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13

PCM1721

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

PCM1721:

For f

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 profes-

sional applications such as broadcast audio for studios, it is

important for total delay time to be less than 2ms.

OUTPUT FILTERING

For testing purposes all dynamic tests are done on the

PCM1721 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

specification numbers.

The performance of the internal low pass filter from DC to

24kHz is shown in Figure 9. The higher frequency rolloff of

the filter is shown in Figure 10. If the user’s application has

the PCM1721 driving a wideband amplifier, it is recom-

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

order filter is shown in Figure 11. For some applications, a

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

FIGURE 10. Low Pass Filter Wideband Frequency Response.

FIGURE 9. Low Pass Filter Frequency Response.

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)

FIGURE 11. 3rd-Order LPF.

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

OPA604