Voltage/current measurement performance and power supply rejection in all-digital Class-D power amplifiers

Low-noise and low-distortion voltage and current acquisition circuits, both analog and digital, are required for the control systems of precision, high-power switch-mode (Class-D) amplifiers used in nanometer-accuracy mechatronic systems. In this paper, the achievable linearity and noise of analog signal processing elements, such as sensors or active filters, is investigated and their performance in a closed-loop control system is demonstrated. Resistive voltage dividers are sufficiently linear but are subject to resistor noise. Nonetheless, a signal-to-noise ratio (SNR) of 130dB is achievable. Shunt resistors can achieve a total harmonic distortion (THD), limited by self-heating, of <;-120dB, whereas galvanically isolated, industrial current sensors achieve an insufficient THD of ≈-90dB. High-order active op-amp filters, which are used, e.g., to amplify and condition a shunt resistor voltage, can achieve a THD of -120dB. Furthermore, as a subsequent signal digitization is required for the control system, it is shown how the SNR of analog-to-digital converters can be improved sufficiently for the discussed application by oversampling, and how this process affects the feedback system's phase margin. In order to reveal the performance of a closed-loop system employing the key parts of the signal acquisition components, the results of the sensor analysis are implemented in a comprehensive computer simulation model of a digitally controlled switch-mode power amplifier. The analyzed 400V, 500VA converter system achieves a THD <;-117dB and an SNR (DC-10kHz) >110dB while operating at switching frequencies up to 500kHz.