Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> Audio
Circuits > APP 3735

Mar 13, 2006

Keywords: integer, non-integer, MAX9850, master
clock
Related
parts

Download,
PDF format (117kB)

[?]

APPLICATION NOTE 3735

Audio-DAC Performance Investigation

Abstract: The
MAX9850 audio DAC can operate from a wide range of
master-clock frequencies with varying levels of performance.
By selecting different master-clock frequencies, designers can
balance performance with ease of design. This application note
analyzes the performance of the MAX9850 with various
master-clock to sample-rate ratios, showing how the
master-clock frequency affects performance.

Attend
this brief webcast by Maxim on
TechOnline

The
MAX9850 is an
audio DAC
that features unique clocking circuitry that allows it to
achieve good audio performance from a wide range of
master-clock frequencies. Typical audio DACs require a master
clock that is an exact multiple of the sample
rate in use. For example, if the sample rate is 48kHz, the
most common master clock is 48kHz x 256 = 12.288MHz. The
MAX9850 can operate from standard audio clock rates, but it
will also operate from commonly available system clocks, such
as 12MHz, with reduced performance. With a wide range of
acceptable master-clock frequencies, the MAX9850 allows
designers select a master-clock frequency that appropriately
balances audio performance with ease of design.

Integer Modes¹
To achieve maximum performance, both
the master and slave integer modes are available. These modes
require a master clock that is an exact integer multiple of
the sample rate in use. Master mode sets the MAX9850 to output
word and bit clocks; slave mode sets the MAX9850 to input word
and bit clocks. Figure 1 shows an FFT
of the MAX9850 operating in master-integer mode. The input
signal is a full-scale 1kHz sine wave sampled at 48kHz. The
master clock is a synchronous 12.288MHz signal.

Figure
1. Master-integer mode. MCLK = 12.288MHz, FS =
48kHz, 0dBFS.

The amplitude of the distortion in
master-integer mode is at or below -90dB at all frequencies.
The resulting THD+N is -85dB. The distortion that does appear
is all harmonic, and is at such a level that it is likely to
be masked from audibility to most listeners. As the MAX9850 is
intended for use in playback systems reproducing audio that
has already been encoded by a lossy algorithm, such as MP3,
the distortion created in this mode is not likely to be the
limiting factor in the sound quality of the replay
chain.

In Figure 2, the MAX9850 operates in
slave mode with the same clock, sample rate, and input signal
as in Figure 1. The performance is unaffected because all the
clocks are synchronous and exact integer multiples of each
other.

Figure
2. Slave-integer mode. MCLK = 12.288MHz, FS =
48kHz, 0dBFS.

Noninteger Modes
While integer mode provides the best
performance, it requires a clock frequency that is an exact
integer multiple if the sample rate (FS—an exact
multiple of 16 x FS, to be exact). Clock
frequencies that are compatible with integer mode are not
commonly used by other parts of a system. In many
portable-audio systems, a commonly available clock frequency
is 12MHz, because it is used by USB.
Because this clock is already available in the system, it is
simpler to use this clock for audio rather than a dedicated
clock. Although 12MHz is not an exact multiple of 16 x
FS for any audio sample rate, it is an integer
multiple of 48kHz. As a result, although integer mode cannot
be used, the noninteger circuitry can easily create
appropriate internal clock rates. Figure 3 shows the
frequency spectrum of a system using a 12MHz clock to
reproduce a 1kHz signal sampled at 48kHz. The performance of
the MAX9850 in this mode is degraded—but not
significantly—from the performance obtainable in integer mode.
The largest harmonic is still roughly 90dB below the signal
amplitude, but now additional nonharmonic distortion elements
appear in the signal. The THD+N is only slightly reduced to
-83dB.

Figure
3. Master noninteger mode. MCLK = 12MHz, FS =
48kHz, 0dBFS.

Switching to slave mode (Figure
4) eliminates the nonharmonic distortion elements and
provides performance comparable to integer mode.

Figure
4. Slave noninteger mode. MCLK = 12MHz, FS = 48kHz,
0dBFS.

Figure 5 shows an FFT of the MAX9850
output operating from a 12MHz clock in master noninteger mode
with input data sampled at 44.1kHz. The MAX9850 is able to
reproduce the 1kHz sine wave, even though the sample rate and
master clock are not integer related. The performance hit
comes in the form of higher distortion. The distortion is also
no longer all harmonic
distortion, due to the data and clock manipulations
required to make the DAC operate from a noninteger clock. The
THD+N is measured at -71dB in this mode.

