High-speed and high-precision integrating analog-to-digital converter based on DPWM

0 Introduction

Digital signal processing can easily implement various advanced adaptive algorithms and complete functions that analog circuits cannot achieve. Therefore, more and more analog signal processing is being digitized. At present, the most widely used analog/digital converters are integral type, successive approximation type and ∑-△ type analog/digital converters. The integral type A/D converter generally adopts a dual slope integration method. Its principle is to convert the input voltage into time (pulse width signal) or frequency (pulse frequency), and then obtain the digital value by the timer/counter. The advantage is that high resolution can be obtained with a simple circuit; the disadvantage is that the conversion accuracy depends on the integration time, and the conversion rate is low. The ∑-△ type A/D converter consists of an integrator, a comparator, a 1-bit D/A converter and a digital filter. Its principle is similar to the integral type. It converts the input voltage into a time (pulse width) signal and obtains a digital value after being processed by a digital filter. The digital part of the circuit is basically easy to chip, so it is easy to achieve high resolution, but the cost is high and the overall chip is difficult. The principle of the existing A/D converter has high requirements on the performance and parameters of related analog devices, which is not easy to integrate. In applications that require A/D conversion, it is generally difficult to integrate high-performance A/Ds, and the corresponding IP cores need to be purchased; the principle of dual-slope integral A/Ds, etc., requires a negative reference voltage for reverse integration, and generally requires dual power supplies or negative voltage references, which brings inconvenience to many applications, affects versatility, and is slow, and generally does not support the coexistence of communication and display. In order to solve the above problems, DPWM technology is used for analog/digital conversion. On the one hand, it provides a convenient solution for applications such as MCU and FPGA that lack A/D resources. On the other hand, this solution has no special requirements for the performance of analog devices, is easy to integrate, can be used for chip manufacturing, and has a low cost. It can be applied to single power supply operation, and the use of a fast search algorithm can increase the conversion rate, while having the coexistence of communication and display.

1 Principle of high-speed and high-precision integral analog-to-digital converter

The basic working principle of the converter used here is to generate a pulse width signal (DPWM) through the DP-WM module. After the signal is filtered by a simple RC low-pass filter, it is compared with the detected signal by the comparator, processed and then sent out by the comparator. Finally, the signal sent by the comparator is picked up and analyzed by the logic operation module to obtain the relevant information of the detected signal and send it to the communication module and the display module. The specific scheme is shown in Figure 1. The converter is implemented using the DPWM principle. The duty cycle of the signal it sends has a certain corresponding relationship with the measured value, avoiding the complexity of the analog circuit design of the high-precision analog/digital converter. The conversion rate can be improved by using a fast search algorithm.

1.1 Digital pulse width modulation module design

The core control part of the converter can be realized by single chip microcomputer, DSP, FPGA, etc. It mainly completes the generation of DPWM, the measurement of analog signal and the display control of A/D conversion result. The design prototype uses Cy-cloneⅡFPGA as the control chip, and the overall structure of its program is shown in Figure 2.

The specific working process is as follows: a high-speed clock is obtained through a phase-locked loop to generate a high-resolution DPWM signal; the DPWM signal with a duty cycle adjusted according to a certain rule is used to control the voltage of the external capacitor and compare it with the input analog signal until the comparator flips. At this time, Duty×Vref is the A/D conversion result. In the system, a 50 MHz clock is input and multiplied to 400 MHz through a phase-locked loop. The A/D conversion accuracy reaches 165μV. The specific design is shown in Figure 3, and its signal functions are shown in Table 1.

1.2 DPWM generator design

The DPWM generation module sends out a high-resolution DPWM signal by updating the duty cycle setting value in real time. In this system, the frequency of the DPWM signal is 20 kHz and the DPWM accuracy is 20,000 clock cycles/duty cycle. As shown in Figure 4, its signal function is shown in Table 2.

1.3 Analog monitor design

The main function of this module is to monitor the comparison result INT in real time, continuously adjust DUTY, and obtain the final conversion result Duty display. The analog switch control signal ASW completes the control of a monitoring process. The process is as follows: capacitor discharge → ground measurement calibration → analog measurement (duty cycle change) → comparator flip to complete the conversion. The above three steps are continuously cycled. The analog measurement module is shown in Figure 5, and its signal function is shown in Table 3.

