The performance of the internal low pass filter from DC to
24kHz is shown in Figure 14. The higher frequency rolloff
of the filter is shown in Figure 15. If the user’s application
has the PCM1717 driving a wideband amplifier, it is recom-
mended to use an external low pass filter. A simple 3rd-
order filter is shown in Figure 16. For some applications, a
passive RC filter or 2nd-order filter may be adequate.
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
PCM1717:
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(20Hz~24kHz, Expanded Scale)
1.0
0.5
TD = 11.125 x 1/fS
For fS = 44.1kHz, TD = 11.125/44.1kHz = 251.4µs
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.
0
–0.5
–1.0
INTERNAL RESET
20
100
1k
10k
24k
Frequency (Hz)
When power is first applied to PCM1717, an automatic reset
function occurs after 1,024 cycles of XTI clock. Refer to
Table I for default conditions. During the first 1,024 cycles
of XTI clock, PCM1717 cannot be programmed (Software
Control). Data can be loaded into the control registers during
this time, and after 1,204 cycles of XTI clock, a "LOW" on
ML (pin 18) will initiate programming.
FIGURE 14. Low Pass Filter Frequency Response.
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(10Hz~10MHz)
10
5
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
OUTPUT FILTERING
For testing purposes all dynamic tests are done on the
PCM1717 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.
10
100
1k
10k
100k
1M
10M
Frequency (Hz)
FIGURE 15. Low Pass Filter Frequency Response.
GAIN vs FREQUENCY
90
6
Gain
–14
–34
–54
–74
–94
0
1500pF
–90
OPA604
+
10kΩ
10kΩ
10kΩ
–180
Phase
VSIN
680pF
100pF
–
–270
–360
100
1k
10k
100k
1M
Frequency (Hz)
FIGURE 16. 3rd-Order LPF.
®
13
PCM1717
BB [ BURR-BROWN CORPORATION ]
