Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-10-05
2002-09-03
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S161000, C341S143000, C341S150000, C341S163000, C341S155000, C341S120000, C341S118000
Reexamination Certificate
active
06445324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for dynamic matching of the elements of an integrated multibit digital-to-analog converter with balanced output for audio applications.
2. Description of the Related Art
As is known, multibit digital-to-analog conversion for audio applications is performed by generating the output analog signal as the sum, at each sampling instant, of a given number of elementary quantities or contributions, which may be, for example, currents supplied by current generators or generated by means of resistors, or charges stored in capacitors.
It is also known that digital-to-analog conversion can be roughly divided into two major categories according to the approach adopted in the conversion. Belonging to the first category are those digital-to-analog conversions performed adopting an approach known in the literature as “thermometric coding”, whereas belonging to the second category are those digital-to-analog conversions performed adopting an approach known in the literature as “binary coding”.
In particular, in thermometric coding the elementary contributions used for generating the output analog signal assume values identical to one another and are generated by distinct generator elements numbering N, where N represents the number of levels of the output analog signal. In addition, in order to obtain a balanced output analog signal, i.e., an output signal of mean value zero able to assume either positive values or negative values that are symmetrical with respect to zero, one half of the generator elements supplies a positive elementary contribution and one half of the generator elements supplies a negative elementary contribution, and the value of each elementary contribution is 2A
MAX
N, where A
MAX
represents the maximum amplitude, either positive or negative, which it is desired that the output analog signal should assume.
In binary coding, instead, the number of distinct generator elements to be implemented for generating the elementary contributions is equal to n, where n represents the number of bits of the digital-to-analog converter and is equal to n=log
2
N, and the dimensions of said generator elements are appropriately scaled in such a way that the elementary contributions generated thereby are submultiples of a power of 2 with respect to the maximum value A
MAX
, in which the least significant bit (LSB) has a weight of 2A
MAX
/N, whilst the most significant bit (MSB) has a weight of A
MAX
.
It may be readily appreciated how the element generating the MSB has a larger area than the element generating the LSB, and hence, in terms of area occupied, binary coding does not differ much from thermometric coding on account of the increase in the size of the generating elements due to the decrease in their number.
In addition, the heavy bearing that the errors on the most significant bits have in binary coding and the relatively high complexity of implementation, in said binary coding, of so-called “scrambling” techniques, i.e., ones through which the generator elements to be activated are appropriately chosen each time from the set of the ones available with the purpose of rendering the conversion error not correlated to the signal to be converted, have recently given a strong impulse to the development of thermometric coding.
At the highest level of abstraction, a digital-to-analog converter, hereinafter designated for reasons of convenience by the term “DAC”, can be represented with the block diagram shown in
FIG. 1
, where s[n] designates the numerical sequence that is obtained from the sampling of the input audio signal, said sequence being then processed by the blocks downstream until the reconstruction s(t) of the signal is obtained at output from the power stage.
In particular, in
FIG. 1
the part of numerical processing of the DAC, designated by
1
, is formed by an interpolator
2
and a noise shaper
4
, and has the purpose of reducing the number of bits with which the signal is encoded, without however worsening the quality thereof in terms of in-band noise level.
In detail, in order to maintain the high audio fidelity, the interpolator
2
performs an oversampling of the input signal s[n]; i.e., it increases the sampling frequency by a factor R, and this technique makes it possible to reduce the intensity in frequency of all spurious spectral components, at the same time eliminating undesired spectral repetitions.
The interpolation is followed by the noise-shaping operation performed by the noise shaper
4
. The said operation consists in differentiating the signal transfer function and the requantization error transfer function: whilst the input signal is transferred from the input to the output unaltered, the requantization error “sees” a transfer typical of a high-pass filtering having a modulus smaller than one within the audio band and greater than one outside. The joint effect of these two operations is that of obtaining a decidedly high precision even in the presence of a quantizer with a very limited number of bits, even just one.
The numerical processing part is then followed by a block
6
which performs the actual analog-to-digital conversion and by an amplification and filtering block
8
. The block
6
that performs the actual analog-to-digital conversion represents the interface between the two domains, the digital one and the analog one, and contained inside it are, as shown in
FIG. 2
, a thermometric encoder
10
and the set of the generator elements
12
, which are known in the literature also as “unitary elements”.
It is moreover known that, in the context of thermometric coding, it is not possible to integrate N generator elements that are perfectly identical to each other, and this entails the need to arrange an additional block downstream of the noise shaper, the said additional block having the purpose of offsetting the effects of the mismatch between the components, represented particularly by a non-linearity of the transfer characteristic and by the consequent distortion of the signal.
The complete block diagram of a DAC with mismatch compensation thus becomes the one shown in
FIG. 3
, in which the compensation block is designated by
14
.
In the case, for instance, in which the generator elements are constituted by current generators formed by MOS transistors, the latter, although designed identical, have different dimensions and characteristics owing to the limited accuracy that characterizes any technological integration process; the causes can be diffusion imprecisions, irregularities in the masks used in the lithographic step, undesirable variations in the thicknesses of the oxide or metal layers, etc.
In addition, one of the technological processes most widely used for integration of current generators, known as Bipolar CMOS DMOS
5
(BCD
5
)—the most recent one among the processes able to integrate logic, linear and power circuits on the same chip—is not a technological process devised exclusively for the integration of MOS signal circuits (as may instead be the technological process known as HCMOS), and thus from said technological process very low tolerance values cannot be expected on the dimensions of the MOS transistors forming the current generators. The mismatches that take place are in fact reflected in a substantially proportional way on the values of the currents supplied by the generators on account of the linearity of the link between the drain current of a MOS transistor and its W/L shape ratio. The error that modifies the behavior of the output with respect to the desired one is strongly correlated to the signal, and consequently worsens the quality of the signal above all in terms of harmonic distortion.
Each of the generator elements supplies an elementary contribution different from the expected nominal contribution, and the error thus generated is transferred unaltered onto the output analog signal in so far as it is the sum of the various elementary contributions.
Given the extent of the latter (bu
Baschirotto Andrea
Botti Edoardo
Ghioni Massimo
Meroni Cristiano
Mai Lam T.
SEED IP Law Group PLLC
Tarleton E. Russell
Tokar Michael
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