PSI - Issue 43

Dagmar Faktorová et al. / Procedia Structural Integrity 43 (2023) 288–293 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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The DDS generator consists of an ACC accumulator with phase increment INC, which periodically increases the value of the accumulator. The upper 9 bits of the accumulator are used as an index to the look-up table (LUT), which contains 512 values of the sine function for one period of the output signal. The double buffer technology is used to transfer values from the LUT table to the DAC converter, where buffer switching and data transfer is controlled by the DMA controller timer. 3.2 Analog circuits The analog part of the impedance converter implementation is shown in Fig. 4. It is connected to two pins of the MCU – the output of the DAC converter (PA4) and the input of the ADC converter (PA5).

Fig. 4. Basic circuit of the analog part of the impedance converter (without calibration and power supply circuits).

A JFET type operational amplifier (LF411) is used as a current-to-voltage (C/V) converter. The sensitivity of the C/V converter is determined by the feedback resistor R ref . An external rail-to-rail operational amplifier (MCP 6272) is connected to the input of the ADC converter, resistor R f together with capacitor C f form a low-pass filter. 3.3 Data processing The result of each measurement cycle is an array of measured values from the ADC converter and the corresponding array of indices to the harmonic function table. The array of indices is important for the reconstruction of the signal waveform at the output of the DDS generator and for generating the values of the tabulated orthogonal sine and cosine functions with the correct phase shift with respect to measured data. The mathematical coprocessor, which is a part of the CPU, can be used for mathematical calculations. The individual parts of the measuring system introduce time delays into the measurement results, which depend on the operating frequencies of the ADC and DAC converters. They also indroduce the phase shifts and amplitude changes, which depend on the generated DDS frequency. At the same time, components can change their properties due to ageing process as well as temperature. For this reason, it is necessary to calibrate the amplitude and phase characteristics of the analyzer. The simplest way to calibrate the measuring system is to use a non-inductive resistor as the reference calibration impedance, which will be connected to the measuring system instead of the measured impedance. 3.4 Properties of the impedance converter implementation The described concept of impedance converter was implemented on MCU STM32L476RG, see STMicroelectronics N.V. (2019), with CPU core frequency 80 MHz. The CPU's own overhead at this frequency to operate the entire impedance converter was less than 20% of the CPU time.