Isolated sigma-delta (Σ-Δ)-based analog-to-digital converters (ADCs), such as those from Analog Devices, have become the preferred method for phase current measurement in high-performance motor and servo drives. In order to extract maximum performance, it’s important to optimize the system, including the demodulation of sigma-delta coded data using sinc filters and implementing sinc filters with HDL code.
One of the most critical parts of a motor control algorithm is the phase current measurement. Modern ADCs such as the ADuM7701 are designed specifically for this function, however, it’s important to incorporate sinc filters in order to synchronize them to the rest of the system. An important note about sinc filters is that they are not a representation of what the input to the sigma-delta ADC is at that instant. Rather, the output is a weighted average of what the input was in an impulse during a windowed period in the past, with the most weight being given to data at the center of the sampling range. In order to correct any present errors with this method, the synchronization of the impulse response to pulse width modulation (PWM) must occur.
The structure of the sinc filters can be improved as well, especially for applications that require tight timing control of the feedback chain. In a traditional sinc filter, all the modulator clock and filter clock are controlled by the system clock, meaning that they operate in an on-off fashion, which can lead to inaccuracies. However, by incorporating a clock generator function that separates the modulator clock from the integrator clock, it becomes possible to continuously clock the ADC while only using the integrator clock when obtaining a measurement. Once the system is configured correctly, accurate measurement, precise control, and application adaptability are normalized.
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