Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1998-06-17
2001-03-06
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S118000
Reexamination Certificate
active
06198419
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital-to-analog converters and, more particularly, to a digital-to-analog converter with an extended spurious free dynamic range (SFDR).
2. Description of Related Art
Many physical devices generate output signals which are analog or continuously varying. Today, signal processing is often accomplished using digital methods, and the processed signal is often converted back to analog form. Many types of converters exist which act as interfaces between analog devices and digital systems. These converters are used in a variety of applications, including testing, measurement, process control, and communications. Digital-to-analog converters (DAC) produce an analog output from a given digital input. The spurious free dynamic range (SFDR) for the DAC is often measured as the difference in decibels (dB) between the amplitude of the spurious distortion, such as intermodulation products, and the amplitude of the highest output signal. The SFDR performance of a DAC varies as a function of the amplitude of the digital signal to be converted. Usually, the greater the amplitude of the digital signal, the smaller the SFDR because as the amplitude of the digital signal increases, the amplitude of the analog output signal increases, but the amplitude of the resulting spurious distortion generally increases more than the amplitude of the output signal. Depending on the application, extending the SFDR of the DAC can enable the DAC to convert a greater range of digital signals to analog signals while maintaining or reducing the level of spurious distortion or to convert a given range of digital signals to analog signals while reducing the level of spurious distortion. For example, for discussion purposes, a 16 bit DAC could have a spurious free dynamic range of 90 dB measured by a high output level of 4 dBm and the spurious distortion level at −86 dBm. By increasing the relative difference between the amplitudes of the worst case spurious distortion and the high output signal, the SFDR of the DAC can be extended.
SUMMARY OF THE INVENTION
The present invention involves a DAC system which provides an expanded SFDR when compared to the SFDR of an individual DAC(s) that makes up the DAC system. The DAC system uses a clipping arrangement which receives the digital input signal and provides an amplitude-limited signal and resulting signal distortion to a first DAC for digital input signals whose amplitudes would result in an insufficient SFDR for the first DAC. By amplitude-limiting the digital input signal, the amplitude of the digital input signal is effectively reduced or “clipped,” thereby improving the SFDR performance for the first DAC. The signal distortion resulting from the amplitude adjustment is routed to a second DAC. The first DAC converts the amplitude-limited digital signal to an analog signal with an expanded SFDR due to the lower amplitude of the adjusted digital signal. The second DAC receives the signal distortion which can be viewed as the clipped amplitude portion of the digital input signal. The second DAC converts the signal distortion, which typically has a lower amplitude than the adjusted digital signal, to an analog signal with at least an acceptable SFDR. The DAC system combines the resulting analog signals from the first and second DACs to produce the desired converted analog signal with an increased relative difference between the amplitudes of the output signal and the spurious distortion, thereby providing an extended SFDR for the DAC system when compared to the SFDR if the digital input signal had been converted by the first DAC without amplitude adjustment.
In certain embodiments, the DAC system receives a digital input signal of N bits. The clipping arrangement involves routing an amplitude-limited portion of the digital input signal comprising the n
2
most significant bits (MSBs) of the digital input signal to the first DAC for converting to analog form. The DAC system routes the clipped amplitude portion of the digital input signal comprising n
1
bits to a second DAC for converting to analog form. The number n
1
of least significant bits (LSBs) which are routed to the second DAC from the total number of N bits of the digital input signal reflects the amount of amplitude “clipping” of the digital input signal. The more LSBs which are routed to the second DAC, the lower the amplitude of the amplitude-limited portion of the digital input signal which is routed to the first DAC. By providing a lower amplitude digital signal with signal distortion to the first DAC, the SFDR of the first DAC is expanded. The SFDR of the second DAC is generally not a problem because the amplitude of the clipped portion or signal distortion portion of the digital input signal can be designed to produce an acceptable spurious distortion level from the second DAC. The DAC system then combines the analog outputs from the first and second DACs to produce the desired converted signal with a reduced spurious distortion level and therefore an expanded SFDR.
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Jean-Pierre Peguy
Lucent Technologies - Inc.
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