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6

PCM1704

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.

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.

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.

DISCUSSION OF KEY

SPECIFICATIONS

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.

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).

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.

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.

OFFSET GAIN AND TEMPERATURE DRIFT

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.