RU2291559C1 - Method for integrating analog-digital transformation of voltage - Google Patents

Abstract

FIELD: informational and measuring equipment, in particular - methods for measuring electric voltage.

SUBSTANCE: method for integrating analog-digital transformation of voltage is based on integration of difference of input voltage and intermediate signal, received by means of impulse modulation of integral of aforementioned difference, and adding of intermediate signals during adjacent transformation cycles, special because digital equivalent of output value of integrator in time moment, appropriate for boundary between two cycles of transformation, multiplied by constant coefficient, is subtracted from result of transformation received in previous cycle, and added to result, received in current cycle, while aforementioned constant coefficient is selected from condition for compensating error from edge effects.

EFFECT: increased precision of transformation of voltage to code due to decreased component of methodic error from edge effects; creation of digital measuring tools and devices for transformation of analog signal to code, providing for measuring of constant and alternating voltages with lesser error and simplification of circuit realization compared to existing methods.

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Description

Currently, the most high-precision ADC voltage are based on the principle of intermediate compensation integrating the conversion of voltage into a signal of one of the types of pulse modulation - PWM, PFM, PIM, IRM (pulse-difference modulation), CMM (pulse-code modulation). The intermediate signal is summed during adjacent conversion cycles, the duration of which is many times longer than the period of pulse modulation. The longer the conversion time, the potentially higher resolution can be achieved. However, there are serious limitations along this path associated with a number of methodological and instrumental errors, of which one of the main ones is the so-called error from edge effects.

The highest accuracy of integrating ADCs (IATsP) is reached when using IRM. A number of foreign companies, including ANALOG DEVICES, BURR-BROWN, INTERSIL, TEXAS INSTRUENTS and others, have mastered the mass production in integrated performance of IACP with a resolution of 8 to 24 binary bits in integrated performance [1, 2]. In foreign literature, such IACs are commonly called ∑Δ-ADCs (in some sources, Δ∑-ADCs).

The present invention is aimed at eliminating the error of the IACP from edge effects. Therefore, we consider the nature of this error in two examples.

In Fig.1, a generalized functional diagram of the IACC is presented, in which any type of pulse modulation can be implemented [3]. The structure of the circuit includes the following nodes: 1 - shaper of the weight function g _o (t), 2 - multiplier of the reference voltage U _o by the weight function g _o (t), 3 - multiplier of the converted voltage U _x by the weight function g _x (t), 4 - shaper of the weight function g _x (t), 5 - adder, 6 - integrator, 7 - comparison device, 8 - threshold level shaper, 9 - control device, 10 - digital integrator (pulse counter), 11 - clock generator. The processes occurring in the IACC are illustrated by the time chart in figure 1, b. The specific type of diagram depends on the type of pulse modulation implemented in the IACP. In this case, phase-modulation is implemented. The threshold level driver 8 changes the polarity of the threshold voltage (in the diagram, the corresponding waveform is denoted as 8 ') whenever the output voltage of the integrator 6 (in the diagram it is denoted as 6') reaches the threshold level. However, this does not happen immediately after the operation of the comparison device 7, but at the time of arrival of the first clock pulse after the operation. These points are marked on the diagram by vertical lines, depicted as dots. The weight function g _o (t), simultaneously with the change in the threshold level, changes the sign of its value (modulo it is equal to unity), as a result of which the polarity of the reference voltage supplied to the input of adder 5 from the output of multiplier 2 changes.

The transformation equation can be represented as follows:

where u _x (t) is the converted voltage; U _o - reference (model) voltage; τ _o - time constant of the integrator 6; g _x (t) and g _o (t) are weight functions; t _n and t _to - the moments of the beginning and end of the integration interval (conversion time); I (t _n ) and I (t _k ) are the values of the output value of the integrator at the beginning and end of the conversion time of the IACP.

As shown in [3], the weight function g _x (t) determines the dynamic properties of the IACS, since in some cases (when it is even or odd-symmetric with respect to the integration interval) it is completely equivalent to the pulse transition function, and in all other cases (g _x (t) is asymmetric) the dynamic properties of the IAPC are determined by the pulse transition function, which is connected with the weight function by a simple relation - it is mirror symmetric with respect to the weight function (in mathematics, such functions are called enantiamorphic). For simplicity, in the algorithm under consideration, the weight function g _x (t) has a constant value equal to 1, as a result of which the IARC has an amplitude-frequency characteristic of the form | Sinω (t _to -t _n ) / ω (t _to -t _n ) | [3], where ω is the frequency of the input action. As you know, this frequency response has zeros at frequencies that are multiples of 1 / (t _to -t _n ), which provides suppression of interference with frequencies equal to and multiples of this frequency.

:The presence on the right side of equation (1) of the difference I (t _k ) -I (t _n ) is the source of the error, which is usually called the error from the edge effects. At the output of the digital integrator 10, a conversion result is generated, expressed by the following relation, obtained by resolving equation (1) with respect to the output quantity

:

where (τ _o / U _о ) I (t _к ), (τ _o / U _о ) I (t _н ) is the absolute value of the error from the edge effects.

