APPLICATION
CONSIDERATIONS
DELAY TIME
There is a finite delay time in delta-sigma converters. In
analog-to-digital converters, this is commonly referred to as
latency. For a delta-sigma DAC, 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 the PCM1727:
t
D
= 11.125
x
1/f
S
For f
S
= 44.1kHz, t
D
= 11.125/44.1kHz = 251.4µs
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.
OUTPUT FILTERING
For testing purposes, all dynamic tests are done on the
PCM1727 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 not audible, it may affect dynamic specification
numbers.
The performance of the internal low-pass filter from DC to
24kHz is shown in Figure 10. The higher frequency rolloff
of the filter is shown in Figure 11. If an application has the
PCM1727 driving a wideband amplifier, it is recommended
to use an external low-pass filter. A simple 3rd-order filter
is shown in Figure 12. For some applications, a passive RC
filter or 2nd-order filter may be adequate.
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(20Hz~24kHz, Expanded Scale)
1.0
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(10Hz~10MHz)
10
5
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
10
100
1k
10k
100k
1M
10M
Frequency (Hz)
0.5
dB
0
–0.5
–1.0
20
100
1k
Frequency (Hz)
10k
24k
FIGURE 10. Low-Pass Filter Frequency Response.
FIGURE 11. Low-Pass Filter Wideband Frequency Response.
dB
GAIN vs FREQUENCY
6
Gain
–14
1500pF
+
V
SIN
–
–74
–270
0
90
10kΩ
10kΩ
680pF
10kΩ
100pF
–54
Phase
–180
–94
100
1k
10k
Frequency (Hz)
100k
1M
–360
FIGURE 12. 3rd-Order Low-Pass Filter.
12
PCM1727
www.ti.com
SBAS077A
Phase (°)
OPA604
Gain (dB)
–34
–90
TI [ TEXAS INSTRUMENTS ]
