A Differential Measurement Probe with High Common Mode Rejection

Abstract: Measuring signals with high frequency common mode components are typically prone to measurement errors due to low common mode rejection at high frequencies. The purpose of this master thesis is to design a measurement probe that presents sufficient common mode rejection to measure current on high voltage motor drives directly at the switching stage, before filtering. Outlined in this work is the design process, on a block diagram and simulation level, culminating in the construction of a prototype. The measurement probe consists of a sensor head and a receiver that are fiber optically isolated from each other. It has a signal chain that splits the signal into a high and a low frequency path. The high frequency path is frequency modulated and the low frequency path is digital. In the receiver, these two paths are joined before being output as an analog signal. A signal chain is simulated that shows a DC-100 MHz response with an amplitude accuracy of 2.9 %, dynamic range of 53 dB, dropping to 38 dB at the end of the passband and, a spurious free dynamic range greater than 40 dB. Furthermore, a prototype is constructed but has not been debugged to a degree that permits meaningful measurement results.