廬
DYNAMIC PERFORMANCE TESTING OF
DIGITAL AUDIO D/A CONVERTERS
By Larry Gaddy and Hajima Kawai
This application bulletin will provide the reader with an
understanding of test methods for audio digital-to-analog
converters (DACs). In particular, delta-sigma (鈭單? DACs
require a special understanding due to the nature of their
architecture. The dynamic specifications of interest for an
audio DAC are THD+N (total harmonic distortion + noise),
dynamic range, channel separation, and idle channel noise
(often referred to as SNR, signal-to-noise ratio). There are
many possible digital audio applications, but only CD-DA
(reproduction of digital audio from a compact disc) has a
standard specification defined by EIAJ (Electronic Industry
Association of Japan). Test methods for evaluating DACs
used in other digital audio applications are somewhat am-
biguous. For these applications, evaluating the DAC is
highly dependent upon the experience and knowledge base
of the design engineer. Digital audio reproduction is based
upon sampling and Nyquist theorems. This application bul-
letin will describe the operation theory of
鈭單?/div>
audio DACs,
and associated test methods for these DACs.
SAMPLING AND NYQUIST THEOREMS
Digital audio can be reproduced when the data obeys the
Nyquist criteria. This criterion states any signal can be
completely described when the bandwidth of the sampled
signal is less than one half of the sampling frequency. For a
DAC, this means the sampling frequency must be at more
than twice the value of the highest frequency for reproduc-
tion. Figure 1a illustrates proper sampling technique, where
the sampling frequency f
S
is more than twice the value of the
frequency of interest f
A
. In figure 1b, f
S
is less than 2f
A
,
violating the Nyquist sampling theorem and creating signal
aliasing. The frequency spectra shown in figures 1a and 1b
represent the reconstructed analog signals. CD players use a
sampling frequency of 44.1kHz, which allows for reproduc-
tion of signals up to 22.05kHz, more than the accepted upper
audio bandwidth limit of 20kHz.
OUTPUT SPECTRA IS DEPENDENT UPON DAC
ARCHITECTURE
There are two primary DAC architectures used in digital
audio applications: R-2R and
鈭單?
Consumer-level and cost-
sensitive applications tend to use
鈭單?/div>
DACs, which have
higher integration and lower cost. The
鈭單?/div>
DAC is also
known as noise-shaping or one-bit DACs. In most cases, the
original 16-bit audio data is digitally filtered and interpo-
lated. This filtering function is external for most R-2R DACs
and internal to most
鈭單?/div>
DACs.
漏
A
(a)
f
S
鈮?/div>
2f
A
max
f
f
A
max
f
S
鈥?f
A
max
A
(b)
f
S
< 2f
A
max
f
S
f
S
+ f
A
max
Aliasing
f
f
S
鈥?f
A
max f
A
max
f
S
FIGURE 1. Sampling Theorem and Nyquist Theorem.
OUTPUT SPECTRA FOR AN R-2R DAC
Early generation digital audio was reproduced using R-2R
type DACs, sometimes called current-steering or ladder
DACs. The architecture for the R-2R DAC is very similar to
instrumentation-grade DACs, where all bits are converted
simultaneously (although these DACs accept standard serial
digital audio data, they utilize internal serial-to-parallel
converters). This architecture is used in Burr-Brown鈥檚 audio
DACs such as PCM56, PCM61, PCM63, PCM1700 and
PCM1702. This architecture has a theoretical noise limit
equivalent to the random quantization noise contained in the
digital audio data. The R-2R DAC does not contain any
internal filtering or noise shaping. In practice, the noise floor
is limited by analog noise sources, such as resistor noise and
1/f noise. These noise sources are typically less than
鈥?00dB, as shown in Figure 2.
D
IN
Conventional
DAC
A
A
OUT
A
Digital
Analog
DAC Noise
f
f
S
f
S
f
FIGURE 2. Output Spectrum for an R-2R DAC.
1997 Burr-Brown Corporation
AB-104
Printed in U.S.A. May, 1997
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