Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
  • H03M1/0617—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence
  • H03M1/0675—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence using redundancy
  • H03M1/069—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence using redundancy by range overlap between successive stages or steps
  • H03M1/0692—Continuously compensating for, or preventing, undesired influence of physical parameters characterised by the use of methods or means not specific to a particular type of detrimental influence using redundancy by range overlap between successive stages or steps using a diminished radix representation, e.g. radix 1.95
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/10—Calibration or testing
  • H03M1/1009—Calibration
  • H03M1/1033—Calibration over the full range of the converter, e.g. for correcting differential non-linearity
  • H03M1/1038—Calibration over the full range of the converter, e.g. for correcting differential non-linearity by storing corrected or correction values in one or more digital look-up tables
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/12—Analogue/digital converters
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/66—Digital/analogue converters

Definitions

• the present invention relates to A/D and D/A converters and converter systems and their calibra ⁇ tion methods.
• Digital-to-analog (D/A) and analog-to-digital (A/D) converters are used generally in electronics, especially in computer interfaces, to convert an analog signal into digital form, and vice versa.
• D/A Digital-to-analog
• A/D analog-to-digital converters
• U is the value of an analog signal
• E is a reference voltage used in the converter
• B- ⁇ ... B n are binary values (0,1)
• C is a constant
• n is the number of the bits/weights of the converter.
• the binary weights 2 ⁇ k of the converters are usually realized by weight or reference elements forming a so-called weighing network.
• the weighing network and the weight elements are usually realized by means of resistors, capacitors and transistors.
• the accuracy of the converter there ⁇ by depends on how accurately the relationship between the values of the weight elements is the right one. This involves very high requirements for the converter manufacturing technique, and in many cases converters have to be tuned after manufacture.
• the accuracy limit of converters is usually about 12 bits. Converters having a greater accuracy than that have to be calibrated or tuned intermittently during use. Converters usually have to be calibrated by adjusting the values of the weight elements and the reference voltages.
• the object of the present invention is to provide a novel A/D or D/A converter which is more accurate, easier to calibrate and less demanding in view of the manufacturing technique than previously.
• the invention is based on the Applicant's discovery that when a number system having a base number smaller than 2 is used in the converter in place of the binary system, the obtained converter comprises number ranges in which the same number is formed by different combinations of bits. Consequent ⁇ ly, deviation can be allowed for the weights without the risk of gaps remaining the number range of the converter.
• the actual accurate value of the weight can be determined by calculation or programmatically with a certain self-calibration procedure and each conversion result can be corrected on the basis of these actual weights. For example, the offset and noise of the comparators of a weighing A/D converter do not cause an error in the conversion, as the same voltage value can be represented at the threshold value points of the comparators with two different bit series in the number system according to the invention. In a normal binary system, inaccuracy in the weights causes a gap in the number range of the converter, and the error caused by the gap is impossible to correct.
• the weights and other parameters of the converter can be allowed to deviate fairly largely within certain limits.
• the structure of the converter is simple, requiring no greater accuracy from the manufacturing technique, which, in turn, reduces costs.
• the speed of the A/D converter can be increased as the sampling frequency is constant and thus the amplifier need not reach its final state before conversion, because such settling time errors will be compensated for in calibration.
• Converters can be calibrated (the weight values can be measured) by software at desired moments.
• the invention is also concerned with a method according to claim 9 for calibrating the A/D converter of the invention.
• the invention further relates to an A/D and D/A system according to claim 12, comprising an A/D and
• the invention is also concerned with a method of calibrating a converter system according to claim
• the input voltage U of the A/D converter or the out ⁇ put voltage of the D/A converter fulfills the follow- ing equation:
• X- j _ ... x n are the weight values of the converter
• B- ⁇ ... B n are bits having a value 0 or 1
• C is a constant containing the offset values of the converter.
• Equation (4) sets a condition that the sum of weight values preceding each weight has to be greater than the value of this particular weight.
• this condition is fulfilled by using, in place of the binary number system with the base number 2, another number system in which the base number is smaller than 2, whereby
• Equation (2) Equation (2)
• the system further comprises a D/A converter 2 (DAC) utilizing a (2- ⁇ 2 ) system.
• the numbers ⁇ and ⁇ 2 are different, as a result of which the base numbers of the number systems of the converters 1 and 2 are different, as calibration by a method 3 to be described later could not be carried out if the base numbers would be equal.
• the base number ( 2- ⁇ _- ) of the converter 1 and the base number (2- ⁇ 2 ) of the converter 2 may vary within the respective converter from one weight to another within a predetermined tolerance area, but the mean values of the base numbers of the two converters have to differ from each other.
