one hand by a digital-to-analog converter comprising a first and a second current output, at least two current sources, the currents of the current sources being summed to form an analog output signal, and assigned to each of the current sources a current switch circuit for connecting the respective current source to the first current output if the current source is selected according to a digital input signal and for connecting the respective current source to the second current output if the current source is not selected according to the digital input signal, each current switch circuit comprising means for creating two overlapping complementary control signals out of a signal indicating whether the current source is selected, while in a first group of the current switch circuits the connection of the respective current source to the first current output is controlled by the first one of the overlapping control signals and the connection of the current source to the second current output is controlled by the second one of the overlapping control signals, and while in a second group of the current switch circuits the connection of the respective current source to the first current output is controlled by the second one of the overlapping control signals and the connection of the current source to the second current output is controlled by the first one of the overlapping control signals, each of the current switch circuits of the second group comprising in addition means for inverting the signal input to the means for creating two overlapping complementary control signals.
On the other hand, the object is reached by a method for reducing harmonic distortion in a digital-to-analog converter comprising a first and a second current output, at least two current sources, the currents of the current sources being summed up to form an analog output signal, and assigned to each of the current sources a current switch circuit for connecting the respective current source to the first current output if the current source is selected according to a digital input signal and for connecting the respective current source to the second current output if the current source is not selected according to the digital input signal, the method comprising
creating for each current source of a first group of current sources two overlapping control signals based on a signal indicat
Cohen & Pontani, Lieberman & Pavane
Jean-Pierre Peguy
Lauture Joseph
Nokia Corporation
LandOfFree
Digital-to-analog converter and method for reducing harmonic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Digital-to-analog converter and method for reducing harmonic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Digital-to-analog converter and method for reducing harmonic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3272249