RU2420867C2 - Method for digital-to-analogue conversion - Google Patents

Abstract

FIELD: information technology.

SUBSTANCE: in the method for digital-to-analogue conversion, comprising reception of a pulsed PWM signal whose period is determined by the length of the converted code and frequency of the main oscillator, and the pulse ratio is inversely proportional to the converted code, normalisation of the amplitude of the obtained signal and subsequent filtration thereof in the low-frequency region, before filtration, the PWM signal is further amplitude modulated by an amplitude-normalised square modulated signal at the frequency of the main oscillator, wherein in one of the half-periods of the modulated signal on the information interval of conversion, the previous value of intermediate conversion is recorded.

EFFECT: shorter conversion time while maintaining accuracy.

3 dwg

Description

FIELD OF THE INVENTION

The invention relates to measuring technique, automation, and also to the technique of converting digital values into analog and can be used to create high-precision analog-to-digital converters and systems for monitoring the parameters of electronic products.

State of the art

There are various methods of digital-to-analog conversion [1], based in the first case on the summation of weighted currents or voltages obtained on the basis of code-controlled resistive matrices of various types, and in the second on the basis of frequency-to-voltage conversion (see [1], a digital-to-analog converter microcircuit ( DAC) KR1108PP1, p. 257).

The disadvantages of the methods of the first type include the technological complexity and high cost of manufacturing resistive matrices, and the second is the lack of accuracy and linearity of the conversion characteristics. It is well known that precision resistive matrices can only be manufactured using thin-film technology, including functional fitting of resistors, and not semiconductor, which determines these disadvantages. Filtering the signals of a wide frequency range with a specific low-pass filter leads to unevenness of the filtered harmonics and, as a result, to insufficient accuracy and linearity of the static conversion characteristic.

A known method of digital-to-analog conversion, implemented, for example, DAC with pulse-width modulation, constructed according to the scheme according to [2], in which the code is converted into a time interval (duty cycle) of several pulse sequences shifted relative to each other by equal time intervals, their subsequent summation and filtering obtained after summing the signal.

The disadvantage of this method is its high complexity, which is confirmed by the DACs constructed by this method [2], which contain a large number of electronic components.

A known method of digital-to-analog conversion with pulse-width modulation (PWM), shown, for example, in [3, 4] (prototype), which includes converting the code into a time interval of the amplitude normalized cyclic sequence and subsequent filtering of this sequence with an low-pass filter.

This method of digital-to-analog conversion with PWM can already be considered a classic. Its disadvantages include a large conversion time, as well as the significant non-linearity of the static conversion characteristics.

SUMMARY OF THE INVENTION

The objective of the invention is to create a method of digital-to-analog conversion, allowing to perform this conversion with a given accuracy in a shorter time.

The problem is achieved due to the fact that in the method of digital-to-analog conversion, which includes receiving a pulse PWM signal, the period of which is determined by the bit depth of the converted code and the frequency of the master oscillator, and the duty cycle is inversely proportional to the converted code, normalization of the amplitude of the received signal and its subsequent filtering in the low-frequency region additionally, before filtering, the amplitude modulation of the PWM signal is performed by the amplitude-normalized rectangular modulated signal at pilots at the master oscillator, wherein one of the half cycles of the modulated signal in the information storing interval conversion is carried out previous value intermediate conversion.

List of drawings

Figure 1 presents a diagram of a digital-to-analog converter that implements the proposed method.

The digital-to-analog converter contains a master clock 1, a two-position controlled switch 2, a code converter in a time interval 3, a bipolar reference voltage source 4, a controlled key 5, an adder 6, a low-pass filter (LPF) 7, an element And 8, a controlled key 9, capacitors 10, output bus 11.

Figure 2 presents a plot of 12-16 voltages at the outputs of the corresponding circuit elements.

