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8

PCM1744

APPLICATION

CONSIDERATIONS

DELAY TIME

There is a finite delay time in delta-sigma converters. In A/D

converters, this is commonly referred to as latency. For a

delta-sigma D/A converter, delay time is determined by the

order number of the FIR filter stage, and the chosen sampling

rate. The following equation expresses the delay time of

PCM1744:

Applications using data from a disc or tape source, such as

CD audio, CD-Interactive, Video CD, DAT, Minidisc,

etc., generally are not affected by delay time. For some

professional applications such as broadcast audio for stu-

dios, it is important for total delay time to be less than 2ms.

OUTPUT FILTERING

For testing purposes all dynamic tests are done on the

PCM1744 using a 20kHz low-pass filter. This filter limits

the measured bandwidth for THD+N, etc. to 20kHz. Failure

to use such a filter will result in higher THD+N and lower

SNR and dynamic range readings than are found in the

specifications. The low-pass filter removes out-of-band noise.

Although it is not audible, it may affect dynamic specifica-

tion numbers.

The performance of the internal low pass filter from DC to

24kHz is shown in Figure 6. The higher frequency roll-off of

the filter is shown in Figure 7. If the user’s application has

the PCM1744 driving a wideband amplifier, it is recom-

mended to use an external low-pass filter. A simple 3rd-

order filter is shown in Figure 8. For some applications, a

passive RC filter or 2nd-order filter may be adequate.

BYPASSING POWER SUPPLIES

The power supplies should be bypassed as close as possible

to the unit. It is also recommended to include a 0.1

µF ceramic

capacitor in parallel with the 10

µF tantalum bypass capacitor.

FIGURE 6. Low-Pass Filter Frequency Response.

FIGURE 7. Low-Pass Filter Wideband Frequency Response.

FIGURE 8. 3rd-Order LPF.

1.0

0.5

0

–0.5

–1.0

20

Frequency (Hz)

100

1k

10k

24k

INTERNAL ANALOG FILTER FREQUENCY RESPONSE

(20Hz~24kHz, Expanded Scale)

10

5

0

–5

–10

–15

–20

–25

–30

–35

–40

–45

–50

–55

10

100

1k

10k

100k

1M

10M

Frequency (Hz)

–60

INTERNAL ANALOG FILTER FREQUENCY RESPONSE

(10Hz~10MHz)

10k

Ω

10k

Ω

10k

Ω

1500pF

100pF

680pF

+

V

SIN

–

90

0

–90

–180

–270

–360

100

1k

10k

100k

1M

GAIN vs FREQUENCY

Frequency (Hz)

6

–14

–34

–54

–74

–94

Gain

Phase

OPA134