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KEY PERFORMANCE PARAMETERS AND MEASUREMENT

Total Harmonic Distortion + Noise

S/PDIF

Receiver

Evaluation Board

f

− 3dB = 54kHz or 108kHz

PCM1742

DEM−DAI1742

2nd−Order

Low−Pass

Filter

Notch Filter

Band Limit

HPF = 22Hz

LPF = 30kHz

f

C = 1kHz

rms Mode

0dBFS,

1kHz Sine Wave

S/PDIF

Output

Analyzer

and

Display

20kHz

Apogee

Filter

Digital

Generator

Dynamic Range

PCM1742

SBAS176A – DECEMBER 2000 – REVISED APRIL 2005

This section provides information on how to measure key dynamic performance parameters for the PCM1742. In

all cases, a System Two™ Cascade audio measurement system by Audio Precision™ or equivalent audio

measurement system is used to perform the testing.

Total harmonic distortion + noise (THD+N) is a significant figure of merit for audio DACs, because it takes into

account both harmonic distortion and all noise sources within a specified measurement bandwidth. The true rms

value of the distortion and noise is referred to as THD+N. Figure 35 shows the test setup for THD+N

measurements.

For the PCM1742, THD+N is measured with a full-scale, 1-kHz digital sine wave as the test stimulus at the input

of the DAC. The digital generator is set to a 24-bit audio word length and a sampling frequency of 44.1 kHz or 96

kHz. The digital generator output is taken from the unbalanced S/PDIF connector of the measurement system.

The S/PDIF data is transmitted via a coaxial cable to the digital audio receiver on the DEM-DAI1742

demonstration board. The receiver is then configured to output 24-bit data in either I

2S or left-justified data

format. The DAC audio interface format is programmed to match the receiver output format. The analog output is

then taken from the DAC post filter and connected to the analog analyzer input of the measurement system. The

analog input is band-limited using filters resident in the analyzer. The resulting THD+N is measured by the

analyzer and displayed by the measurement system.

Figure 35. Test Setup for THD+N Measurements

Dynamic range is specified as A-weighted, THD+N measured with a –60-dBFS, 1-kHz digital sine wave stimulus

at the input of the DAC. This measurement is designed to give a good indicator of the DAC performance given a

low-level input signal.

The measurement setup for the dynamic range measurement is shown in Figure 36 and is similar to the THD+N

test setup discussed previously. The differences include the band limit filter selection, the additional A-weighting

filter, and the –60-dBFS input level.

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