Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-01-14
2001-01-23
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S144000, C341S150000
Reexamination Certificate
active
06177896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital/analog converter, and more particularly, to an oversampling digital/analog converter using an Interpolated Finite Impulse Response (IFIR) filter.
2. Background of the Related Art
A related art oversampling digital/analog converter will now be described.
FIG. 1
illustrates a system block diagram of the related art digital/analog converter (DAC). A sigma delta DAC is generally used as a digital/analog converter.
Referring to
FIG. 1
, related art the sigma delta digital/analog converter includes an interpolation filter
11
, a digital noise shaper
12
and an FIR reconstruction filter
13
. The interpolation filter
11
receives a multibit digital word of first sampling frequency and converts it to a multibit digital word of a second sampling frequency higher than the first sampling frequency. The second sampling frequency multibit digital word is then converted into a single-bit word in the digital noise shaper
12
. The single-bit quantization used for the conversion in the digital noise shaper
12
shifts a quantization noise from a low frequency band to a high frequency band (noise shaping). The FIR reconstruction filter
13
has low pass filters, which either have a switch-capacitor or have a resistor and a capacitor. The low pass filter with the switch-capacitor, which has a non-linear phase response, is embodied as a CMOS integrated circuit. The low pass filter with the resistor and the capacitor provides a wider dynamic range, but it requires a precise matching between components for precise filtering.
FIG. 2
illustrates a related art Finite Impulse Response (FIR) type reconstruction filter. Referring to
FIG. 2
, the related art FIR type reconstruction filter includes a plurality of one bit shift registers SR
1
, SR
2
, SR
3
, . . . , SRn connected in series, a plurality of current sources CS
1
, CS
2
, CS
3
, . . . , CSn that each supply a respective current to an I-V converter part
21
or drain the respective current to ground in response to a signal from the shift register SR
1
, SR
2
, SR
3
, . . . , SRn. The I-V converter part
21
converts the selectively received current depending on respective outputs of the shift registers SR
1
, SR
2
, SR
3
, . . . , SRn to a voltage.
The operation of the related art oversampling digital/analog converter will now be described. First, the related art oversampling digital/analog converter subjects a one bit data stream to low-pass filtering using a FIR semi-digital reconstruction filter and subjects a resulting current to I-V conversion. That is, as shown in
FIGS. 1 and 2
, a noise shaped digital data provided to the FIR reconstruction filter
13
is converted into an analog signal. In other words, the noise shaped digital data stream is provided to the shift registers SR
1
, SR
2
, . . . , SRn in the FIR reconstruction filter
13
. If the shift register provides a data “0”, the current from a current source of the shift register is connected to ground. For example, if the first shift register SR
1
provides a data “1”, the second shift register SR
2
provides a data “0” and the third shift register SR
3
provides a data “1”, paths of the first and third current sources CS
1
and CS
3
are established toward the I-V converter part
21
and a path of the second current source CS
2
is established to the ground. Therefore, the currents from the first current source CS
1
and the third current source CS
3
are together provided to the I-V converter part
21
. The I-V converter part
21
converts the received current into a voltage corresponding to the digital data stream received at the shift registers. In this instance, to convert the digital data into a voltage corresponding to the digital data with more precision, more current sources are required. That is, the more orders the filter is extended, the more exact the analog output can be obtained.
In summary, as shown in
FIG. 1
, digital data is provided to the FIR reconstruction filter
13
through the interpolation filter
11
and the digital noise shaper
12
. The FIR reconstruction filter
13
subjects the digital data to low pass filtering according to a transmission function. The transmission function of the FIR reconstruction filter can be expressed as equation 1 as follows.
H
(
Z
)=
a
1
z
−1
+a
2
z
−2
+, . . . , a
n
z
−n
(1)
Therefore, when the noise shaped digital data is passed through the FIR reconstruction filter
13
, a high frequency component in the noise shaped digital data is removed, which leaves a baseband signal.
However, as described above the related art oversampling digital/analog converter has various problems. First, the high order of FIR reconstruction filter required for conversion of a signal from digital to analog results in an increase of occupied area due to the filter system. Second, the error in a filter coefficient caused by process change coming from increased order degrades a filter performance. Third, the current to the I-V converter part being at least greater than “0” at the minimum and smaller than a sum of all current sources at the maximum places a limitation on a dynamic range of the converted voltage signal.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a digital/analog converter that substantially obviates one or more of the problems caused by limitations and disadvantages of the related art.
Another object of the present invention is to provide a digital/analog converter that occupies a reduced area.
Another object of the present invention is to provide a digital/analog converter that has an increased dynamic range of a converted voltage signal.
To achieve at least these objects and other advantages in a whole or in parts and in accordance with the purpose of the present invention, as embodied and broadly described, an over sampling digital/analog converter includes an interpolation filter that receives a digital signal and oversamples the digital signal to generate a multibit digital signal; a digital noise shaper that quantizes noise in the multibit digital signal to output a noise shaped digital signal; and an Interpolated Finite Impulse Response (IFIR) reconstruction filter that converts the noise shaped digital signal to a corresponding analog signal.
To further achieve the above objects in a whole or in part, there is provided a digital/analog converter according to the present invention that includes a plurality of shift registers coupled in series that receive a noise shaped digital signal; a plurality of current sources, wherein each of the current sources is connected to an output terminal of a unit of the shift registers, wherein each unit comprises more than one shift register; and an I-V converter part that selectively receives currents from the current sources based on an output signal from a corresponding unit, wherein the I-V converter part converts the received currents to a voltage.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
REFERENCES:
patent: 5392042 (1995-02-01), Pellon
patent: 5712635 (1998-01-01), Wilson et al.
patent: 5821892 (1998-10-01), Smith
patent: 5995030 (1999-11-01), Cabler
David K. Su et al., A CMOS Oversampling D/A Converter with a Current-Mode Semidigital Reconstruction Filter, Dec. 1993, IEEE Journal of Solid State Circuits, vol. 28, No. 12, pp. 1224-1233.
David K. Su, A CMOS Oversampling D/A Converter with a Current-Mode Semidigital Reconstruction Filter Dec. 1993, IEEE vol. 28, No. 12.
Fleshner & Kim LLP
Hyundai Electronics Industries Co,. Ltd.
Nguyen John
Young Brian
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