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PCM1702

DISCUSSION OF

SPECIFICATIONS

DYNAMIC SPECIFICATIONS

Total Harmonic Distortion + Noise

The key specifications for the PCM1702 is total harmonic

distortion plus noise (THD+N).

Digital data words are read into the PCM1702 at eight times

the standard compact disk audio sampling frequency of

44.1kHz (352.8kHz) so that a sine wave output of 1002Hz

is realized.

For production testing, the output of the DAC goes to an

I to V converter, then through a 40kHz low pass filter, and

then to a programmable gain amplifier to provide gain at

lower signal output test levels before being fed into an

analog-type distortion analyzer. Figure 1 shows a block

diagram of the production THD+N test setup.

For the audio bandwidth, THD+N of the PCM1702 is

essentially flat for all frequencies. The typical performance

curve, “THD+N vs Frequency”, shows four different output

signal levels: 0dB, –20dB, –40dB, and –60dB. The test

signals are derived from a special compact test disk (the

CBS CD-1). It is interesting to note that the –20dB signal

falls only about 10dB below the full scale signal instead of

the expected 20dB. This is primarily due to the superior low

level signal performance of the advanced sign magnitude

architecture of the PCM1702.

In terms of signal measurement, THD+N is the ratio of

Distortion

PCM1702, THD+N is 100% tested at all three specified

output levels using the test setup shown in Figure 1. It is

significant to note that this test setup does not include any

output deglitching circuitry. All specifications are achieved

without the use of external deglitchers.

Dynamic Range

Dynamic range in audio converters is specified as the mea-

sure of THD+N at an effective output signal level of –60dB

referred to 0dB. Resolution is commonly used as a theoreti-

cal measure of dynamic range, but it does not take into

account the effects of distortion and noise at low signal

levels. The advanced sign magnitude architecture of the

PCM1702, with its ideal performance around bipolar zero,

provides a more usable dynamic range, even using the strict

audio definition, than any previously available D/A con-

verter.

THEORY OF OPERATION

ADVANCED SIGN MAGNITUDE

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 at the major carry bipolar

zero transition. More recently, DACs employing a different

architecture which utilizes noise shaping techniques and

very high over-sampling frequencies, have been introduced

(“Bitstream”, “MASH”, or 1-bit DAC). These DACs over-

come the low level linearity problem, but only at the expense

of signal-to-noise performance, and often to the detriment of

channel separation and intermodulation distortion if the

succeeding circuitry is not carefully designed.

The PCM1702 is a new solution to the problem. It combines

all the advantages of a conventional DAC (excellent full

scale performance, high signal-to-noise ratio and ease of

use) with superior low-level performance. Two DACs are

combined in a complementary arrangement to produce an

extremely linear output. The two DACs share a common

reference, and a common R-2R ladder for bit current sources

by dual balanced current segments to ensure perfect tracking

under all conditions. By interleaving the individual bits of

each DAC and employing precise laser trimming of resis-

tors, the highly accurate match required between DACs is

achieved.

This new, complementary linear or advanced sign magni-

tude approach, 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 PCM1702 is such that real 20-bit reso-

lution can be realized, especially around the critical bipolar

zero point.

Table 1 shows the conversion made by the internal logic of

the PCM1702 from binary two’s complement (BTC). Also,

the resulting internal codes to the upper and lower DACs

(see front page block diagram) are listed. Notice that only

the LSB portions of either internal DAC are changing

around bipolar zero. This accounts for the superlative per-

formance of the PCM1702 in this area of operation.

INPUT CODE

LOWER DAC CODE

UPPER DAC CODE

ANALOG OUTPUT

(20-bit Binary Two's Complement)

(19-bit Straight Binary)

(19-bit Straight Binary)

+Full Scale

011...111

111...111

+Full Scale –1LSB

011...110

111...110

Bipolar Zero +2LSB

000...010

000...010

Bipolar Zero +1LSB

000...001

000...001

Bipolar Zero

000...000

000...000

Bipolar Zero –1LSB

111...111

111...111

000...000

Bipolar Zero –2LSB

111...110

111...110

000...000

–Full Scale +LSB

100...001

000...001

000...000

–Full Scale

100...000

000...000

000...000

NOTE: (1) The extra weight of 1LSB is added at this point to make the transfer function symmetrical around bipolar zero.

TABLE I. Binary Two's Complement to Sign Magnitude Conversion Chart.