Diagram (time diagram) 12 - signal at the output of the code converter in the time interval 3, when converting the binary code N = 000 ... 001, equal to one; timing diagram 13 - signal at the output of the on-off controlled switch 2; voltage plot 14: 14 ₁ - at the output of the adder 6, operating without load, in code conversion mode N = 000 ... 001; 14 ₂ - at the output of the adder 6, operating with a load, in code conversion mode N = 000 ... 001; timing diagram 15 - the signal at the output of the code converter in the time interval 3, when converting the binary code N = 000 ... 010, equal to two; voltage plot 16: 16 ₁ - at the output of the adder 6, operating without load, in code conversion mode N = 000 ... 010; 16 ₂ - at the output of the adder 6, operating with a load, in code conversion mode N = 000 ... 010.

Figure 3 presents diagrams of 17 voltages U _{N = 100 ... 000} and U _{N = 111 ... 111} , obtained after PWM conversion of the corresponding code N and explaining the operation of the low-pass filter (LPF) when a rectangular pulse-width signal is applied to its input.

Features

Distinctive features of the claimed method in comparison with the prototype method are:

1. Before filtering, the PWM signal is amplitude-modulated by rectangular pulses of the reference frequency normalized in amplitude.

2. Before filtering, the previous value is stored in one of the half-periods of the modulated signal, in the interval of the PWM pulse (conversion information interval).

Information confirming the possibility of implementation

The essence of the invention is illustrated by the drawings of figures 1 ÷ 3.

The digital-to-analog Converter, shown in figure 1, performs the conversion of the N code into an analog signal according to the proposed method, works as follows.

The code converter in the time interval 3, synchronized by the master oscillator 1, generates a PWM signal (Fig. 2, diagrams 12, 15), the duration of which is N _dec × 1 / f _t , on a time interval equal to the duration T _c of the conversion cycle, where N _ten decimal equivalent of the binary code N. Plot 12 corresponds to the duration of the PWM signal for code N = 000 ... 001, and diagram 15 - code for N = 000 ... 010.

The on-off controlled switch 2 performs the role of a modulator, generating an alternating pulse signal in the range from (-U _op ) to (+ U _op ) with a pulse frequency of the master oscillator (diagram 13), sends this signal to the first input of adder 6, to the second input of which pulse signal from the key 5, normalized in amplitude and duration (the duration corresponds to the duration of the PWM signals (figure 2, diagrams 12, 15)). At the output of the adder 6, signals are generated (plots 14 ₁ , 16 ₁ ) obtained as a result of PWM, and then amplitude modulation. These signals are processed by elements 9, 10 and 7 of the circuit of FIG. 1. The key 9 is closed in each cycle of the information part of the PWM signals for half the period: 1 / 2f _t . As a result, a voltage is generated at the input of the low-pass filter 7, which is shown in FIG. 2 for the corresponding conversion codes by diagrams 14 ₂ , 16 ₂ . As a result of averaging this signal, the low-pass filter 7 received on the output bus 11, the signal is the result of digital-to-analog conversion.

According to the prototype method, the PWM conversion can be represented by diagrams 17, FIG. 3, where the OABCD diagram corresponds to the code N = 111 ... 111, and the OAD diagram corresponds to the code N = 100 ... 000, without taking into account the linearity error. In this case, in the range of T _c, rectangular PWM signals have different durations and, consequently, different spectral components, which makes it almost impossible to accurately calculate the parameters of the optimal low-pass filter.

In the proposed method of digital-to-analog conversion, a twice-modulated signal (see diagrams 14 ₁ , 16 ₁ ) has a strictly defined discrete decomposition on the spectral plane in the entire interval T _c , for example, according to [5]:

Table 1 Component frequency 0 ω ₀ 2ω ₀ 3ω ₀ 4ω ₀ 5ω ₀ Harmonic amplitude U _op (1); 0 (2) 4U _op / π 0 4U _op / 3π 0 4U _op / 5π

In table 1, ω ₀ = 2πf _t ; at zero frequency, the value with index (1) corresponds to the information part of the signal, i.e. on the length of the PWM pulse of the diagrams 14 ₁ , 16 ₁ , and with the index (2) - non-information part; all the harmonic components of the discrete frequency spectrum, both for the information part and for the non-information one, completely coincide, which cannot be achieved using the prototype method.