• the converters 1 and 2 are connected to a control unit 3, preferably a computer, controlling them through data buses 5 and 6 and a common control bus 7.
• data to be transferred over the data bus 5 is in the number system (2- ⁇ --_)
• data to be transferred over the data bus 6 is in the number system (2- ⁇ 2 ).
• the control unit 3 is also connected to external systems through a control bus 9 and a binary data bus 8.
• An analog input 12 in the A/D converter 1 is connectable by a change-over switch SI to an analog input voltage U IN , to a reference voltage E corresponding to the largest value of the number range of the converter, to another reference value (e.g.
• the analog output 13 of the D/A converter 2 is connectable by means of a change-over switch S2 to an analog output U Q U T of the converter system or to the analog input 12 of the A/D converter 1.
• the control unit 3 controls the operation of the switches SI and S2 and the voltage generator 4, as is illustrated by broken lines 10 and 11.
• the converter system of the invention calibrates itself automatically by means of a soft ⁇ ware. In normal use, the analog input of the A/D converter 1 is connected to the analog input U IN of the system and correspondingly the analog output 13 of the D/A converter 2 is connected to the analog output U ou ⁇ of the system.
• the A/D converter 1 performs an analog-to-digital conversion and applies the digital numbers in the (2- ⁇ - ⁇ ) system over the data bus 5 to the control unit 3, which corrects the conversion results by means of calibration parameters calculated in accordance with a predetermined calibration procedure and converts the corrected results into the binary system.
• the control unit 3 corrects control words to the D/A con- verter 2 by means of calibration parameters calculated for this converter and applies the control words over the data bus 6 to the converter 2.
• the inaccurate values of the weights of the converters 1 and 2 are accurately known, and each conversion result from the A/D converter 1 is corrected automatically and cor ⁇ respondingly each control word to the D/A converter 2 is corrected in such a way that an analog voltage obtained as a result of the conversion is accurately correct.
• control unit 3 may contain a search-table type memory to which a corrected value corresponding to the bit pattern supplied by the A/D converter 1 has been stored at the calibration stage.
• control unit 3 may contain an ⁇ other search table in which a corrected control word to be fed to the D/A converter 2 is stored for each control word intended for the converter 2. These search tables may also perform conversions into the binary system and vice versa.
• the operations of the control unit 3 may also be partly or wholly integrated in the converters 1 and 2.
• C- ⁇ is a constant containing the offset voltages of the converter 1.
• the A/D converter 1 may be calibrated in several ways by means of the reference voltages E and 0 V and the voltage generator 4 and the D/A converter 2.
• the weights X k of the A/D converter 1 can be measured by using the calibration generator 4 having an output voltage U f varying with time according to a known function, which can also be measured.
• the function Uf is
• the function U f may be e.g.
• the input of the A/D converter 1 is first connected to the reference voltages E and 0 V and then to the output 13 of the D/A converter 2.
• the control unit 3 applies several numbers to the converter 2, which numbers are subjected to D/A con ⁇ version by the converter 2 and then to A/D conversion by the converter 1.
• the obtained measuring results are stored.
• the numbers to be applied to the converter 2 may be predetermined or alternately generated at random.
• Each conversion to be carried out by the converters 1 and 2 thus requires that the control unit 3 performs a calculation according to Equation (6) or (13).
• the load of the control unit 3 e.g. a computer
• the control unit 3 thereby still performs the calibra ⁇ tion, but stores the calculated calibration parameters in the correcting memories of the converters.
• differences between the assumed and actual values of the weights of the converter can be determined by calibration, and these differences are used as calibration parameters when correcting the conversion results.
• the calibration methods 2 and 3 are computationally very heavy, they are basic calibrations to be performed relatively seldom.
• the more rapid offsets caused e.g. by drifting or temperature variation and having a similar effect on all weights of the converter can be calibrated more often with rapid (about 1,000 times more rapid) and computa ⁇ tionally light intermediate calibration methods.
• the analog input 12 of the A/D converter 1 is connected in succession to the above-mentioned reference voltages E and 0V.
• the offset correction required for the weights is denoted with the calibra- tion parameter Z, the following equation group is obtained on the basis of measurements

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• Engineering & Computer Science (AREA)
• Theoretical Computer Science (AREA)
• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Analogue/Digital Conversion (AREA)

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Citations (1)

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• 1990
  • 1990-07-02 FI FI903334A patent/FI86120C/fi active IP Right Grant
• 1991
  • 1991-07-02 EP EP91912609A patent/EP0553090B1/en not_active Expired - Lifetime
  • 1991-07-02 US US07/965,405 patent/US5331321A/en not_active Expired - Lifetime
  • 1991-07-02 DE DE69122404T patent/DE69122404T2/de not_active Expired - Fee Related
  • 1991-07-02 AU AU81828/91A patent/AU8182891A/en not_active Abandoned
  • 1991-07-02 WO PCT/FI1991/000209 patent/WO1992000630A1/en active IP Right Grant

Patent Citations (1)

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