1.4 Display Controller Module Design

This module mainly completes the task of notifying the main module to update the display. In this design, the update cycle is 100 ms. The display control module is shown in Figure 6, and its signal functions are shown in Table 4.

2 Advantages of high-speed and high-precision integral analog/digital converters

2.1 Meet the single power supply conditions

In many applications, a single power supply is used for cost or system reliability considerations. Traditional integrating A/D converters require positive and negative dual power supplies. The integrator integrates the input voltage within a fixed time interval, which usually corresponds to the maximum number of internal counting units. After the time is reached, the counter is reset and the integrator input is connected to the negative power supply voltage. Under the action of this reverse polarity signal, the integrator is "reverse integrated" until the output returns to zero, and the counter is terminated and the integrator is reset. The accuracy of integrating A/D converters can be very high, but their sampling speed and bandwidth are very low.

The proposed integral analog-to-digital converter based on the DPWM principle can realize single power supply +5 V. When the polarity of the measured signal DPWM signal is the same, the feasibility of using a single power supply is shown. If the polarity of the measured signal is opposite to that of the DPWM signal, the op amp inverting amplifier method can be used to perform polarity conversion under single power supply conditions. Therefore, the scheme can work under single power supply conditions without adding an additional negative power supply. The principle is shown in Figure 7. At this time, VO=-R2Vin/R1, since Vin0, meeting the single power supply condition.

2.2 Easy chip integration

The integral analog/digital converter is implemented using the DPWM principle, with very few analog devices. Its main implementation method is that it only needs to generate a DPWM module, and only needs to add an ordinary analog operational amplifier and necessary logic units such as communication and operation externally; and it is easy to implement in FP-GA, and its code can be conveniently used in integrated chip design. In comparison, it is much easier to manufacture high-precision and high-linearity analog units than traditional analog/digital converters. The converter has a reasonable design and a simple structure. The duty cycle of the signal it sends has a definite corresponding relationship with the measured value, avoiding the complexity of the analog circuit design of the high-precision analog/digital converter, facilitating the integrated chip design, and can be used for chip manufacturing, and the cost is low, and it is also convenient for the implementation of single-chip microcomputers and programmable gate arrays.

2.3 Fast search algorithm improves A/D conversion speed

The initial search uses finite-step binary search, golden section method or random search (such as Monte Carlo method) to quickly determine the search range and then perform duty cycle traversal, which can greatly improve the A/D conversion speed.

2.4 Using dithering method to improve DPWM accuracy

Without taking any additional measures, the accuracy of DPWM depends on the switching frequency and the main frequency of the FPGA. To pursue higher accuracy, the main frequency can be increased or the switching frequency can be reduced. It is not realistic to simply increase the main frequency, and significantly reducing the switching frequency will affect the conversion speed. The dithering method can easily improve the accuracy of DPWM. This method can reduce the main frequency and power consumption, thereby reducing costs. In addition, performance can be improved at the same cost.

2.5 Several issues needing attention

The comparator will oscillate in the critical state, and the hysteresis comparison and logic blocking methods can be considered. The PWM reference requires a relatively stable reference source; the contradiction between accuracy and conversion speed can be coordinated according to specific needs; the influence of digital switching noise requires careful wiring and filtering to suppress; automatic gain technology can be appropriately used to improve low-voltage measurement accuracy.

3 Experimental Results

The DPWM output waveform of this paper is shown in Figure 8. Its integral test waveform after low-pass filter is shown in Figure 9. The analog-to-digital converter has high integral linearity and a resolution of 165μV.

4 Conclusion

Powered by a single power supply, the high-speed, high-precision, integral analog/digital converter based on the DPWM principle can be easily realized by a single-chip microcomputer, DSP, FPGA, etc. No external analog/digital converter is required, and it is easy to design integrated chips, avoiding the complexity of analog circuit design of high-precision A/D converters, and can provide a beneficial method for integration and related IC design. The use of a fast search algorithm can increase the conversion rate, and it has the functions of both communication and display, which is suitable for a wider range of applications.