If, for example, a conversion is performed according to the prototype method of the code N = 100 ... 000, provided that the point B of diagram 17, FIG. 3 is ordinate by U _op , then the decomposition into harmonic components of such a PWM signal can be represented according to Table .2.

table 2 Component frequency 0 ω ₁ 2ω ₁ 3ω ₁ 4ω ₁ 5ω ₁ Harmonic amplitude U _op / 2 2U _op / π 0 2U _op / 3π 0 2U _op / 5π

In table 2, ω ₁ = 2πf _T , where f _T = 1 / T _c , and T _c - the duration of the conversion cycle. As can be seen from the above expressions and time diagrams of figure 2-3, the harmonics of the decomposition of the signal obtained by the prototype method, lower frequency. Therefore, the settling time for filtering is less for the proposed method. In addition, when converting another code, for example, the code N = 111 ... 111 by the prototype method, the number of spectral components increases significantly, which, in turn, leads to additional non-linearity of the conversion. The proposed method allows to obtain a signal in the entire conversion range, the decomposition of which into spectral components is presented in table 1. The discrete spectrum of harmonic components does not change depending on the code being transformed, which determines the best linearity of the final transformation.

The proposed method of digital-to-analog conversion can be explained by the operation of a device that implements this method. In the device of FIG. 1, which implements the proposed method, the elements: key 9 and capacitor 10, which store information in the previous cycle at the interval of the PWM pulse, may not be a sampling and storage device (UX) with strictly normalized drift parameters (decay rate ) The presence of such an error does not interfere with the operation of the proposed DAC. In general, the key 9 and the capacitor 10 constitute an additional low pass filter whose time constant is dependent on the state of the key 9. If the key 9 is closed, the time constant τ _of this additional filter should be much smaller than the time constant τ at the _time open key. In this case, the quantum h of the DAC conversion is determined by the voltage difference across the capacitor 10 in the conversion cycle, equal in duration to 1 / f _t . The refinement of the conversion quantum, or the full-scale DAC to the set value, can be performed, for example, by adjusting the transfer coefficient of the active filter 7 with a constant component.

Thus, the same filtering conditions in the entire conversion range provided by the proposed method allow us to find the optimal low-pass filter parameters and provide higher performance for a given conversion linearity.

Information sources

1. Fedorkov B.G., Taurus V.A. DAC and ADC chips: operation, parameters, application. - M .: Energoatomizdat, 1990 .-- 320 p.

2. A.S. No. 1735999 of the USSR, Н03М 1/66. Digital-to-analog converter / G.S. Vlasov, S.E. Lyakh and V.G. Saraev. // Publ. 1992, bull. No. 19.

3. Microchips ADC and DAC. - M.: Dodeka-XXI Publishing House, 2005. - 432 p., P.13.

4. Metrological tools for checking digital devices / Devices, automation equipment and control systems: TS-5, Issue 3. - M .: INIITEI instrument making, 1982. - 62 p., P.16.

5. Scherbakov V.I., Grezdov G.I. Electronic circuits on operational amplifiers: Reference. - K .: Technika, 1983.- 213 p., P. 90-91.

Claims (1)

A method of digital-to-analog conversion, including obtaining a pulse PWM signal, the period of which is determined by the bit depth of the converted code and the frequency of the master oscillator, and the duty cycle is inversely proportional to the converted code, normalization of the amplitude of the received signal and its subsequent filtering in the low-frequency region, characterized in that before filtering, is performed the amplitude modulation of the PWM signal by the amplitude-normalized rectangular modulated signal at the frequency of the master oscillator, and in one and half cycles of the modulated signal in the information storing interval conversion performed previous value intermediate conversion.

Images