Figure
5. Master noninteger mode. MCLK = 12MHz, FS =
44.1kHz, 0dBFS.

Although in integer mode, switching
between master and slave mode has no effect on the
performance; in noninteger mode, this is no longer the case.
Switching from master to slave operation now further reduces
the performance of the converter, as shown in Figure 6.
The THD+N increases to -65dB.

Figure
6. Slave noninteger mode. MCLK = 12MHz, FS =
44.1kHz, 0dBFS.

Distortion as a Function of Input Amplitude
While at
large signal amplitudes, the signal can mask some distortion.
As the signal amplitude decreases, the distortion must
decrease as well to maintain good sound quality. Figures
7 to 11 show the MAX9850's output spectrum for
-30dBFS and -60dBFS signal levels. For the modes where there
is an integer relationship between the master clock and the
sample rate, all distortion elements are below the noise floor
at signal amplitudes of -30dBFS. For noninteger-related sample
rates and master clocks, such as 44.1kHz and 12MHz, only low
amplitude distortion remains. At -60dBFS levels, distortion
that is inherent in the architecture becomes visible in the
spectrum. This distortion is not directly related to the
sample rate or master clock, so as a result is identical in
all modes.

Figure
7. Master/slave integer mode. MCLK = 12.288MHz, FS
= 48kHz, -30dBFS and -60dBFS.

Figure
8. Master noninteger mode. MCLK = 12MHz, FS =
48kHz, -30dBFS and -60dBFS.

Figure
9. Slave noninteger mode. MCLK = 12MHz, FS = 48kHz,
-30dBFS, -60dBFS.

Figure
10. Master noninteger mode. MCLK = 12MHz, FS =
44.1kHz, -30dBFS, -60dBFS.

Figure
11. Slave noninteger mode. MCLK = 12MHz, FS =
44.1kHz, -30dBFS, -60dBFS.

Conclusion
At full-scale single levels, significant
distortion can be measured in some of the modes. Although it
is measurable, it is not particularly audible because the
human ear picks out the loud signal and the quieter distortion
elements are ignored. This same concept is used in many lossy
compression algorithms (such as MP3 and AAC) where part of the
compression is achieved by eliminating signals that are masked
by more powerful signals. In both cases, significant amounts
of data can be eliminated without significantly sacrificing
sound quality. Similarly, a converter can create some amount
of distortion without adversely affecting sound
quality.

When the human ear listens to low-level
signals, it is much better at detecting distortion in the
signal, as there is no longer a large signal to provide
masking. At these lower levels the MAX9850 produces distortion
near or below the noise floor, maintaining good sound quality.

The MAX9850 also achieves good signal-to-noise
ratio and dynamic
range, regardless of operating mode. Because the spectra
created by a -60dBFS input signal are the same regardless of
operating mode, the dynamic range is identical for all modes.
Similarly, the noise floor is the same for all operating
modes, so the signal-to-noise ratio is the same in all
modes.

Even in the lowest performance mode, the MAX9850
is able to achieve useful results. In systems where sound
quality is not of paramount importance, any available system
clock can be connected to the device and it will function. For
systems that demand better audio quality, the option still
exists to provide a dedicated audio clock and operate the DAC
in a fully synchronous mode.

Appendix: Measurement Settings

Circuit Board

MAX9850EVKIT

Signal Generator

Audio Precision SYS-2722 with PSIA-2722Bit depth: 16
bits

Signal Analyzer

Audio Precision SYS-2722 with PSIA-2722
22Hz to
22kHz bandwidth
A-weighted filtering
FFT input
A/D: HiRes A/D
@ 65536
FFT Samples: 32768
FFT Window:
Blackman-Harris
Averages: 4

Power Supply

Agilent E3630A triple-output DC power supply
AVDD = 3.0V, PVDD = 3.0V

Clock Source

12.288MHz generated by the PSIA-2722
12MHz
generated by the EV kit's crystal
oscillator

¹Refer to the MAX9850 datasheet
for complete details on configuring the MAX9850.

Related parts

MAX9850

Stereo Audio DAC with
DirectDrive® Headphone Amplifier

Free
samples

MAX9851

Stereo Audio CODECs
with Microphone, DirectDrive Headphones, Speaker
Amplifiers, or Line Outputs

Free
samples

MAX9853

Stereo Audio CODECs
with Microphone, DirectDrive Headphones, Speaker
Amplifiers, or Line Outputs

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (117kB)
APP 3735: Mar 13, 2006

AN3735, AN 3735, APP3735,
Appnote3735, Appnote 3735

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products