An error from the edge effects is inevitable in the implementation of any known algorithms for integrating a sweep transform with an intermediate transform into a pulse modulation signal. For example, there are algorithms for converting voltage into a PWM signal, which in statics provide a value of ΔI = 0, but in dynamics ΔI ≠ 0.

It was indicated above that ∑Δ-ADCs have the highest conversion accuracy today. This is achieved thanks to the measures adopted in them to reduce the error from edge effects. Consider one example of ∑Δ-ADC.

Figure 2 presents the functional diagram of the simplest variety of ∑Δ-ADC [1]. The circuit includes an adder 1, integrator 2, a device 3 for comparing the output voltage of integrator 2 with a zero level, a clock trigger 4, a digital filter 5, at the output of which a conversion result is generated, and a switch 6 for the polarity of the reference voltage U _o . The conversion algorithm is illustrated by the time chart in figure 2, b. Whenever the integrator output voltage crosses the zero level, the polarity of the reference voltage is switched to the first clock time after the comparison device is triggered. These points in the diagram are shown by dotted vertical lines. For a better understanding of the operation of the IACC, the diagram of FIG. 2, b displays the processes in the IACC for the case of changing the polarity of the input voltage, this happens at the time t _{+ -} (waveform 7). As in the previously considered algorithm, the difference ΔI = I (t _k ) -I (t _n ) is nonzero, which is a source of error from edge effects. This error is reduced (almost almost eliminated) due to the use of digital filtering at the stage of obtaining the digital equivalent of the output value of the IACP. The presence of a digital filter significantly complicates the circuit implementation of IACS, although at the current level of technology of integrated circuits this drawback is not considered very significant (a digital filter is implemented using PLM). Nevertheless, any simplification of the circuit increases its reliability. Therefore, regardless of the level of technology development, simpler technical solutions will always be preferable.

The present invention is aimed at eliminating errors from edge effects using a method that simplifies the algorithm and a circuit implementation of IACP. This is achieved due to the fact that in the process of integrating analog-to-digital voltage conversion, based on integrating the difference between the input voltage and the intermediate signal obtained by pulse modulation of the integral from the specified difference, and summing the intermediate signals during adjacent conversion cycles, the digital equivalent of the integrator output value at the point in time corresponding to the boundary between two transformation cycles, subtract from the result of the transformation obtained in the previous uyuschem cycle, and added to the result obtained in the current cycle.

To clarify the essence of the method, we rewrite formula (2), which is the result of the conversion of the IACP, taking into account the error from the edge effects:

(для этого надо получить цифровой или аналоговый эквивалент выходной величины интегратора I(t_н) на границе между (n-1)-м и n-м циклами и умножить результат на постоянный коэффициент

). Прибавим эту поправку к результату n-го цикла. Тогда результат преобразования в n-м цикле будет выражаться следующим соотношением:Consider three consecutive conversion cycles: the (n-1) th, n-th and (n + 1) -th. At the boundary between the (n-1) th and nth cycles of the IACP conversion, the output value of the integrator for the (n-1) th cycle is the value of I (t _k ), and for the nth (let's call it current), the value I (t _n ). Let us get the correction value equal to

(for this it is necessary to obtain the digital or analogue equivalent of the output value of the integrator I (t _n ) at the boundary between the (n-1) th and nth cycles and multiply the result by a constant coefficient

) Add this correction to the result of the nth cycle. Then the result of the transformation in the nth cycle will be expressed by the following relation:

и вычтем из результата, полученного в n-м цикле. Получим результат преобразования в n-м циклеLet us further obtain the digital equivalent of the output value of the integrator I (t _k ) on the boundary between the nth and (n + 1) -th cycles. Multiply it by a constant coefficient

and subtract from the result obtained in the nth cycle. We get the result of the conversion in the nth cycle

free from the error generated by edge effects.

Figure 3, a presents a functional diagram of one of the possible options for implementing the proposed method, which differs from the scheme of figure 1, and the fact that it is supplemented by several additional blocks. The scheme of figure 3, a includes: 1 - the shaper of the weight function g _o (t), 2 - the multiplier of the reference voltage U _about the weight function g _o (t), 3 - the multiplier of the converted voltage U _x the weight function g _x (t) 4 - shaper of the weight function g _x (t), 5 - shaper of the weight function of the correction channel, 6 - first adder, 7 - second adder, 8 - key, 9 - third adder, 10 - main integrator, 11 - additional integrator, 12 - comparison device, 13 - threshold level driver, 14 - additional comparison device, 15 - control device, 16 - qi rovoy integrator (counter pulses), 17 - clock.

Figure 3, b and c shows fragments of the time diagram of the processes occurring in the considered IACP at the end of each complete conversion cycle. The thick vertical line marks the boundary between two adjacent complete conversion cycles. In the diagram, I (t) is the output signal of the main integrator And, u _n (t) is the signal generated at the output of the FPU and sets the threshold level for the CSS comparison device.

