Avago Technologies

One of the difficult challenges that designers may face is to isolate precision analog signals in an extremely noisy environment. A good example is monitoring the motor phase current in a high-performance motor drive. A typical 3-phase induction motor drive operates from high-voltage DC power supply rails and converts the DC voltage to AC power to drive the motor. Pulse-width modulation (PWM) is commonly used to generate a variable frequency to control the power amp stage. The difficulty arises from the large voltage transients that are generated by the switching of the inverter transistors. These very large transients exhibit extremely fast rates of change (greater than 10 kV/ms), making it extremely difficult to sense the current flowing through each of the motor phases. Besides phase current, other key feedback information on bus current and voltage are also required to achieve smooth and stable motion control.