Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2001-10-22
2003-04-01
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S102000, C341S103000, C341S159000
Reexamination Certificate
active
06542104
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates generally to analog and digital signal processing. More specifically, the invention relates to the implementation of binary encoders for use in analog-to-digital converters.
2. Related Art
In the art of mixed-mode signal processing, so-called “thermometer-to-binary” coders are used in “Flash” analog-to-digital (A/D) converters to encode the output of a set of 2
n
−1 comparators into an n-bit binary code. A typical flash A/D converter is depicted in FIG.
1
. The input (represented as a voltage signal) is compared to a set of thresholds (generated from a positive reference and negative reference by a resistor ladder or other means). The comparators generate a so-called thermometer code, in which the value of the input signal is represented by a binary code in which increasing numbers of bits are turned on, as shown in Table 1 of the Appendix. For digital signal processing applications, the more compact binary code is ordinarily used to represent the signal. The A/D converter in such cases must include an encoder to convert from one format to another. However, as shown below, conventional encoders consume a significant amount of power and die area.
FIG. 2
shows a conventional thermometer-to-binary coder. The coder
200
first converts from the binary code to a “one hot” code, which is then used to address a read-only memory to generate the binary code. The one-hot code is a code in which each symbol is represented by a vector that has one element equal to one while all of the other elements are equal to zero. Thus, in a one-hot code, there is a bit position for every symbol and the number of symbols determines the length of each codeword. While such encoding is easy to decode (since only one bit needs to be discovered), it can often add significant cost when implemented in digital circuitry.
Referring to
FIG. 2
, at each of the AND gates
210
, the thermometer code bit is combined with the inverse of the next higher bit. Thus, the bits T
0
and ~T
1
are combined by an AND logic gate from AND gates
210
to yield the one-hot code C
0
. Likewise, the one-hot code C
1
is obtained by combining the bit T
1
with the inverse of bit T
2
(~T
2
) at another parallel AND logic gate. In general, the intermediate one-hot code Ci is obtained from Ti∩~Ti+1. The one-hot code Ci is used to address the memory bank (shown implemented in MOS transistors) that then read out the corresponding binary code B
0
, B
1
, B
2
, B
3
. The number of transistors used for each one-hot code varies, as depicted in
FIG. 2. A
thermometer to binary encoder if implemented in a CMOS technology would use a total of at least 2
N−1
(N+12)+2N−6 transistors, where N is the number of bits in the resulting binary code. If implemented in bipolar technology, such an encoder would use at least 2
N−1
(N+18)+3N−9 transistors. One problem with the encoder of
FIG. 2
is its susceptibility to so-called “bubble” codes, which are invalid thermometer codes where the high signals are not contiguous. Bubble codes occur as a result of thermal noise in the comparators.
FIG. 3
illustrates another conventional thermometer-to-binary coder. This encoder is not as sensitive as that of
FIG. 2
to “bubble codes”. The response of the encoders of
FIGS. 2 and 3
, to various bubble codes is shown in Table 2 of the Appendix. The reduced sensitivity of the encoder of
FIG. 3
is evident from the fact that the encoded value is always one of the two ambiguous interpretations of the bubble code. However, the
FIG. 3
encoder is expensive in terms of the number of transistors used. It uses a minimum of 2
N−1
(N+16)+2N−6 transistors in CMOS technology, or a minimum of 2
N−1
(N+26)+3N−17 transistors in bipolar technology. It also uses a more expensive logic than the coder of
FIG. 2
in that three-input AND gates and various inverting terminals are used in the one-hot code determination.
Thus, there is a need for a thermometer to binary code converter which is less expensive than conventional converters and is also less sensitive to bubble thermometers than conventional converters.
SUMMARY
The invention consists of an improved thermometer-to-binary coder in which the bits of the thermometer code are used to directly generate the binary code without using an intermediate one-hot code. The improved coder manipulates the observable common features between the thermometer code and the resulting binary code. The thermometer code bits are grouped according these observable common features and then a selection process which operates in layers, selects thermometer code bits that can be utilized as binary code bits.
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Nguyen Khai
Pillsbury & Winthrop LLP
Santel Networks, Inc.
Tokar Michael
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