AN165

www.DataSheet4U.com AN165 Application Note CS4297A/CS4299 EMI REDUCTION TECHNIQUES 1. INTRODUCTION The CS4297A and CS4299 AC ’97 audio codecs are based on a new and faster fabrication process, and certain precautions in the analog I/O circuitry may be required to prevent 49.1 MHz commonmode radiation. Note that 49.1 MHz is 2 times the CS4297A internal clock frequency of 24.576 MHz. 3. EMI SHUNTING CIRCUIT The key to reducing common-mode emissions is to add decoupling (called shunting) at the audio I/O connectors. The shunt capacitors must be connected to a "clean" (free of digital noise) I/O ground. The recommended EMI shunting circuit is shown in the following diagram, and includes ferrite beads and 1000 pF NPO capacitors. The ferrite bead values are typically selected for an impedance of 100 ohms at 100 MHz. The ferrite bead and capacitor circuit create a low pass filter to attenuate frequencies above 1.6 MHz. Additionally, the following PCB layout checklist includes generally accepted practices for reducing both differential-mode and common-mode radiated emissions, while maintaining audio quality. 2. DESCRIPTION Common-mode radiation is the result of undesired voltage drops due to voltage differentials in the I/O ground system. The CS4297A clock transitions result in a rush of current that may induce the digital ground plane to "bounce". When external speaker or microphone cables are connected to the audio system, any common-mode voltage potential on the analog ground ...