SBAA515A June 2021 – September 2022 AMC3301 , AMC3301-Q1 , AMC3302 , AMC3302-Q1 , AMC3306M05 , AMC3306M25 , AMC3330 , AMC3330-Q1 , AMC3336 , AMC3336-Q1
This document demonstrates how printed circuit board (PCB) input trace or cable design affects radiated emissions electromagnetic interference (EMI) performance for Texas Instruments’ AMC3301 precision isolated amplifier with integrated DC/DC converter. The AMC3301 family as shown in Table 6-1 does not produce excessive radiated emissions by themselves and are capable of passing CISPR 11 class B without additional components as shown in Figure 2-2 if the length of the input traces connected to the device are short. For designs requiring additional radiated emissions attenuation, ferrite bead and common-mode choke selection and placement recommendations are provided.
Several industrial and automotive applications require some type of isolation to protect the digital circuitry from the high-voltage circuit performing a function. Texas Instruments has an extensive portfolio of isolated amplifiers and converters featuring a SiO2 isolation barrier to help customers address their isolated data conversion needs. Texas instruments’ SiO2 isolation barrier allows for exceptional reliability, often over 100 years of operation. For more information on TI's SiO2 isolation barrier, please review the Isolation link. EMI testing is common in these applications to verify the system does not produce radiated emissions that exceed the defined levels which may negatively impact other components or circuits in the system. Please see this application note for a more in-depth description of EMI. The magnitude of acceptable radiation and testing procedure for radiated emissions is put in place by the Comité International Spécial des Perturbations Radio, also known as CISPR. Industrial applications measure according to the CISPR 11 standard, while automotive applications measure to the CISPR 25 standard. For more information on the CISPR standards and their respective magnitudes over frequency, please see this application note.
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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.
CISPR 11 peak measurements were performed with various input cable lengths and Texas Instruments’ AMC3301. The input cable lengths tested are a 1.5 m input, a 30 cm input and an input shorted at the input terminal of the evaluation module (EVM). The same AMC3301EVM was used for all tests and powered from an external battery. All measurements shown are in the horizontal, or worst-case, orientation. Refer to the test setups in Figure 2-1 and CISPR 11 radiated emissions EMI plots in Figure 2-2 and Figure 2-3.
Figure 2-2 shows the radiated emissions performance of the AMC3301 with an input short shown in blue. The AMC3301 shows very little radiated emissions above the noise floor in red – demonstrating that the AMC3301 does not produce excessive radiated emissions if the input traces or cables to the device are short.
Figure 2-3 shows the radiated emissions measurement for the 1.5 m input in blue, 30 cm in red and input short in green. Longer input traces and cables connected to the AMC3301 increase the magnitude of radiated emissions as shown by the 1.5 m input and 30 cm input test cases compared to the input short.