For production testing, the output of the DAC is connected
to a current-to-voltage (I/V) converter. The output of the
I/V converter is then connected to a 40kHz, 3rd-order GIC
low-pass filter. The filter output is then passed on to a
programmable gain amplifier to provide gain for low-level
test signals before being fed into an analog distortion
analyzer (Shiba Soku Model 725 or equivalent).
THEORY OF OPERATION
SIGN-MAGNITUDE ARCHITECTURE
Digital audio systems have traditionally used laser-trimmed,
current-source DACs in order to achieve sufficient accuracy.
However, even the best of these suffer from potential low-
level nonlinearity due to errors in the major carry bipolar
zero transition. Current systems have turned to oversampling
data converters, such as the popular delta-sigma architec-
tures, to correct the linearity problems. This is done, how-
ever, at the expense of signal-to-noise performance, and the
noise shaping techniques utilized by these converters creates
a considerable amount of out-of-band noise. If the outputs
are not properly filtered, dynamic performance of the overall
system will be adversely effected.
For the audio bandwidth, the THD+N for the PCM1704 is
essentially flat for all frequencies.
DYNAMIC RANGE
Dynamic range in data converters is specified as the measure
of THD+N at an effective output signal level of –60dBFS
(conforms to EIAJ method with A-weighting applied). Reso-
lution is commonly used as a theoretical measure of dy-
namic range, but it does not take into account the effects of
distortion and noise at low signal levels. The sign-magnitude
architecture of the PCM1704, with its ideal performance
around bipolar zero, provides a more usable dynamic range
(even with the strict audio definition) than any other previ-
ously available D/A converter.
The PCM1704 employs an innovative architecture which
combines the advantages of traditional DACs (e.g., excellent
full-scale performance, high signal-to-noise ratio, and ease
of use) with superior low-level performance. This architec-
ture is referred to as sign-magnitude. Two DACs are com-
bined in a complementary arrangement to produce an ex-
tremely linear output. The two DACs share a common
reference, and a common R-2R ladder for bit current sources.
The R-2R ladder utilizes dual balanced current segments to
ensure ideal tracking under all conditions. By interleaving
the individual bits of each DAC and employing precision
laser-trimming of resistors, a highly accurate match between
the two DACs is achieved.
IDLE CHANNEL SIGNAL-TO-NOISE RATIO (SNR)
Another important specification for a digital audio converter
is idle channel signal-to-noise ratio (Idle Channel SNR).
This is the ratio of the noise on the DAC output at bipolar
zero compared to the full-scale range of the D/A converter.
To make this measurement, the digital input is continually
fed the code for bipolar zero, while the output of the DAC
is band limited from 20Hz to 20kHz and A-weighting is
applied. The ideal channel SNR for the PCM1704 is typi-
cally greater than 120dB, making it ideal for low noise
applications.
The sign-magnitude architecture, which steps away from
zero with small steps in both directions, avoids any glitching
or large linearity errors, and provides an absolute current
output. The low-level performance of the PCM1704 is such
that true 24-bit resolution can be realized around the critical
bipolar zero point.
OFFSET GAIN AND TEMPERATURE DRIFT
DISCUSSION OF KEY
SPECIFICATIONS
Although the PCM1704’s primary application is in high
performance digital audio systems where dynamic specifica-
tions are most important, specifications are also given for
more traditional DC parameters. These include gain error,
bipolar zero offset, temperature gain and offset drift. These
specifications are important in test and measurement sys-
tems, which is the other main systems application for the
PCM1704.
TOTAL HARMONIC DISTORTION + NOISE (THD+N)
This is the key specification for the PCM1704. Digital data
words are read into the PCM1704 at eight times the standard
DVD audio sampling frequency of 96kHz (e.g., 8 x 96kHz =
768kHz) to create a sinewave output of 1100Hz. The output
of the DAC is then passed through analog signal conditioning
circuitry before being input to a distortion analyzer.
®
6
PCM1704
TI [ TEXAS INSTRUMENTS ]
