Cirrus Logic AN263 - Manual

Copyright © Cirrus Logic, Inc. 2004
(All Rights Reserved)
http://www.cirrus.com
AN263
Audio Conversion Systems Noise Calculations and
Requirements
Steve Green
Technical Marketing Manager
Mixed-Signal Products
1. Introduction
The skills required to predict the dynamic range of a combined Analog-to-Digital converter (ADC) and Dig-
ital-to-Analog converter (DAC) system, as well as determine the noise requirements for the analog input
and output stages, should be considered essential for an audio systems designer. The techniques re-
quired are relatively basic in that they are generally covered in the first analog circuit analysis class in
most engineering programs. However, applying these techniques to the conversion processes often gen-
erates some level of confusion. This discussion will detail the steps required to apply these techniques
and determine this critical system performance parameter.
Figure 1. Conversion System Block Diagram
The ADC and DAC system, Figure 1, contains four primary noise sources that must be considered. These
include the analog input buffer, ADC, DAC and the analog output buffer/ filter stage. Noise generated in
each of these stages adds as the “square root of the sum of the squares” as shown below. This funda-
mental relationship will be used throughout this discussion.
The analysis requires several initial assumptions and setting of limits.
1) The bandwidth is set to 20 kHz. This constraint is purely for convenience and allows the use of a com-
mon converter data sheet specification. The results can easily be scaled to other bandwidths as long
as assumption #2 remains valid.
2) The spectral content of the noise is “white”. This assumes that the contribution of 1/f noise is negligible
and the noise sources within the converters remain “white” to the upper limit of the analysis. A word
Buffer DAC ADC Filter
V
total
V
1
2
V
2
2
V
3
2
V
4
2
+ + + =
Oct ‘04
AN263REV1