Реализуется это следующим образом. На интервале времени от конца полного цикла преобразования до момента, когда выходное напряжение основного интегратора достигает положительного порогового уровня, на дополнительный интегратор 11 (И_д) через ключ 8 и третий сумматор 9 подается тот же сигнал, что и на вход основного интегратора (ключ 8 открывается устройством управления 15 в течение интервала {t_к, t₁}). После этого на вход дополнительного интегратора 11 с выхода формирователя 5 подается через третий сумматор 9 произведение g_кU_o, где g_к - корректирующая весовая функция (проще говоря - постоянный коэффициент). Найдем уравнение преобразования, выполняемого дополнительным каналом преобразования, включающим блоки 5, 11 и 14:According to the proposed method for the correction of errors from edge effects, it is necessary to subtract the correction value from the result of the transformation (3)

It is implemented as follows. In the time interval from the end of a complete conversion cycle until the output voltage of the principal integrator reaches the positive threshold level, a further integrator 11 (and _d) through the switch 8 and a third adder 9 is applied the same signal as the input to principal integrator (key 8 opens by the control device 15 during the interval {t _to , t ₁ }). After that, the input of the additional integrator 11 from the output of the shaper 5 is fed through the third adder 9, the product g _to U _o , where g _to is the correcting weight function (in other words, a constant coefficient). Find the equation of the conversion performed by an additional conversion channel, including blocks 5, 11 and 14:

where τ _k is the time constant of the additional integrator.

Since on the interval {t ₁ , t ₂ } we have g _x = 0 and g _o (t) = 0 (key 8 is open only on the interval {t _to , t ₁ }, then equation (6) can be rewritten in the form

If the relation τ _k = τ _{о is} satisfied, then, obviously, the first two terms express the increment of the output voltage of the main integrator over the interval {t _k , t ₁ }, i.e. the value of I (t _to ). With this in mind, we will solve equation (7) regarding the value of the informative interval ΔT = t ₂ -t ₁

The right side of the expression (5) contains the value I (t _k ), which allows us to use the ΔT value as a correction, which is introduced into the result (2) in order to eliminate the error from the edge effects. With the introduction of the amendment, expression (3) takes the form

погрешности от краевых эффектов будет отсутствовать при выполнении условияFrom the expression (9) it follows that the component

errors from edge effects will be absent when the condition

When this condition is satisfied, expression (9) takes the form

, но с противоположным знаком, то результат будет полностью свободным от погрешности, порождаемой краевыми эффектами. В схеме фиг.3, а это осуществляется следующим образом. В течение интервала ΔT через второй сумматор 7 на вход основного интегратора 10 подается выходная величина g_кU_o формирователя 5, в результате чего выходная величина основного интегратора 10 получит приращение, равное g_кU_oΔT, т.е. выражение (11) можно переписать в видеIf at the beginning of the cycle to which the result (11) relates, introduce a correction equal to the component

, but with the opposite sign, the result will be completely free from the error generated by the edge effects. In the diagram of figure 3, and this is as follows. During the interval ΔT, through the second adder 7, the output value g _to U _{o of the} former 5 is supplied to the input of the main integrator 10, as a result of which the output value of the main integrator 10 will receive an increment equal to g _to U _o ΔT, i.e. expression (11) can be rewritten as

We substitute expression (8) for ΔT in (12), but keep in mind that for the current cycle, the quantity I (t _к ) is the initial value of the output value of the main integrator, therefore, instead of I (t _к ) in formula (8) it should. figure the value of I (t _n ). As a result, we get

It was previously indicated that to compensate for the first component from edge effects, it was necessary to fulfill the condition τ _k = τ _o . Given this condition, expression (13) takes the form

Thus, in the considered embodiment of the circuit implementation, the error from the edge effects is completely (except for the sampling discreteness) eliminated. Note that the first component of the error from the edge effects is corrected by entering the correction in digital form, and the second component by entering the correction in analog form.

Literature

1. Nikamin V.A. Analog-to-digital and digital-to-analog converters. Directory. M .: Altex-A, 2003 - 224 p.

2. Gubner G.B., Gutnikov B.C. The use of Δ∑ modulation in measuring devices. Sat Proceedings: Microprocessor-based measuring instruments. St. Petersburg. 1998, - p. 3-14.

3. Shakhov E.K., Mikhotin V.D. Integrating Deployment Converters. M .: Energoatomizdat. 1986 - 144 p.

Claims (1)

A method of integrating analog-to-digital voltage conversion, based on integrating the difference between the input voltage and the intermediate signal obtained by pulse modulation of the integral from the specified difference, and summing the intermediate signals during adjacent conversion cycles, characterized in that the digital equivalent of the output value of the integrator at a time the corresponding boundary between two conversion cycles, multiplied by a constant coefficient, is subtracted from the conversion result, obtained in the previous cycle, and added to the result obtained in the current cycle, and the said constant coefficient is selected from the condition of compensation for errors from edge effects.

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