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10

PCM1728

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

PCM1728:

TD = 30 x 1/fS

For fS = 44.1kHz, TD = 30/44.1kHz = 680µs

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

CD audio, DVD audio, 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

PCM1728 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

40kHz is shown in Figure 10. The higher frequency roll-off

of the filter is shown in Figure 11. If the user’s application

has the PCM1728 driving a wideband amplifier, it is recom-

mended to use an external low pass filter.

BYPASSING POWER SUPPLIES

The power supplies should be bypassed as close as possible

to the unit. Refer to Figure 12 for optimal values of bypass

capacitors.

1

10

100

1k

10k

100k

1

0.5

0

–0.5

–1

Log Frequency (Hz)

FIGURE 10. Low Pass Filter Response.

FIGURE 11. Low Pass Filter Response.

POWER SUPPLY

CONNECTIONS

PCM1728 has four power supply pin for digital (VDD), and

analog (VCC). Each connection also has a separate ground. If

the power supplies turn on at different times, there is a

possibility of a latch-up condition. To avoid this condition,

it is recommended to have a common connection between

the digital and analog power supplies. If separate supplies

are used without a common connection, the delta between

the two supplies during ramp-up time must be less than

0.1V.

1

10

100

1k

10k

100k

10M

1M

20

0

–20

–40

–60

–80

–100

Log Frequency (Hz)