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Technical Papers
The Effects and Reduction of Common-Mode Noise and Electromagnetic Interference in High-Resolution Digital Audio Transmission

Presented at the Audio Engineering Society Convention, New York October 2003

By Jon D. Paul, Vice President
Scientific Conversion, Inc.


High-resolution digital audio systems are especially susceptible to sources of electromagnetic noise from the environment, for example, crosstalk from adjacent cables. The noise can induce errors and increase jitter in the recovered clock signal.

We discuss the most important noise sources and their characteristics. Next, we analyze the noise susceptibility of typical transmitter and receiver circuits. Test results are provided for a system with induced common-mode noise. The paper concludes with circuit design, component and application considerations.


Transmitter Common-Mode Noise/EMI Current Paths

Simulation Results: CMRR vs. Frequency
without transformer, 15 pF Cp-s and 0.5 pF Cp-s transformer

High Capacitance vs. Low Capacitance Transformers 
30 dB CMRR difference at 10 MHz!

Improved Receiver Circuit: Beads, CM Choke and Shielded Transformer



Fig. 1 Unbalanced Transmission System

Fig. 2 Balanced Transmission System

Fig. 3 Balanced Transmission System with Noise Sources

Fig. 4 Normal-Mode and Common-Mode Currents

Fig. 5 Resistive Coupled Interference (“ground loop”)

Fig. 6 Capacitive (Electric) Coupled Interference

Fig. 7 Inductive (magnetic) Coupled Interference

Fig. 8 Common-Mode to Differential-Mode Conversion

Fig. 9 RS-485 Output Driver and IC Manufacturer’s Suggested Circuits

Fig. 10 Transmitter Common-Mode Noise/EMI Current Paths

Fig. 11 RX IC Manufacturer’s Suggested Circuits

Fig. 12 RX and RS-485 Input Circuit, Transfer Function

Fig. 13 Balanced RX Common-Mode Noise Current

Fig. 14 Receiver with Reclocking Stage Common-Mode Noise Current Paths

Fig. 15 Simulation Model: Common-mode Gain of Transformer + Diff Amp.

Fig. 16 Transformer and Differential Amp Simulation of CMRR vs. Frequency

Fig. 17 RX Differential Signal Eye Pattern with Common-mode Noise

Fig. 18 Common-mode Interference Test Fixture

Fig. 19 RMS Jitter and Histogram Test Equipment

Fig. 20 Common Mode Noise at TX

Fig. 21 RX Inputs With CM Interference

Fig. 22 TX IC Power CM current Hi Cap Trsf X vs. Shielded Low Cap Trsf S2

Fig. 23 RX Induced Jitter No Transformer Jitter 1.505 ns RMS

Fig. 24 RX Induced Jitter Transformer X Jitter 3.104 ns RMS

Fig. 25 RX Induced Jitter TX Transformer Y, Jitter 7.83 ns RMS

Fig. 26 RX Induced Jitter Low Capacitance Transformer S1, Jitter 297 ps RMS

Fig. 27 TX Induced Jitter No Transformer Jitter 19.367 ns RMS

Fig. 28 TX Induced Jitter Hi Capacitance Transformer X, Jitter 84.5 ns RMS

Fig. 29 TX Induced Jitter Lo Capacitance Transformer S1, Jitter 1.64 ns RMS

Fig. 30 TX Induced Jitter Shielded Transformer S2, Jitter 302 ps RMS

Fig. 31 Jitter Histogram Interpretations

Fig. 32 Jitter Histogram Interpretations

Fig. 33 Reclocking RX with Transformer Isolation

Fig. 34  Balanced Transmission System Using Shielded Transformers

Fig. 35  Application of Transformers to Unbalanced System

Fig. 36 Transformer Parameter Comparison

Fig. 37 Transformer Construction

Fig. 38 Improved Receiver Circuit with Shield Beads, Choke & Transformer

Fig. 39 Common-mode Chokes

Fig. 40 Suggested PCB Groundplane  for Transformer & Receiver

Fig. 41 Double Termination

Fig. 42 Double Termination vs. Secondary Termination Reflection Coefficient

Summary and Conclusions

The circuitry of a high-resolution digital audio interface is especially sensitive to common-mode noise, crosstalk and electromagnetic interference. The design of interface circuits and printed circuit board layouts must consider this interference susceptibility to realize high quality, low jitter transmission of digital audio signals. 

New product designs should be tested for interference susceptibility to detect and debug the effects of noise and interference on clock jitter and data errors.

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Copyright © 2008 Scientific Conversion, Inc. All rights reserved. Information is subject to change without notice.


Last revised: 8 March 2008

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