1 Introduction

The AMC3301 family of devices has two sources of radiated emissions, as shown in Figure 1-1, the capacitive data path shown below in red and the integrated DC/DC converter shown in blue. The radiated emissions performance of the data path is the same as the AMC1300B-Q1 and contributes very little radiated emissions as shown in this Best in Class Radiated Emissions EMI Performance with the AMC1300B-Q1 Isolated Amplifier technical write paper. The second and largest source of radiated emissions for the AMC3301 family is the integrated DC/DC converter that operates at a frequency of 30 MHz with spread spectrum modulation. The coils of the internal DC/DC converter have a parasitic capacitance from the primary (user) side to the secondary (high) side of the isolation barrier. The primary driver generates a common-mode voltage between the isolated grounds, HGND and GND that has a quasi-resonant nature and generates harmonics to higher frequencies. Because of the nature of the isolation barrier, the energy is unable to find a conductor to return to the source. With no path back to the source, the energy radiates from the device pins (and any traces or PCB planes they are connected to) in the form of radiated emissions.

Input traces and cables that are connected to the isolated amplifier or converter act as antennas for the electro-magnetic energy injected between HGND and GND. The size and shape of the traces and cables directly affect the magnitude of the radiated emissions over frequency. As a general rule, shorter antennas radiate more effectively at higher frequencies, while longer antennas radiate more effectively at lower frequencies. When designing with the AMC3301 family, input traces and cables should be kept as short as possible to limit the magnitude of radiated emissions.