Low-Noise Isolated Digital Shunt for Precision Class-D Power Amplifiers

Power electronic converters regularly employ isolated analog-to-digital converters (ADCs) for the acquisition of voltage/current signals that are required for their digital control systems. In precision power converters, such as those required in nanometer-accuracy positioning systems for integrated circuit manufacturing, the noise content of the measured signals is critical since it leads to undesired noise in the power converter output signals due to the feedback system's high sensor noise sensitivity. This letter illustrates how sampling jitter, i.e., stochastic deviations of the ADC's sampling instants from their ideal occurrences, which is mainly caused by the digital signal isolators used to isolate the ADC control signals, introduces a measurement error and critically influences the signal to noise ratio (SNR) of an isolated current measurement at the output of a half-bridge. Jitter figures of commonly used digital signal isolators are measured, as these values are rarely available in the literature. It is revealed that RMS jitter varies significantly between the analyzed devices, from 13 ps up to 250 ps, even among those that utilize similar isolation techniques. Thus, in a second step, it is analytically derived how the half-bridge inductor, and the related half-bridge current ripple, directly influences the SNR of the current measurement. Using the resulting simple design equations, which are also verified with numeric simulations, the half-bridge inductor can be designed to significantly reduce the half-bridge current measurement noise, by a factor of up to ≈ 0.6 in common measurement configurations.